[PATCH v4 0/4] x86: Improve Minimum Alternate Stack Size

[PATCH v4 0/4] x86: Improve Minimum Alternate Stack Size

[Chang S. Bae via Libc-alpha]

During signal entry, the kernel pushes data onto the normal userspace
stack. On x86, the data pushed onto the user stack includes XSAVE state,
which has grown over time as new features and larger registers have been
added to the architecture.

MINSIGSTKSZ is a constant provided in the kernel signal.h headers and
typically distributed in lib-dev(el) packages, e.g. [1]. Its value is
compiled into programs and is part of the user/kernel ABI. The MINSIGSTKSZ
constant indicates to userspace how much data the kernel expects to push on
the user stack, [2][3].

However, this constant is much too small and does not reflect recent
additions to the architecture. For instance, when AVX-512 states are in
use, the signal frame size can be 3.5KB while MINSIGSTKSZ remains 2KB.

The bug report [4] explains this as an ABI issue. The small MINSIGSTKSZ can
cause user stack overflow when delivering a signal.

In this series, we suggest a couple of things:
1. Provide a variable minimum stack size to userspace, as a similar
   approach to [5]
2. Avoid using a too-small alternate stack

Changes from v3 [8]:
* Updated the changelog (Borislav Petkov)
* Revised the test messages again (Borislav Petkov)

Changes from v2 [7]:
* Simplified the sigaltstack overflow prevention (Jann Horn)
* Renamed fpstate size helper with cleanup (Borislav Petkov)
* Cleaned up the signframe struct size defines (Borislav Petkov)
* Revised the selftest messages (Borislav Petkov)
* Revised a changelog (Borislav Petkov)

Changes from v1 [6]:
* Took stack alignment into account for sigframe size (Dave Martin)

[1]: https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/bits/sigstack.h;h=b9dca794da093dc4d41d39db9851d444e1b54d9b;hb=HEAD
[2]: https://www.gnu.org/software/libc/manual/html_node/Signal-Stack.html
[3]: https://man7.org/linux/man-pages/man2/sigaltstack.2.html
[4]: https://bugzilla.kernel.org/show_bug.cgi?id=153531
[5]: https://blog.linuxplumbersconf.org/2017/ocw/system/presentations/4671/original/plumbers-dm-2017.pdf
[6]: https://lore.kernel.org/lkml/20200929205746.6763-1-chang.seok.bae@intel.com/
[7]: https://lore.kernel.org/lkml/20201119190237.626-1-chang.seok.bae@intel.com/
[8]: https://lore.kernel.org/lkml/20201223015312.4882-1-chang.seok.bae@intel.com/

Chang S. Bae (4):
  x86/signal: Introduce helpers to get the maximum signal frame size
  x86/elf: Support a new ELF aux vector AT_MINSIGSTKSZ
  x86/signal: Detect and prevent an alternate signal stack overflow
  selftest/x86/signal: Include test cases for validating sigaltstack

 arch/x86/include/asm/elf.h                |   4 +
 arch/x86/include/asm/fpu/signal.h         |   2 +
 arch/x86/include/asm/sigframe.h           |   2 +
 arch/x86/include/uapi/asm/auxvec.h        |   6 +-
 arch/x86/kernel/cpu/common.c              |   3 +
 arch/x86/kernel/fpu/signal.c              |  19 ++++
 arch/x86/kernel/signal.c                  |  69 +++++++++++-
 tools/testing/selftests/x86/Makefile      |   2 +-
 tools/testing/selftests/x86/sigaltstack.c | 128 ++++++++++++++++++++++
 9 files changed, 228 insertions(+), 7 deletions(-)
 create mode 100644 tools/testing/selftests/x86/sigaltstack.c

-- 
2.17.1

[PATCH v4 1/4] x86/signal: Introduce helpers to get the maximum signal frame size

[Chang S. Bae via Libc-alpha]

Signal frames do not have a fixed format and can vary in size when a number
of things change: support XSAVE features, 32 vs. 64-bit apps. Add the code
to support a runtime method for userspace to dynamically discover how large
a signal stack needs to be.

Introduce a new variable, max_frame_size, and helper functions for the
calculation to be used in a new user interface. Set max_frame_size to a
system-wide worst-case value, instead of storing multiple app-specific
values.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Len Brown <len.brown@intel.com>
Acked-by: H.J. Lu <hjl.tools@gmail.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: x86@kernel.org
Cc: linux-kernel@vger.kernel.org
---
Changes from v2:
* Renamed the fpstate size helper with cleanup (Borislav Petkov)
* Moved the sigframe struct size defines to where used (Borislav Petkov)
* Removed unneeded sentence in the changelog (Borislav Petkov)

Change from v1:
* Took stack alignment into account for sigframe size (Dave Martin)
---
 arch/x86/include/asm/fpu/signal.h |  2 ++
 arch/x86/include/asm/sigframe.h   |  2 ++
 arch/x86/kernel/cpu/common.c      |  3 ++
 arch/x86/kernel/fpu/signal.c      | 19 +++++++++++
 arch/x86/kernel/signal.c          | 57 +++++++++++++++++++++++++++++--
 5 files changed, 81 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/fpu/signal.h b/arch/x86/include/asm/fpu/signal.h
index 7fb516b6893a..8b6631dffefd 100644
--- a/arch/x86/include/asm/fpu/signal.h
+++ b/arch/x86/include/asm/fpu/signal.h
@@ -29,6 +29,8 @@ unsigned long
 fpu__alloc_mathframe(unsigned long sp, int ia32_frame,
 		     unsigned long *buf_fx, unsigned long *size);
 
+unsigned long fpu__get_fpstate_size(void);
+
 extern void fpu__init_prepare_fx_sw_frame(void);
 
 #endif /* _ASM_X86_FPU_SIGNAL_H */
diff --git a/arch/x86/include/asm/sigframe.h b/arch/x86/include/asm/sigframe.h
index 84eab2724875..5b1ed650b124 100644
--- a/arch/x86/include/asm/sigframe.h
+++ b/arch/x86/include/asm/sigframe.h
@@ -85,4 +85,6 @@ struct rt_sigframe_x32 {
 
 #endif /* CONFIG_X86_64 */
 
+void __init init_sigframe_size(void);
+
 #endif /* _ASM_X86_SIGFRAME_H */
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 35ad8480c464..6954932272d5 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -58,6 +58,7 @@
 #include <asm/intel-family.h>
 #include <asm/cpu_device_id.h>
 #include <asm/uv/uv.h>
+#include <asm/sigframe.h>
 
 #include "cpu.h"
 
@@ -1331,6 +1332,8 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
 
 	fpu__init_system(c);
 
+	init_sigframe_size();
+
 #ifdef CONFIG_X86_32
 	/*
 	 * Regardless of whether PCID is enumerated, the SDM says
diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index a4ec65317a7f..dbb304e48f16 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -507,6 +507,25 @@ fpu__alloc_mathframe(unsigned long sp, int ia32_frame,
 
 	return sp;
 }
+
+unsigned long fpu__get_fpstate_size(void)
+{
+	unsigned long ret = xstate_sigframe_size();
+
+	/*
+	 * This space is needed on (most) 32-bit kernels, or when a 32-bit
+	 * app is running on a 64-bit kernel. To keep things simple, just
+	 * assume the worst case and always include space for 'freg_state',
+	 * even for 64-bit apps on 64-bit kernels. This wastes a bit of
+	 * space, but keeps the code simple.
+	 */
+	if ((IS_ENABLED(CONFIG_IA32_EMULATION) ||
+	     IS_ENABLED(CONFIG_X86_32)) && use_fxsr())
+		ret += sizeof(struct fregs_state);
+
+	return ret;
+}
+
 /*
  * Prepare the SW reserved portion of the fxsave memory layout, indicating
  * the presence of the extended state information in the memory layout
diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index be0d7d4152ec..138a9f5b78d8 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -212,6 +212,11 @@ do {									\
  * Set up a signal frame.
  */
 
+/* x86 ABI requires 16-byte alignment */
+#define FRAME_ALIGNMENT	16UL
+
+#define MAX_FRAME_PADDING	(FRAME_ALIGNMENT - 1)
+
 /*
  * Determine which stack to use..
  */
@@ -222,9 +227,9 @@ static unsigned long align_sigframe(unsigned long sp)
 	 * Align the stack pointer according to the i386 ABI,
 	 * i.e. so that on function entry ((sp + 4) & 15) == 0.
 	 */
-	sp = ((sp + 4) & -16ul) - 4;
+	sp = ((sp + 4) & -FRAME_ALIGNMENT) - 4;
 #else /* !CONFIG_X86_32 */
-	sp = round_down(sp, 16) - 8;
+	sp = round_down(sp, FRAME_ALIGNMENT) - 8;
 #endif
 	return sp;
 }
@@ -663,6 +668,54 @@ SYSCALL_DEFINE0(rt_sigreturn)
 	return 0;
 }
 
+/*
+ * There are four different struct types for signal frame: sigframe_ia32,
+ * rt_sigframe_ia32, rt_sigframe_x32, and rt_sigframe. Use the worst case
+ * -- the largest size. It means the size for 64-bit apps is a bit more
+ * than needed, but this keeps the code simple.
+ */
+#if defined(CONFIG_X86_32) || defined(CONFIG_IA32_EMULATION)
+# define MAX_FRAME_SIGINFO_UCTXT_SIZE	sizeof(struct sigframe_ia32)
+#else
+# define MAX_FRAME_SIGINFO_UCTXT_SIZE	sizeof(struct rt_sigframe)
+#endif
+
+/*
+ * The FP state frame contains an XSAVE buffer which must be 64-byte aligned.
+ * If a signal frame starts at an unaligned address, extra space is required.
+ * This is the max alignment padding, conservatively.
+ */
+#define MAX_XSAVE_PADDING	63UL
+
+/*
+ * The frame data is composed of the following areas and laid out as:
+ *
+ * -------------------------
+ * | alignment padding     |
+ * -------------------------
+ * | (f)xsave frame        |
+ * -------------------------
+ * | fsave header          |
+ * -------------------------
+ * | alignment padding     |
+ * -------------------------
+ * | siginfo + ucontext    |
+ * -------------------------
+ */
+
+/* max_frame_size tells userspace the worst case signal stack size. */
+static unsigned long __ro_after_init max_frame_size;
+
+void __init init_sigframe_size(void)
+{
+	max_frame_size = MAX_FRAME_SIGINFO_UCTXT_SIZE + MAX_FRAME_PADDING;
+
+	max_frame_size += fpu__get_fpstate_size() + MAX_XSAVE_PADDING;
+
+	/* Userspace expects an aligned size. */
+	max_frame_size = round_up(max_frame_size, FRAME_ALIGNMENT);
+}
+
 static inline int is_ia32_compat_frame(struct ksignal *ksig)
 {
 	return IS_ENABLED(CONFIG_IA32_EMULATION) &&
-- 
2.17.1

[PATCH v4 2/4] x86/elf: Support a new ELF aux vector AT_MINSIGSTKSZ

[Chang S. Bae via Libc-alpha]

Historically, signal.h defines MINSIGSTKSZ (2KB) and SIGSTKSZ (8KB), for
use by all architectures with sigaltstack(2). Over time, the hardware state
size grew, but these constants did not evolve. Today, literal use of these
constants on several architectures may result in signal stack overflow, and
thus user data corruption.

A few years ago, the ARM team addressed this issue by establishing
getauxval(AT_MINSIGSTKSZ), such that the kernel can supply at runtime value
that is an appropriate replacement on the current and future hardware.

Add getauxval(AT_MINSIGSTKSZ) support to x86, analogous to the support
added for ARM in commit 94b07c1f8c39 ("arm64: signal: Report signal frame
size to userspace via auxv").

Reported-by: Florian Weimer <fweimer@redhat.com>
Fixes: c2bc11f10a39 ("x86, AVX-512: Enable AVX-512 States Context Switch")
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Len Brown <len.brown@intel.com>
Cc: H.J. Lu <hjl.tools@gmail.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Dave Martin <Dave.Martin@arm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: x86@kernel.org
Cc: libc-alpha@sourceware.org
Cc: linux-arch@vger.kernel.org
Cc: linux-api@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Link: https://bugzilla.kernel.org/show_bug.cgi?id=153531
---
 arch/x86/include/asm/elf.h         | 4 ++++
 arch/x86/include/uapi/asm/auxvec.h | 6 ++++--
 arch/x86/kernel/signal.c           | 5 +++++
 3 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index b9a5d488f1a5..044b024abea1 100644
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -311,6 +311,7 @@ do {									\
 		NEW_AUX_ENT(AT_SYSINFO,	VDSO_ENTRY);			\
 		NEW_AUX_ENT(AT_SYSINFO_EHDR, VDSO_CURRENT_BASE);	\
 	}								\
+	NEW_AUX_ENT(AT_MINSIGSTKSZ, get_sigframe_size());			\
 } while (0)
 
 /*
@@ -327,6 +328,7 @@ extern unsigned long task_size_32bit(void);
 extern unsigned long task_size_64bit(int full_addr_space);
 extern unsigned long get_mmap_base(int is_legacy);
 extern bool mmap_address_hint_valid(unsigned long addr, unsigned long len);
+extern unsigned long get_sigframe_size(void);
 
 #ifdef CONFIG_X86_32
 
@@ -348,6 +350,7 @@ do {									\
 	if (vdso64_enabled)						\
 		NEW_AUX_ENT(AT_SYSINFO_EHDR,				\
 			    (unsigned long __force)current->mm->context.vdso); \
+	NEW_AUX_ENT(AT_MINSIGSTKSZ, get_sigframe_size());			\
 } while (0)
 
 /* As a historical oddity, the x32 and x86_64 vDSOs are controlled together. */
@@ -356,6 +359,7 @@ do {									\
 	if (vdso64_enabled)						\
 		NEW_AUX_ENT(AT_SYSINFO_EHDR,				\
 			    (unsigned long __force)current->mm->context.vdso); \
+	NEW_AUX_ENT(AT_MINSIGSTKSZ, get_sigframe_size());			\
 } while (0)
 
 #define AT_SYSINFO		32
diff --git a/arch/x86/include/uapi/asm/auxvec.h b/arch/x86/include/uapi/asm/auxvec.h
index 580e3c567046..edd7808060e6 100644
--- a/arch/x86/include/uapi/asm/auxvec.h
+++ b/arch/x86/include/uapi/asm/auxvec.h
@@ -10,11 +10,13 @@
 #endif
 #define AT_SYSINFO_EHDR		33
 
+#define AT_MINSIGSTKSZ		51
+
 /* entries in ARCH_DLINFO: */
 #if defined(CONFIG_IA32_EMULATION) || !defined(CONFIG_X86_64)
-# define AT_VECTOR_SIZE_ARCH 2
+# define AT_VECTOR_SIZE_ARCH 3
 #else /* else it's non-compat x86-64 */
-# define AT_VECTOR_SIZE_ARCH 1
+# define AT_VECTOR_SIZE_ARCH 2
 #endif
 
 #endif /* _ASM_X86_AUXVEC_H */
diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index 138a9f5b78d8..761d856f8ef7 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -716,6 +716,11 @@ void __init init_sigframe_size(void)
 	max_frame_size = round_up(max_frame_size, FRAME_ALIGNMENT);
 }
 
+unsigned long get_sigframe_size(void)
+{
+	return max_frame_size;
+}
+
 static inline int is_ia32_compat_frame(struct ksignal *ksig)
 {
 	return IS_ENABLED(CONFIG_IA32_EMULATION) &&
-- 
2.17.1

[PATCH v4 3/4] x86/signal: Detect and prevent an alternate signal stack overflow

[Chang S. Bae via Libc-alpha]

The kernel pushes context on to the userspace stack to prepare for the
user's signal handler. When the user has supplied an alternate signal
stack, via sigaltstack(2), it is easy for the kernel to verify that the
stack size is sufficient for the current hardware context.

Check if writing the hardware context to the alternate stack will exceed
it's size. If yes, then instead of corrupting user-data and proceeding with
the original signal handler, an immediate SIGSEGV signal is delivered.

While previous patches in this series allow new source code to discover and
use a sufficient alternate signal stack size, this check is still necessary
to protect binaries with insufficient alternate signal stack size from data
corruption.

Suggested-by: Jann Horn <jannh@google.com>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Len Brown <len.brown@intel.com>
Reviewed-by: Jann Horn <jannh@google.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Jann Horn <jannh@google.com>
Cc: x86@kernel.org
Cc: linux-kernel@vger.kernel.org
---
Changes from v3:
* Updated the changelog (Borislav Petkov)

Changes from v2:
* Simplified the implementation (Jann Horn)
---
 arch/x86/kernel/signal.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index 761d856f8ef7..91056a940271 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -242,7 +242,7 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
 	unsigned long math_size = 0;
 	unsigned long sp = regs->sp;
 	unsigned long buf_fx = 0;
-	int onsigstack = on_sig_stack(sp);
+	bool onsigstack = on_sig_stack(sp);
 	int ret;
 
 	/* redzone */
@@ -251,8 +251,11 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size,
 
 	/* This is the X/Open sanctioned signal stack switching.  */
 	if (ka->sa.sa_flags & SA_ONSTACK) {
-		if (sas_ss_flags(sp) == 0)
+		if (sas_ss_flags(sp) == 0) {
 			sp = current->sas_ss_sp + current->sas_ss_size;
+			/* On the alternate signal stack */
+			onsigstack = true;
+		}
 	} else if (IS_ENABLED(CONFIG_X86_32) &&
 		   !onsigstack &&
 		   regs->ss != __USER_DS &&
-- 
2.17.1

[PATCH v4 4/4] selftest/x86/signal: Include test cases for validating sigaltstack

[Chang S. Bae via Libc-alpha]

The test measures the kernel's signal delivery with different (enough vs.
insufficient) stack sizes.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Len Brown <len.brown@intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: x86@kernel.org
Cc: linux-kselftest@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
Changes from v3:
* Revised test messages again (Borislav Petkov)

Changes from v2:
* Revised test messages (Borislav Petkov)
---
 tools/testing/selftests/x86/Makefile      |   2 +-
 tools/testing/selftests/x86/sigaltstack.c | 128 ++++++++++++++++++++++
 2 files changed, 129 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/x86/sigaltstack.c

diff --git a/tools/testing/selftests/x86/Makefile b/tools/testing/selftests/x86/Makefile
index 6703c7906b71..e0c52e5ab49e 100644
--- a/tools/testing/selftests/x86/Makefile
+++ b/tools/testing/selftests/x86/Makefile
@@ -13,7 +13,7 @@ CAN_BUILD_WITH_NOPIE := $(shell ./check_cc.sh $(CC) trivial_program.c -no-pie)
 TARGETS_C_BOTHBITS := single_step_syscall sysret_ss_attrs syscall_nt test_mremap_vdso \
 			check_initial_reg_state sigreturn iopl ioperm \
 			test_vdso test_vsyscall mov_ss_trap \
-			syscall_arg_fault fsgsbase_restore
+			syscall_arg_fault fsgsbase_restore sigaltstack
 TARGETS_C_32BIT_ONLY := entry_from_vm86 test_syscall_vdso unwind_vdso \
 			test_FCMOV test_FCOMI test_FISTTP \
 			vdso_restorer
diff --git a/tools/testing/selftests/x86/sigaltstack.c b/tools/testing/selftests/x86/sigaltstack.c
new file mode 100644
index 000000000000..f689af75e979
--- /dev/null
+++ b/tools/testing/selftests/x86/sigaltstack.c
@@ -0,0 +1,128 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#define _GNU_SOURCE
+#include <signal.h>
+#include <stdio.h>
+#include <stdbool.h>
+#include <string.h>
+#include <err.h>
+#include <errno.h>
+#include <limits.h>
+#include <sys/mman.h>
+#include <sys/auxv.h>
+#include <sys/prctl.h>
+#include <sys/resource.h>
+#include <setjmp.h>
+
+/* sigaltstack()-enforced minimum stack */
+#define ENFORCED_MINSIGSTKSZ	2048
+
+#ifndef AT_MINSIGSTKSZ
+#  define AT_MINSIGSTKSZ	51
+#endif
+
+static int nerrs;
+
+static bool sigalrm_expected;
+
+static unsigned long at_minstack_size;
+
+static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
+		       int flags)
+{
+	struct sigaction sa;
+
+	memset(&sa, 0, sizeof(sa));
+	sa.sa_sigaction = handler;
+	sa.sa_flags = SA_SIGINFO | flags;
+	sigemptyset(&sa.sa_mask);
+	if (sigaction(sig, &sa, 0))
+		err(1, "sigaction");
+}
+
+static void clearhandler(int sig)
+{
+	struct sigaction sa;
+
+	memset(&sa, 0, sizeof(sa));
+	sa.sa_handler = SIG_DFL;
+	sigemptyset(&sa.sa_mask);
+	if (sigaction(sig, &sa, 0))
+		err(1, "sigaction");
+}
+
+static int setup_altstack(void *start, unsigned long size)
+{
+	stack_t ss;
+
+	memset(&ss, 0, sizeof(ss));
+	ss.ss_size = size;
+	ss.ss_sp = start;
+
+	return sigaltstack(&ss, NULL);
+}
+
+static jmp_buf jmpbuf;
+
+static void sigsegv(int sig, siginfo_t *info, void *ctx_void)
+{
+	if (sigalrm_expected) {
+		printf("[FAIL]\tWrong signal delivered: SIGSEGV (expected SIGALRM).");
+		nerrs++;
+	} else {
+		printf("[OK]\tSIGSEGV signal delivered.\n");
+	}
+
+	siglongjmp(jmpbuf, 1);
+}
+
+static void sigalrm(int sig, siginfo_t *info, void *ctx_void)
+{
+	if (!sigalrm_expected) {
+		printf("[FAIL]\tWrong signal delivered: SIGALRM (expected SIGSEGV).");
+		nerrs++;
+	} else {
+		printf("[OK]\tSIGALRM signal delivered.\n");
+	}
+}
+
+static void test_sigaltstack(void *altstack, unsigned long size)
+{
+	if (setup_altstack(altstack, size))
+		err(1, "sigaltstack()");
+
+	sigalrm_expected = (size > at_minstack_size) ? true : false;
+
+	sethandler(SIGSEGV, sigsegv, 0);
+	sethandler(SIGALRM, sigalrm, SA_ONSTACK);
+
+	if (!sigsetjmp(jmpbuf, 1)) {
+		printf("[RUN]\tTest an alternate signal stack of %ssufficient size.\n",
+		       sigalrm_expected ? "" : "in");
+		printf("\tRaise SIGALRM. %s is expected to be delivered.\n",
+		       sigalrm_expected ? "It" : "SIGSEGV");
+		raise(SIGALRM);
+	}
+
+	clearhandler(SIGALRM);
+	clearhandler(SIGSEGV);
+}
+
+int main(void)
+{
+	void *altstack;
+
+	at_minstack_size = getauxval(AT_MINSIGSTKSZ);
+
+	altstack = mmap(NULL, at_minstack_size + SIGSTKSZ, PROT_READ | PROT_WRITE,
+			MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
+	if (altstack == MAP_FAILED)
+		err(1, "mmap()");
+
+	if ((ENFORCED_MINSIGSTKSZ + 1) < at_minstack_size)
+		test_sigaltstack(altstack, ENFORCED_MINSIGSTKSZ + 1);
+
+	test_sigaltstack(altstack, at_minstack_size + SIGSTKSZ);
+
+	return nerrs == 0 ? 0 : 1;
+}
-- 
2.17.1

Re: [PATCH v4 2/4] x86/elf: Support a new ELF aux vector AT_MINSIGSTKSZ

[Carlos O'Donell via Libc-alpha]

On 1/15/21 4:10 PM, Chang S. Bae via Libc-alpha wrote:
> Historically, signal.h defines MINSIGSTKSZ (2KB) and SIGSTKSZ (8KB), for
> use by all architectures with sigaltstack(2). Over time, the hardware state
> size grew, but these constants did not evolve. Today, literal use of these
> constants on several architectures may result in signal stack overflow, and
> thus user data corruption.
> 
> A few years ago, the ARM team addressed this issue by establishing
> getauxval(AT_MINSIGSTKSZ), such that the kernel can supply at runtime value
> that is an appropriate replacement on the current and future hardware.
> 
> Add getauxval(AT_MINSIGSTKSZ) support to x86, analogous to the support
> added for ARM in commit 94b07c1f8c39 ("arm64: signal: Report signal frame
> size to userspace via auxv").

The value of AT_MINSIGSTKSZ has been generic ABI for all architectures since
2018 when it was added to glibc's generic elf.h because of arm64.

Rather than define this just for x86 may we please put this into the generic
headers, since it has been there since 2018?

Any userspace code that might be inspecting for AT_MINSIGSTKSZ is going to
expect a value of 51.

No other architecture has a define for value 51. This way we avoid any accidents
with the value being different for architectures.
 
> Reported-by: Florian Weimer <fweimer@redhat.com>
> Fixes: c2bc11f10a39 ("x86, AVX-512: Enable AVX-512 States Context Switch")
> Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
> Reviewed-by: Len Brown <len.brown@intel.com>
> Cc: H.J. Lu <hjl.tools@gmail.com>
> Cc: Fenghua Yu <fenghua.yu@intel.com>
> Cc: Dave Martin <Dave.Martin@arm.com>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: x86@kernel.org
> Cc: libc-alpha@sourceware.org
> Cc: linux-arch@vger.kernel.org
> Cc: linux-api@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=153531
> ---
>  arch/x86/include/asm/elf.h         | 4 ++++
>  arch/x86/include/uapi/asm/auxvec.h | 6 ++++--
>  arch/x86/kernel/signal.c           | 5 +++++
>  3 files changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
> index b9a5d488f1a5..044b024abea1 100644
> --- a/arch/x86/include/asm/elf.h
> +++ b/arch/x86/include/asm/elf.h
> @@ -311,6 +311,7 @@ do {									\
>  		NEW_AUX_ENT(AT_SYSINFO,	VDSO_ENTRY);			\
>  		NEW_AUX_ENT(AT_SYSINFO_EHDR, VDSO_CURRENT_BASE);	\
>  	}								\
> +	NEW_AUX_ENT(AT_MINSIGSTKSZ, get_sigframe_size());			\
>  } while (0)
>  
>  /*
> @@ -327,6 +328,7 @@ extern unsigned long task_size_32bit(void);
>  extern unsigned long task_size_64bit(int full_addr_space);
>  extern unsigned long get_mmap_base(int is_legacy);
>  extern bool mmap_address_hint_valid(unsigned long addr, unsigned long len);
> +extern unsigned long get_sigframe_size(void);
>  
>  #ifdef CONFIG_X86_32
>  
> @@ -348,6 +350,7 @@ do {									\
>  	if (vdso64_enabled)						\
>  		NEW_AUX_ENT(AT_SYSINFO_EHDR,				\
>  			    (unsigned long __force)current->mm->context.vdso); \
> +	NEW_AUX_ENT(AT_MINSIGSTKSZ, get_sigframe_size());			\
>  } while (0)
>  
>  /* As a historical oddity, the x32 and x86_64 vDSOs are controlled together. */
> @@ -356,6 +359,7 @@ do {									\
>  	if (vdso64_enabled)						\
>  		NEW_AUX_ENT(AT_SYSINFO_EHDR,				\
>  			    (unsigned long __force)current->mm->context.vdso); \
> +	NEW_AUX_ENT(AT_MINSIGSTKSZ, get_sigframe_size());			\
>  } while (0)
>  
>  #define AT_SYSINFO		32
> diff --git a/arch/x86/include/uapi/asm/auxvec.h b/arch/x86/include/uapi/asm/auxvec.h
> index 580e3c567046..edd7808060e6 100644
> --- a/arch/x86/include/uapi/asm/auxvec.h
> +++ b/arch/x86/include/uapi/asm/auxvec.h
> @@ -10,11 +10,13 @@
>  #endif
>  #define AT_SYSINFO_EHDR		33
>  
> +#define AT_MINSIGSTKSZ		51

This definition should go into the generic auxvec.h.

We could also remove the arm64 define, but that is orthogonal arch cleanup.

> +
>  /* entries in ARCH_DLINFO: */
>  #if defined(CONFIG_IA32_EMULATION) || !defined(CONFIG_X86_64)
> -# define AT_VECTOR_SIZE_ARCH 2
> +# define AT_VECTOR_SIZE_ARCH 3
>  #else /* else it's non-compat x86-64 */
> -# define AT_VECTOR_SIZE_ARCH 1
> +# define AT_VECTOR_SIZE_ARCH 2
>  #endif
>  
>  #endif /* _ASM_X86_AUXVEC_H */
> diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
> index 138a9f5b78d8..761d856f8ef7 100644
> --- a/arch/x86/kernel/signal.c
> +++ b/arch/x86/kernel/signal.c
> @@ -716,6 +716,11 @@ void __init init_sigframe_size(void)
>  	max_frame_size = round_up(max_frame_size, FRAME_ALIGNMENT);
>  }
>  
> +unsigned long get_sigframe_size(void)
> +{
> +	return max_frame_size;
> +}
> +
>  static inline int is_ia32_compat_frame(struct ksignal *ksig)
>  {
>  	return IS_ENABLED(CONFIG_IA32_EMULATION) &&
> 


-- 
Cheers,
Carlos.