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* [PATCH v5 0/4] arm64: Enable BTI for the executable as well as the interpreter
@ 2021-07-19 16:45 Mark Brown via Libc-alpha
  2021-07-19 16:45 ` [PATCH v5 1/4] elf: Allow architectures to parse properties on the main executable Mark Brown via Libc-alpha
                   ` (4 more replies)
  0 siblings, 5 replies; 6+ messages in thread
From: Mark Brown via Libc-alpha @ 2021-07-19 16:45 UTC (permalink / raw)
  To: Catalin Marinas, Will Deacon
  Cc: linux-arch, Yu-cheng Yu, libc-alpha, Szabolcs Nagy,
	Jeremy Linton, Mark Brown, Dave Martin, linux-arm-kernel

Deployments of BTI on arm64 have run into issues interacting with
systemd's MemoryDenyWriteExecute feature.  Currently for dynamically
linked executables the kernel will only handle architecture specific
properties like BTI for the interpreter, the expectation is that the
interpreter will then handle any properties on the main executable.
For BTI this means remapping the executable segments PROT_EXEC |
PROT_BTI.

This interacts poorly with MemoryDenyWriteExecute since that is
implemented using a seccomp filter which prevents setting PROT_EXEC on
already mapped memory and lacks the context to be able to detect that
memory is already mapped with PROT_EXEC.  This series resolves this by
handling the BTI property for both the interpreter and the main
executable.

This does mean that we may get more code with BTI enabled if running on
a system without BTI support in the dynamic linker, this is expected to
be a safe configuration and testing seems to confirm that. It also
reduces the flexibility userspace has to disable BTI but it is expected
that for cases where there are problems which require BTI to be disabled
it is more likely that it will need to be disabled on a system level.

v5:
 - Rebase onto v5.14-rc2.
 - Tweak changelog on patch 1.
 - Use the helper for interpreter/executable flag in elf.h as well.
v4:
 - Rebase onto v5.14-rc1.
v3:
 - Fix passing of properties for parsing by the main executable.
 - Drop has_interp from arch_parse_elf_property().
 - Coding style tweaks.
v2:
 - Add a patch dropping has_interp from arch_adjust_elf_prot()
 - Fix bisection issue with static executables on arm64 in the first
   patch.

Mark Brown (4):
  elf: Allow architectures to parse properties on the main executable
  arm64: Enable BTI for main executable as well as the interpreter
  elf: Remove has_interp property from arch_adjust_elf_prot()
  elf: Remove has_interp property from arch_parse_elf_property()

 arch/arm64/include/asm/elf.h | 14 ++++++++++++--
 arch/arm64/kernel/process.c  | 16 +++-------------
 fs/binfmt_elf.c              | 29 ++++++++++++++++++++---------
 include/linux/elf.h          |  8 +++++---
 4 files changed, 40 insertions(+), 27 deletions(-)


base-commit: 2734d6c1b1a089fb593ef6a23d4b70903526fe0c
-- 
2.20.1


^ permalink raw reply	[flat|nested] 6+ messages in thread

* [PATCH v5 1/4] elf: Allow architectures to parse properties on the main executable
  2021-07-19 16:45 [PATCH v5 0/4] arm64: Enable BTI for the executable as well as the interpreter Mark Brown via Libc-alpha
@ 2021-07-19 16:45 ` Mark Brown via Libc-alpha
  2021-07-19 16:45 ` [PATCH v5 2/4] arm64: Enable BTI for main executable as well as the interpreter Mark Brown via Libc-alpha
                   ` (3 subsequent siblings)
  4 siblings, 0 replies; 6+ messages in thread
From: Mark Brown via Libc-alpha @ 2021-07-19 16:45 UTC (permalink / raw)
  To: Catalin Marinas, Will Deacon
  Cc: linux-arch, Yu-cheng Yu, libc-alpha, Szabolcs Nagy,
	Jeremy Linton, Mark Brown, Dave Martin, linux-arm-kernel

Currently the ELF code only attempts to parse properties on the image
that will start execution, either the interpreter or for statically linked
executables the main executable. The expectation is that any property
handling for the main executable will be done by the interpreter. This is
a bit inconsistent since we do map the executable and is causing problems
for the arm64 BTI support when used in conjunction with systemd's use of
seccomp to implement MemoryDenyWriteExecute which stops the dynamic linker
adjusting the permissions of executable segments.

Allow architectures to handle properties for both the dynamic linker and
main executable, adjusting arch_parse_elf_properties() to have a new
flag is_interp flag as with arch_elf_adjust_prot() and calling it for
both the main executable and any intepreter.

The user of this code, arm64, is adapted to ensure that there is no
functional change.

Signed-off-by: Mark Brown <broonie@kernel.org>
Tested-by: Jeremy Linton <jeremy.linton@arm.com>
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
---
 arch/arm64/include/asm/elf.h |  3 ++-
 fs/binfmt_elf.c              | 27 +++++++++++++++++++--------
 include/linux/elf.h          |  4 +++-
 3 files changed, 24 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
index 8d1c8dcb87fd..a488a1329b16 100644
--- a/arch/arm64/include/asm/elf.h
+++ b/arch/arm64/include/asm/elf.h
@@ -261,6 +261,7 @@ struct arch_elf_state {
 
 static inline int arch_parse_elf_property(u32 type, const void *data,
 					  size_t datasz, bool compat,
+					  bool has_interp, bool is_interp,
 					  struct arch_elf_state *arch)
 {
 	/* No known properties for AArch32 yet */
@@ -273,7 +274,7 @@ static inline int arch_parse_elf_property(u32 type, const void *data,
 		if (datasz != sizeof(*p))
 			return -ENOEXEC;
 
-		if (system_supports_bti() &&
+		if (system_supports_bti() && has_interp == is_interp &&
 		    (*p & GNU_PROPERTY_AARCH64_FEATURE_1_BTI))
 			arch->flags |= ARM64_ELF_BTI;
 	}
diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 439ed81e755a..28eaf36f145b 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -716,8 +716,9 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
  */
 
 static int parse_elf_property(const char *data, size_t *off, size_t datasz,
-			      struct arch_elf_state *arch,
-			      bool have_prev_type, u32 *prev_type)
+			      struct arch_elf_state *arch, bool has_interp,
+			      bool is_interp, bool have_prev_type,
+			      u32 *prev_type)
 {
 	size_t o, step;
 	const struct gnu_property *pr;
@@ -751,7 +752,8 @@ static int parse_elf_property(const char *data, size_t *off, size_t datasz,
 	*prev_type = pr->pr_type;
 
 	ret = arch_parse_elf_property(pr->pr_type, data + o,
-				      pr->pr_datasz, ELF_COMPAT, arch);
+				      pr->pr_datasz, ELF_COMPAT,
+				      has_interp, is_interp, arch);
 	if (ret)
 		return ret;
 
@@ -764,6 +766,7 @@ static int parse_elf_property(const char *data, size_t *off, size_t datasz,
 #define NOTE_NAME_SZ (sizeof(GNU_PROPERTY_TYPE_0_NAME))
 
 static int parse_elf_properties(struct file *f, const struct elf_phdr *phdr,
+				bool has_interp, bool is_interp,
 				struct arch_elf_state *arch)
 {
 	union {
@@ -813,7 +816,8 @@ static int parse_elf_properties(struct file *f, const struct elf_phdr *phdr,
 	have_prev_type = false;
 	do {
 		ret = parse_elf_property(note.data, &off, datasz, arch,
-					 have_prev_type, &prev_type);
+					 has_interp, is_interp, have_prev_type,
+					 &prev_type);
 		have_prev_type = true;
 	} while (!ret);
 
@@ -828,6 +832,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
 	unsigned long error;
 	struct elf_phdr *elf_ppnt, *elf_phdata, *interp_elf_phdata = NULL;
 	struct elf_phdr *elf_property_phdata = NULL;
+	struct elf_phdr *interp_elf_property_phdata = NULL;
 	unsigned long elf_bss, elf_brk;
 	int bss_prot = 0;
 	int retval, i;
@@ -963,12 +968,11 @@ static int load_elf_binary(struct linux_binprm *bprm)
 			goto out_free_dentry;
 
 		/* Pass PT_LOPROC..PT_HIPROC headers to arch code */
-		elf_property_phdata = NULL;
 		elf_ppnt = interp_elf_phdata;
 		for (i = 0; i < interp_elf_ex->e_phnum; i++, elf_ppnt++)
 			switch (elf_ppnt->p_type) {
 			case PT_GNU_PROPERTY:
-				elf_property_phdata = elf_ppnt;
+				interp_elf_property_phdata = elf_ppnt;
 				break;
 
 			case PT_LOPROC ... PT_HIPROC:
@@ -979,10 +983,17 @@ static int load_elf_binary(struct linux_binprm *bprm)
 					goto out_free_dentry;
 				break;
 			}
+
+		retval = parse_elf_properties(interpreter,
+					      interp_elf_property_phdata,
+					      true, true, &arch_state);
+		if (retval)
+			goto out_free_dentry;
+
 	}
 
-	retval = parse_elf_properties(interpreter ?: bprm->file,
-				      elf_property_phdata, &arch_state);
+	retval = parse_elf_properties(bprm->file, elf_property_phdata,
+				      interpreter, false, &arch_state);
 	if (retval)
 		goto out_free_dentry;
 
diff --git a/include/linux/elf.h b/include/linux/elf.h
index c9a46c4e183b..1c45ecf29147 100644
--- a/include/linux/elf.h
+++ b/include/linux/elf.h
@@ -88,13 +88,15 @@ struct arch_elf_state;
 #ifndef CONFIG_ARCH_USE_GNU_PROPERTY
 static inline int arch_parse_elf_property(u32 type, const void *data,
 					  size_t datasz, bool compat,
+					  bool has_interp, bool is_interp,
 					  struct arch_elf_state *arch)
 {
 	return 0;
 }
 #else
 extern int arch_parse_elf_property(u32 type, const void *data, size_t datasz,
-				   bool compat, struct arch_elf_state *arch);
+				   bool compat, bool has_interp, bool is_interp,
+				   struct arch_elf_state *arch);
 #endif
 
 #ifdef CONFIG_ARCH_HAVE_ELF_PROT
-- 
2.20.1


^ permalink raw reply	[flat|nested] 6+ messages in thread

* [PATCH v5 2/4] arm64: Enable BTI for main executable as well as the interpreter
  2021-07-19 16:45 [PATCH v5 0/4] arm64: Enable BTI for the executable as well as the interpreter Mark Brown via Libc-alpha
  2021-07-19 16:45 ` [PATCH v5 1/4] elf: Allow architectures to parse properties on the main executable Mark Brown via Libc-alpha
@ 2021-07-19 16:45 ` Mark Brown via Libc-alpha
  2021-07-19 16:45 ` [PATCH v5 3/4] elf: Remove has_interp property from arch_adjust_elf_prot() Mark Brown via Libc-alpha
                   ` (2 subsequent siblings)
  4 siblings, 0 replies; 6+ messages in thread
From: Mark Brown via Libc-alpha @ 2021-07-19 16:45 UTC (permalink / raw)
  To: Catalin Marinas, Will Deacon
  Cc: linux-arch, Yu-cheng Yu, libc-alpha, Szabolcs Nagy,
	Jeremy Linton, Mark Brown, Dave Martin, linux-arm-kernel

Currently for dynamically linked ELF executables we only enable BTI for
the interpreter, expecting the interpreter to do this for the main
executable. This is a bit inconsistent since we do map main executable and
is causing issues with systemd's MemoryDenyWriteExecute feature which is
implemented using a seccomp filter which prevents setting PROT_EXEC on
already mapped memory and lacks the context to be able to detect that
memory is already mapped with PROT_EXEC.

Resolve this by checking the BTI property for the main executable and
enabling BTI if it is present when doing the initial mapping. This does
mean that we may get more code with BTI enabled if running on a system
without BTI support in the dynamic linker, this is expected to be a safe
configuration and testing seems to confirm that. It also reduces the
flexibility userspace has to disable BTI but it is expected that for cases
where there are problems which require BTI to be disabled it is more likely
that it will need to be disabled on a system level.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Tested-by: Jeremy Linton <jeremy.linton@arm.com>
---
 arch/arm64/include/asm/elf.h | 15 ++++++++++++---
 arch/arm64/kernel/process.c  | 14 ++------------
 2 files changed, 14 insertions(+), 15 deletions(-)

diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
index a488a1329b16..43b10fd04467 100644
--- a/arch/arm64/include/asm/elf.h
+++ b/arch/arm64/include/asm/elf.h
@@ -253,12 +253,21 @@ struct arch_elf_state {
 	int flags;
 };
 
-#define ARM64_ELF_BTI		(1 << 0)
+#define ARM64_ELF_INTERP_BTI		(1 << 0)
+#define ARM64_ELF_EXEC_BTI		(1 << 1)
 
 #define INIT_ARCH_ELF_STATE {			\
 	.flags = 0,				\
 }
 
+static inline int arm64_elf_bti_flag(bool is_interp)
+{
+	if (is_interp)
+		return ARM64_ELF_INTERP_BTI;
+	else
+		return ARM64_ELF_EXEC_BTI;
+}
+
 static inline int arch_parse_elf_property(u32 type, const void *data,
 					  size_t datasz, bool compat,
 					  bool has_interp, bool is_interp,
@@ -274,9 +283,9 @@ static inline int arch_parse_elf_property(u32 type, const void *data,
 		if (datasz != sizeof(*p))
 			return -ENOEXEC;
 
-		if (system_supports_bti() && has_interp == is_interp &&
+		if (system_supports_bti() &&
 		    (*p & GNU_PROPERTY_AARCH64_FEATURE_1_BTI))
-			arch->flags |= ARM64_ELF_BTI;
+			arch->flags |= arm64_elf_bti_flag(is_interp);
 	}
 
 	return 0;
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index c8989b999250..2645bc79b35e 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -686,18 +686,8 @@ core_initcall(tagged_addr_init);
 int arch_elf_adjust_prot(int prot, const struct arch_elf_state *state,
 			 bool has_interp, bool is_interp)
 {
-	/*
-	 * For dynamically linked executables the interpreter is
-	 * responsible for setting PROT_BTI on everything except
-	 * itself.
-	 */
-	if (is_interp != has_interp)
-		return prot;
-
-	if (!(state->flags & ARM64_ELF_BTI))
-		return prot;
-
-	if (prot & PROT_EXEC)
+	if ((prot & PROT_EXEC) &&
+	    (state->flags & arm64_elf_bti_flag(is_interp)))
 		prot |= PROT_BTI;
 
 	return prot;
-- 
2.20.1


^ permalink raw reply	[flat|nested] 6+ messages in thread

* [PATCH v5 3/4] elf: Remove has_interp property from arch_adjust_elf_prot()
  2021-07-19 16:45 [PATCH v5 0/4] arm64: Enable BTI for the executable as well as the interpreter Mark Brown via Libc-alpha
  2021-07-19 16:45 ` [PATCH v5 1/4] elf: Allow architectures to parse properties on the main executable Mark Brown via Libc-alpha
  2021-07-19 16:45 ` [PATCH v5 2/4] arm64: Enable BTI for main executable as well as the interpreter Mark Brown via Libc-alpha
@ 2021-07-19 16:45 ` Mark Brown via Libc-alpha
  2021-07-19 16:45 ` [PATCH v5 4/4] elf: Remove has_interp property from arch_parse_elf_property() Mark Brown via Libc-alpha
  2021-08-01 14:56 ` [PATCH v5 0/4] arm64: Enable BTI for the executable as well as the interpreter Catalin Marinas via Libc-alpha
  4 siblings, 0 replies; 6+ messages in thread
From: Mark Brown via Libc-alpha @ 2021-07-19 16:45 UTC (permalink / raw)
  To: Catalin Marinas, Will Deacon
  Cc: linux-arch, Yu-cheng Yu, libc-alpha, Szabolcs Nagy,
	Jeremy Linton, Mark Brown, Dave Martin, linux-arm-kernel

Since we have added an is_interp flag to arch_parse_elf_property() we can
drop the has_interp flag from arch_elf_adjust_prot(), the only user was
the arm64 code which no longer needs it and any future users will be able
to use arch_parse_elf_properties() to determine if an interpreter is in
use.

Signed-off-by: Mark Brown <broonie@kernel.org>
Tested-by: Jeremy Linton <jeremy.linton@arm.com>
Reviewed-By: Dave Martin <Dave.Martin@arm.com>
---
 arch/arm64/kernel/process.c | 2 +-
 fs/binfmt_elf.c             | 2 +-
 include/linux/elf.h         | 4 ++--
 3 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 2645bc79b35e..14478c5b52d3 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -684,7 +684,7 @@ core_initcall(tagged_addr_init);
 
 #ifdef CONFIG_BINFMT_ELF
 int arch_elf_adjust_prot(int prot, const struct arch_elf_state *state,
-			 bool has_interp, bool is_interp)
+			 bool is_interp)
 {
 	if ((prot & PROT_EXEC) &&
 	    (state->flags & arm64_elf_bti_flag(is_interp)))
diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 28eaf36f145b..4f3bea1a697c 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -580,7 +580,7 @@ static inline int make_prot(u32 p_flags, struct arch_elf_state *arch_state,
 	if (p_flags & PF_X)
 		prot |= PROT_EXEC;
 
-	return arch_elf_adjust_prot(prot, arch_state, has_interp, is_interp);
+	return arch_elf_adjust_prot(prot, arch_state, is_interp);
 }
 
 /* This is much more generalized than the library routine read function,
diff --git a/include/linux/elf.h b/include/linux/elf.h
index 1c45ecf29147..d8392531899d 100644
--- a/include/linux/elf.h
+++ b/include/linux/elf.h
@@ -101,11 +101,11 @@ extern int arch_parse_elf_property(u32 type, const void *data, size_t datasz,
 
 #ifdef CONFIG_ARCH_HAVE_ELF_PROT
 int arch_elf_adjust_prot(int prot, const struct arch_elf_state *state,
-			 bool has_interp, bool is_interp);
+			 bool is_interp);
 #else
 static inline int arch_elf_adjust_prot(int prot,
 				       const struct arch_elf_state *state,
-				       bool has_interp, bool is_interp)
+				       bool is_interp)
 {
 	return prot;
 }
-- 
2.20.1


^ permalink raw reply	[flat|nested] 6+ messages in thread

* [PATCH v5 4/4] elf: Remove has_interp property from arch_parse_elf_property()
  2021-07-19 16:45 [PATCH v5 0/4] arm64: Enable BTI for the executable as well as the interpreter Mark Brown via Libc-alpha
                   ` (2 preceding siblings ...)
  2021-07-19 16:45 ` [PATCH v5 3/4] elf: Remove has_interp property from arch_adjust_elf_prot() Mark Brown via Libc-alpha
@ 2021-07-19 16:45 ` Mark Brown via Libc-alpha
  2021-08-01 14:56 ` [PATCH v5 0/4] arm64: Enable BTI for the executable as well as the interpreter Catalin Marinas via Libc-alpha
  4 siblings, 0 replies; 6+ messages in thread
From: Mark Brown via Libc-alpha @ 2021-07-19 16:45 UTC (permalink / raw)
  To: Catalin Marinas, Will Deacon
  Cc: linux-arch, Yu-cheng Yu, libc-alpha, Szabolcs Nagy,
	Jeremy Linton, Mark Brown, Dave Martin, linux-arm-kernel

Since all current users of arch_parse_elf_property() are able to treat the
interpreter and main executable orthogonaly the has_interp argument is now
redundant so remove it.

Signed-off-by: Mark Brown <broonie@kernel.org>
Tested-by: Jeremy Linton <jeremy.linton@arm.com>
Reviewed-By: Dave Martin <Dave.Martin@arm.com>
---
 arch/arm64/include/asm/elf.h | 2 +-
 fs/binfmt_elf.c              | 2 +-
 include/linux/elf.h          | 4 ++--
 3 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
index 43b10fd04467..43f2ee69a7de 100644
--- a/arch/arm64/include/asm/elf.h
+++ b/arch/arm64/include/asm/elf.h
@@ -270,7 +270,7 @@ static inline int arm64_elf_bti_flag(bool is_interp)
 
 static inline int arch_parse_elf_property(u32 type, const void *data,
 					  size_t datasz, bool compat,
-					  bool has_interp, bool is_interp,
+					  bool is_interp,
 					  struct arch_elf_state *arch)
 {
 	/* No known properties for AArch32 yet */
diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 4f3bea1a697c..e3beaf4345cd 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -753,7 +753,7 @@ static int parse_elf_property(const char *data, size_t *off, size_t datasz,
 
 	ret = arch_parse_elf_property(pr->pr_type, data + o,
 				      pr->pr_datasz, ELF_COMPAT,
-				      has_interp, is_interp, arch);
+				      is_interp, arch);
 	if (ret)
 		return ret;
 
diff --git a/include/linux/elf.h b/include/linux/elf.h
index d8392531899d..cdb080d4b34a 100644
--- a/include/linux/elf.h
+++ b/include/linux/elf.h
@@ -88,14 +88,14 @@ struct arch_elf_state;
 #ifndef CONFIG_ARCH_USE_GNU_PROPERTY
 static inline int arch_parse_elf_property(u32 type, const void *data,
 					  size_t datasz, bool compat,
-					  bool has_interp, bool is_interp,
+					  bool is_interp,
 					  struct arch_elf_state *arch)
 {
 	return 0;
 }
 #else
 extern int arch_parse_elf_property(u32 type, const void *data, size_t datasz,
-				   bool compat, bool has_interp, bool is_interp,
+				   bool compat, bool is_interp,
 				   struct arch_elf_state *arch);
 #endif
 
-- 
2.20.1


^ permalink raw reply	[flat|nested] 6+ messages in thread

* Re: [PATCH v5 0/4] arm64: Enable BTI for the executable as well as the interpreter
  2021-07-19 16:45 [PATCH v5 0/4] arm64: Enable BTI for the executable as well as the interpreter Mark Brown via Libc-alpha
                   ` (3 preceding siblings ...)
  2021-07-19 16:45 ` [PATCH v5 4/4] elf: Remove has_interp property from arch_parse_elf_property() Mark Brown via Libc-alpha
@ 2021-08-01 14:56 ` Catalin Marinas via Libc-alpha
  4 siblings, 0 replies; 6+ messages in thread
From: Catalin Marinas via Libc-alpha @ 2021-08-01 14:56 UTC (permalink / raw)
  To: Mark Brown
  Cc: linux-arch, Yu-cheng Yu, libc-alpha, Florian Weimer,
	Szabolcs Nagy, Jeremy Linton, Topi Miettinen, Lennart Poettering,
	Mark Rutland, Will Deacon, Dave Martin, linux-arm-kernel

On Mon, Jul 19, 2021 at 05:45:32PM +0100, Mark Brown wrote:
> Deployments of BTI on arm64 have run into issues interacting with
> systemd's MemoryDenyWriteExecute feature.  Currently for dynamically
> linked executables the kernel will only handle architecture specific
> properties like BTI for the interpreter, the expectation is that the
> interpreter will then handle any properties on the main executable.
> For BTI this means remapping the executable segments PROT_EXEC |
> PROT_BTI.
> 
> This interacts poorly with MemoryDenyWriteExecute since that is
> implemented using a seccomp filter which prevents setting PROT_EXEC on
> already mapped memory and lacks the context to be able to detect that
> memory is already mapped with PROT_EXEC.  This series resolves this by
> handling the BTI property for both the interpreter and the main
> executable.
> 
> This does mean that we may get more code with BTI enabled if running on
> a system without BTI support in the dynamic linker, this is expected to
> be a safe configuration and testing seems to confirm that. It also
> reduces the flexibility userspace has to disable BTI but it is expected
> that for cases where there are problems which require BTI to be disabled
> it is more likely that it will need to be disabled on a system level.

One last call on this series as I'd like to at least have it in -next
for a few weeks if we are to upstream it. Adding people that have been
involved on the previous longer discussion:

https://lore.kernel.org/r/8584c14f-5c28-9d70-c054-7c78127d84ea@arm.com

The glibc issue has been solved in the sense that if mprotect() fails,
the error is ignored and we end up with BTI disabled for the main
executable.

This series aims to change the responsibility for PROT_BTI on the main
(dynamic) executable from the dynamic linker to the kernel. It doesn't
come without risks though. Suppose that the dynamic linker in the future
decides that it's not safe to mix BTI/non-BTI for some objects, it won't
have a way to disable BTI on the main executable if MDWX is enabled.

It's a trade-off between the risks introduced by this series vs
the benefits of MDWX. If the risk is non-negligible, the next step is
assessing the importance of MDWX vs BTI on arm64 and whether we should
look for alternatives to MDWX in the future (ideally which don't involve
new flags to mprotect()).

Thanks.

-- 
Catalin

^ permalink raw reply	[flat|nested] 6+ messages in thread

end of thread, other threads:[~2021-08-01 14:57 UTC | newest]

Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-07-19 16:45 [PATCH v5 0/4] arm64: Enable BTI for the executable as well as the interpreter Mark Brown via Libc-alpha
2021-07-19 16:45 ` [PATCH v5 1/4] elf: Allow architectures to parse properties on the main executable Mark Brown via Libc-alpha
2021-07-19 16:45 ` [PATCH v5 2/4] arm64: Enable BTI for main executable as well as the interpreter Mark Brown via Libc-alpha
2021-07-19 16:45 ` [PATCH v5 3/4] elf: Remove has_interp property from arch_adjust_elf_prot() Mark Brown via Libc-alpha
2021-07-19 16:45 ` [PATCH v5 4/4] elf: Remove has_interp property from arch_parse_elf_property() Mark Brown via Libc-alpha
2021-08-01 14:56 ` [PATCH v5 0/4] arm64: Enable BTI for the executable as well as the interpreter Catalin Marinas via Libc-alpha

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