Re: [PATCH 3/3] Linux: Require properly configured /dev/pts for PTYs

[Adhemerval Zanella via Libc-alpha <libc-alpha@sourceware.org>]

On 27/05/2020 07:14, Florian Weimer via Libc-alpha wrote:
> Current systems do not have BSD terminals, so the fallback code in
> posix_openpt/getpt does not do anything.  Also remove the file system
> check for /dev/pts.  Current systems always have a devpts file system
> mounted there if /dev/ptmx exists.
> 
> grantpt is now essentially a no-op.  It only verifies that the
> argument is a ptmx-descriptor.  Therefore, this change indirectly
> addresses bug 24941.

LGTM with some comments below, thanks.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

> ---
>  NEWS                              | 12 ++++
>  sysdeps/unix/sysv/linux/getpt.c   | 67 +---------------------
>  sysdeps/unix/sysv/linux/grantpt.c | 73 ++++++++++++------------
>  sysdeps/unix/sysv/linux/ptsname.c | 95 ++-----------------------------
>  4 files changed, 55 insertions(+), 192 deletions(-)
> 
> diff --git a/NEWS b/NEWS
> index 55389b8466..b8e0408a56 100644
> --- a/NEWS
> +++ b/NEWS
> @@ -52,6 +52,18 @@ Changes to build and runtime requirements:
>  * powerpc64le requires GCC 7.4 or newer.  This is required for supporting
>    long double redirects.
>  
> +* On Linux, the system administrator needs to configure /dev/pts with
> +  the intended access modes for pseudo-terminals.  glibc no longer
> +  attemps to adjust permissions of terminal devices.  The previous glibc
> +  defaults ("tty" group, user read/write and group write) already
> +  corresponded to what most systems used, so that grantpt did not
> +  perform any adjustments.
> +
> +* On Linux, the posix_openpt and getpt functions no longer attempt to
> +  use legacy (BSD) pseudo-terminals and assume that if /dev/ptmx exists
> +  (and pseudo-terminals are supported), a devpts file system is mounted
> +  on /dev/pts.  Current systems already meet these requirements.
> +
>  Security related changes:
>  
>    CVE-2020-10029: Trigonometric functions on x86 targets suffered from stack

Ok, although "Current systems" is somewhat vague (does it refer to minimum
kernel version or a common practice?).

> diff --git a/sysdeps/unix/sysv/linux/getpt.c b/sysdeps/unix/sysv/linux/getpt.c
> index 1803b232c9..3cc745e11a 100644
> --- a/sysdeps/unix/sysv/linux/getpt.c
> +++ b/sysdeps/unix/sysv/linux/getpt.c
> @@ -16,69 +16,18 @@
>     License along with the GNU C Library; if not, see
>     <https://www.gnu.org/licenses/>.  */
>  
> -#include <errno.h>
>  #include <fcntl.h>
> -#include <stdlib.h>
>  #include <unistd.h>
>  #include <paths.h>
> -#include <sys/statfs.h>
> -
> -#include "linux_fsinfo.h"
>  
>  /* Path to the master pseudo terminal cloning device.  */
>  #define _PATH_DEVPTMX _PATH_DEV "ptmx"
> -/* Directory containing the UNIX98 pseudo terminals.  */
> -#define _PATH_DEVPTS _PATH_DEV "pts"
> -
> -/* Prototype for function that opens BSD-style master pseudo-terminals.  */
> -extern int __bsd_getpt (void) attribute_hidden;
>  
>  /* Open a master pseudo terminal and return its file descriptor.  */
>  int
>  __posix_openpt (int oflag)
>  {
> -  static int have_no_dev_ptmx;
> -  int fd;
> -
> -  if (!have_no_dev_ptmx)
> -    {
> -      fd = __open (_PATH_DEVPTMX, oflag);
> -      if (fd != -1)
> -	{
> -	  struct statfs fsbuf;
> -	  static int devpts_mounted;
> -
> -	  /* Check that the /dev/pts filesystem is mounted
> -	     or if /dev is a devfs filesystem (this implies /dev/pts).  */
> -	  if (devpts_mounted
> -	      || (__statfs (_PATH_DEVPTS, &fsbuf) == 0
> -		  && fsbuf.f_type == DEVPTS_SUPER_MAGIC)
> -	      || (__statfs (_PATH_DEV, &fsbuf) == 0
> -		  && fsbuf.f_type == DEVFS_SUPER_MAGIC))
> -	    {
> -	      /* Everything is ok.  */
> -	      devpts_mounted = 1;
> -	      return fd;
> -	    }
> -
> -	  /* If /dev/pts is not mounted then the UNIX98 pseudo terminals
> -	     are not usable.  */
> -	  __close (fd);
> -	  have_no_dev_ptmx = 1;
> -	  __set_errno (ENOENT);
> -	}
> -      else
> -	{
> -	  if (errno == ENOENT || errno == ENODEV)
> -	    have_no_dev_ptmx = 1;
> -	  else
> -	    return -1;
> -	}
> -    }
> -  else
> -    __set_errno (ENOENT);
> -
> -  return -1;
> +  return __open (_PATH_DEVPTMX, oflag);
>  }
>  weak_alias (__posix_openpt, posix_openpt)
>  

Ok.  As side note I think we should change its prototype at include/stdlib.h
from attribute_hidden to a proper hidden_def/hidden_proto.

> @@ -86,16 +35,6 @@ weak_alias (__posix_openpt, posix_openpt)
>  int
>  __getpt (void)
>  {
> -  int fd = __posix_openpt (O_RDWR);
> -  if (fd == -1)
> -    fd = __bsd_getpt ();
> -  return fd;
> +  return __posix_openpt (O_RDWR);
>  }
> -
> -
> -#define PTYNAME1 "pqrstuvwxyzabcde";
> -#define PTYNAME2 "0123456789abcdef";
> -
> -#define __getpt __bsd_getpt
> -#define HAVE_POSIX_OPENPT
> -#include <sysdeps/unix/bsd/getpt.c>
> +weak_alias (__getpt, getpt)

Ok.

> diff --git a/sysdeps/unix/sysv/linux/grantpt.c b/sysdeps/unix/sysv/linux/grantpt.c
> index 2030e07fa6..43122f9a76 100644
> --- a/sysdeps/unix/sysv/linux/grantpt.c
> +++ b/sysdeps/unix/sysv/linux/grantpt.c
> @@ -1,44 +1,41 @@
> -#include <assert.h>
> -#include <ctype.h>
> -#include <dirent.h>
> -#include <errno.h>
> -#include <fcntl.h>
> -#include <paths.h>
> -#include <stdlib.h>
> -#include <unistd.h>
> +/* grantpt implementation for Linux.
> +   Copyright (C) 1998-2020 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +   Contributed by Zack Weinberg <zack@rabi.phys.columbia.edu>, 1998.

Should we keep copying the 'Contributed by' in this case? Specially
for the case where the implementation is really a stripped down
version?

>  
> -#include <not-cancel.h>
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
>  
> -#include "pty-private.h"
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
>  
> -#if HAVE_PT_CHOWN
> -/* Close all file descriptors except the one specified.  */
> -static void
> -close_all_fds (void)
> -{
> -  DIR *dir = __opendir ("/proc/self/fd");
> -  if (dir != NULL)
> -    {
> -      struct dirent64 *d;
> -      while ((d = __readdir64 (dir)) != NULL)
> -	if (isdigit (d->d_name[0]))
> -	  {
> -	    char *endp;
> -	    long int fd = strtol (d->d_name, &endp, 10);
> -	    if (*endp == '\0' && fd != PTY_FILENO && fd != dirfd (dir))
> -	      __close_nocancel_nostatus (fd);
> -	  }
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#include <errno.h>
> +#include <stdlib.h>
> +#include <sys/ioctl.h>
> +#include <termios.h>
>  
> -      __closedir (dir);
> +int
> +grantpt (int fd)
> +{
> +  /* Without pt_chown on Linux, we have delegated the creation of the
> +     pty node with the right group and permission mode to the kernel, and
> +     non-root users are unlikely to be able to change it. Therefore let's
> +     consider that POSIX enforcement is the responsibility of the whole
> +     system and not only the GNU libc.   */
>  
> -      int nullfd = __open_nocancel (_PATH_DEVNULL, O_RDONLY);
> -      assert (nullfd == STDIN_FILENO);
> -      nullfd = __open_nocancel (_PATH_DEVNULL, O_WRONLY);
> -      assert (nullfd == STDOUT_FILENO);
> -      __dup2 (STDOUT_FILENO, STDERR_FILENO);
> -    }
> +  /* Verify that fd refers to a ptmx descriptor.  */
> +  unsigned int ptyno;
> +  int ret = __ioctl (fd, TIOCGPTN, &ptyno);
> +  if (ret != 0 && errno == ENOTTY)
> +    /* POSIX requires EINVAL instead of ENOTTY provided by the kernel.  */
> +    __set_errno (EINVAL);
> +  return ret;
>  }
> -# define CLOSE_ALL_FDS() close_all_fds()
> -#endif
> -
> -#include <sysdeps/unix/grantpt.c>

Ok.

> diff --git a/sysdeps/unix/sysv/linux/ptsname.c b/sysdeps/unix/sysv/linux/ptsname.c
> index 81d9d26f1e..3e9be3f0d4 100644
> --- a/sysdeps/unix/sysv/linux/ptsname.c
> +++ b/sysdeps/unix/sysv/linux/ptsname.c
> @@ -21,39 +21,14 @@
>  #include <stdlib.h>
>  #include <string.h>
>  #include <sys/ioctl.h>
> -#include <sys/stat.h>
> -#include <sys/sysmacros.h>
>  #include <termios.h>
>  #include <unistd.h>
>  
>  #include <_itoa.h>
>  
> -/* Check if DEV corresponds to a master pseudo terminal device.  */
> -#define MASTER_P(Dev)							\
> -  (__gnu_dev_major ((Dev)) == 2						\
> -   || (__gnu_dev_major ((Dev)) == 4					\
> -       && __gnu_dev_minor ((Dev)) >= 128 && __gnu_dev_minor ((Dev)) < 192) \
> -   || (__gnu_dev_major ((Dev)) >= 128 && __gnu_dev_major ((Dev)) < 136))
> -
> -/* Check if DEV corresponds to a slave pseudo terminal device.  */
> -#define SLAVE_P(Dev)							\
> -  (__gnu_dev_major ((Dev)) == 3						\
> -   || (__gnu_dev_major ((Dev)) == 4					\
> -       && __gnu_dev_minor ((Dev)) >= 192 && __gnu_dev_minor ((Dev)) < 256) \
> -   || (__gnu_dev_major ((Dev)) >= 136 && __gnu_dev_major ((Dev)) < 144))
> -
> -/* Note that major number 4 corresponds to the old BSD style pseudo
> -   terminal devices.  As of Linux 2.1.115 these are no longer
> -   supported.  They have been replaced by major numbers 2 (masters)
> -   and 3 (slaves).  */
> -
>  /* Directory where we can find the slave pty nodes.  */
>  #define _PATH_DEVPTS "/dev/pts/"
>  
> -/* The are declared in getpt.c.  */
> -extern const char __libc_ptyname1[] attribute_hidden;
> -extern const char __libc_ptyname2[] attribute_hidden;
> -
>  /* Static buffer for `ptsname'.  */
>  static char buffer[sizeof (_PATH_DEVPTS) + 20];
>  
> @@ -68,19 +43,15 @@ ptsname (int fd)
>  }
>  

Ok.

>  
> +/* Store at most BUFLEN characters of the pathname of the slave pseudo
> +   terminal associated with the master FD is open on in BUF.
> +   Return 0 on success, otherwise an error number.  */
>  int
> -__ptsname_internal (int fd, char *buf, size_t buflen, struct stat64 *stp)
> +__ptsname_r (int fd, char *buf, size_t buflen)
>  {
>    int save_errno = errno;
>    unsigned int ptyno;
>  
> -  if (!__isatty (fd))
> -    {
> -      __set_errno (ENOTTY);
> -      return ENOTTY;
> -    }
> -
> -#ifdef TIOCGPTN
>    if (__ioctl (fd, TIOCGPTN, &ptyno) == 0)
>      {
>        /* Buffer we use to print the number in.  For a maximum size for
> @@ -101,67 +72,11 @@ __ptsname_internal (int fd, char *buf, size_t buflen, struct stat64 *stp)
>  
>        memcpy (__stpcpy (buf, devpts), p, &numbuf[sizeof (numbuf)] - p);
>      }
> -  else if (errno != EINVAL)
> -    return errno;
>    else
> -#endif
> -    {
> -      char *p;
> -
> -      if (buflen < strlen (_PATH_TTY) + 3)
> -	{
> -	  __set_errno (ERANGE);
> -	  return ERANGE;
> -	}
> -
> -      if (__fxstat64 (_STAT_VER, fd, stp) < 0)
> -	return errno;
> -
> -      /* Check if FD really is a master pseudo terminal.  */
> -      if (! MASTER_P (stp->st_rdev))
> -	{
> -	  __set_errno (ENOTTY);
> -	  return ENOTTY;
> -	}
> -
> -      ptyno = __gnu_dev_minor (stp->st_rdev);
> -
> -      if (ptyno / 16 >= strlen (__libc_ptyname1))
> -	{
> -	  __set_errno (ENOTTY);
> -	  return ENOTTY;
> -	}
> -
> -      p = __stpcpy (buf, _PATH_TTY);
> -      p[0] = __libc_ptyname1[ptyno / 16];
> -      p[1] = __libc_ptyname2[ptyno % 16];
> -      p[2] = '\0';
> -    }
> -
> -  if (__xstat64 (_STAT_VER, buf, stp) < 0)
> +    /* Bad file descriptor, or not a ptmx descriptor.  */
>      return errno;
>  
> -  /* Check if the name we're about to return really corresponds to a
> -     slave pseudo terminal.  */
> -  if (! S_ISCHR (stp->st_mode) || ! SLAVE_P (stp->st_rdev))
> -    {
> -      /* This really is a configuration problem.  */
> -      __set_errno (ENOTTY);
> -      return ENOTTY;
> -    }
> -
>    __set_errno (save_errno);
>    return 0;
>  }
> -
> -
> -/* Store at most BUFLEN characters of the pathname of the slave pseudo
> -   terminal associated with the master FD is open on in BUF.
> -   Return 0 on success, otherwise an error number.  */
> -int
> -__ptsname_r (int fd, char *buf, size_t buflen)
> -{
> -  struct stat64 st;
> -  return __ptsname_internal (fd, buf, buflen, &st);
> -}
>  weak_alias (__ptsname_r, ptsname_r)
> 

Ok.