[Bug libc/28769] Off-by-one buffer overflow/underflow in getcwd() (CVE-2021-3999)

["cvs-commit at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=28769

--- Comment #5 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
The release/2.33/master branch has been updated by Aurelien Jarno
<aurel32@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=bcdde07537d733eafe1f28cea3084003e140f5ff

commit bcdde07537d733eafe1f28cea3084003e140f5ff
Author: Siddhesh Poyarekar <siddhesh@sourceware.org>
Date:   Fri Jan 21 23:32:56 2022 +0530

    getcwd: Set errno to ERANGE for size == 1 (CVE-2021-3999)

    No valid path returned by getcwd would fit into 1 byte, so reject the
    size early and return NULL with errno set to ERANGE.  This change is
    prompted by CVE-2021-3999, which describes a single byte buffer
    underflow and overflow when all of the following conditions are met:

    - The buffer size (i.e. the second argument of getcwd) is 1 byte
    - The current working directory is too long
    - '/' is also mounted on the current working directory

    Sequence of events:

    - In sysdeps/unix/sysv/linux/getcwd.c, the syscall returns ENAMETOOLONG
      because the linux kernel checks for name length before it checks
      buffer size

    - The code falls back to the generic getcwd in sysdeps/posix

    - In the generic func, the buf[0] is set to '\0' on line 250

    - this while loop on line 262 is bypassed:

        while (!(thisdev == rootdev && thisino == rootino))

      since the rootfs (/) is bind mounted onto the directory and the flow
      goes on to line 449, where it puts a '/' in the byte before the
      buffer.

    - Finally on line 458, it moves 2 bytes (the underflowed byte and the
      '\0') to the buf[0] and buf[1], resulting in a 1 byte buffer overflow.

    - buf is returned on line 469 and errno is not set.

    This resolves BZ #28769.

    Reviewed-by: Andreas Schwab <schwab@linux-m68k.org>
    Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
    Signed-off-by: Qualys Security Advisory <qsa@qualys.com>
    Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
    (cherry picked from commit 23e0e8f5f1fb5ed150253d986ecccdc90c2dcd5e)

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