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)

1 files changed, 7 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index 5352599d7f..cc026f03b1 100644
--- a/NEWS
+++ b/NEWS
@@ -30,6 +30,12 @@ Security related changes:
   function could result in a memory leak and potential access of
   uninitialized memory.  Reported by Qualys.
 
+  CVE-2021-3999: Passing a buffer of size exactly 1 byte to the getcwd
+  function may result in an off-by-one buffer underflow and overflow
+  when the current working directory is longer than PATH_MAX and also
+  corresponds to the / directory through an unprivileged mount
+  namespace.  Reported by Qualys.
+
 The following bugs are resolved with this release:
 
   [15271] dlfcn function failure after dlmopen terminates process
@@ -50,6 +56,7 @@ The following bugs are resolved with this release:
   [28524] Conversion from ISO-2022-JP-3 with iconv may emit spurious NULs
   [28532] powerpc64[le]: CFI for assembly templated syscalls is incorrect
   [28768] CVE-2022-23218: Buffer overflow in sunrpc svcunix_create
+  [28769] CVE-2021-3999: Off-by-one buffer overflow/underflow in getcwd()
   [28770] CVE-2021-3998: Unexpected return value from realpath() for too long results
 
 Version 2.33