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authorSiddhesh Poyarekar <siddhesh@sourceware.org>2022-01-19 09:43:26 +0530
committerSiddhesh Poyarekar <siddhesh@sourceware.org>2022-01-19 09:49:49 +0530
commit5af236968a7a5aff01d877ecb1f086bd743c571b (patch)
tree825b9a870c806783ba46e55fbbeba218a2690202 /features.h
parentc9c20ec3b0e9831c1d0c47c9c51330c3e1685eae (diff)
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getcwd: Set errno to ERANGE for size == 1 (CVE-2021-3999) siddhesh/realpath-and-getcwd
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.

Signed-off-by: Qualys Security Advisory <qsa@qualys.com>
Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Diffstat (limited to 'features.h')
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