[glibc/google/grte/v5-2.27/master] getcwd: Set errno to ERANGE for size == 1 (CVE-2021-3999)

[Pranav Kant <pranavk@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=5643a977d0e2cca3877775f27a41df08e98d833a

commit 5643a977d0e2cca3877775f27a41df08e98d833a
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)
    
    Cherry-picked from 23e0e8f5f1fb5ed150253d986ecccdc90c2dcd5e in main branch.
    Test included with this commit is not cherry-picked because it requires more
    changes.
    
    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>

Diff:
---
 NEWS                   | 6 ++++++
 sysdeps/posix/getcwd.c | 8 ++++++++
 2 files changed, 14 insertions(+)

diff --git a/NEWS b/NEWS
index 46b5a2d8d8..e0937786c8 100644
--- a/NEWS
+++ b/NEWS
@@ -28,6 +28,12 @@ Deprecated and removed features, and other changes affecting compatibility:
 
 Security related changes:
 
+  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.
+
   CVE-2016-10739: The getaddrinfo function could successfully parse IPv4
   addresses with arbitrary trailing characters, potentially leading to data
   or command injection issues in applications.
diff --git a/sysdeps/posix/getcwd.c b/sysdeps/posix/getcwd.c
index b53433a2dc..154b9846a5 100644
--- a/sysdeps/posix/getcwd.c
+++ b/sysdeps/posix/getcwd.c
@@ -241,6 +241,14 @@ __getcwd (char *buf, size_t size)
   char *path;
 #ifndef NO_ALLOCATION
   size_t allocated = size;
+
+  /* A size of 1 byte is never useful.  */
+  if (allocated == 1)
+    {
+      __set_errno (ERANGE);
+      return NULL;
+    }
+
   if (size == 0)
     {
       if (buf != NULL)