Backport of:

From 472e799a5f2102bc0c3206dbd5a801765fceb39c Mon Sep 17 00:00:00 2001
From: Siddhesh Poyarekar <siddhesh@sourceware.org>
Date: Fri, 21 Jan 2022 23:32:56 +0530
Subject: [PATCH] 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)
---
 NEWS                                          |   6 +
 sysdeps/posix/getcwd.c                        |   7 +
 sysdeps/unix/sysv/linux/Makefile              |   7 +-
 .../unix/sysv/linux/tst-getcwd-smallbuff.c    | 241 ++++++++++++++++++
 4 files changed, 260 insertions(+), 1 deletion(-)
 create mode 100644 sysdeps/unix/sysv/linux/tst-getcwd-smallbuff.c

#diff --git a/NEWS b/NEWS
#index b4f81c2668..8d7467d2c1 100644
#--- a/NEWS
#+++ b/NEWS
#@@ -20,6 +20,12 @@ 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:
# 
#   [12889] nptl: Fix race between pthread_kill and thread exit
--- 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)