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CVE-2021-3999.patch
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)