From 472e799a5f2102bc0c3206dbd5a801765fceb39c Mon Sep 17 00:00:00 2001 From: Siddhesh Poyarekar 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 269 - this while loop on line 282 is bypassed: while (!(thisdev == rootdev && thisino == rootino)) since the rootfs (/) is bind mounted onto the directory and the flow goes on to line 492, where it puts a '/' in the byte before the buffer. - Finally on line 500, 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 514 and errno is not set. This resolves BZ #28769. Reviewed-by: Andreas Schwab Reviewed-by: Adhemerval Zanella Signed-off-by: Qualys Security Advisory Signed-off-by: Siddhesh Poyarekar (cherry picked from commit 23e0e8f5f1fb5ed150253d986ecccdc90c2dcd5e) --- sysdeps/posix/getcwd.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/sysdeps/posix/getcwd.c b/sysdeps/posix/getcwd.c index b53433a2..e3c1d77c 100644 --- a/sysdeps/posix/getcwd.c +++ b/sysdeps/posix/getcwd.c @@ -239,6 +239,14 @@ __getcwd (char *buf, size_t size) int fd = AT_FDCWD; char *path; + + /* A size of 1 byte is never useful. */ + if (size == 1) + { + __set_errno (ERANGE); + return NULL; + } + #ifndef NO_ALLOCATION size_t allocated = size; if (size == 0) -- 2.27.0