For very long file names, the getcwd syscall returns ENAMETOOLONG, resulting in a call to __getcwd_generic. That function in turn assumes a size that's suficient for at least a '/', so a size of 1 results in access that's one byte beyond bounds of buf. We always need a minimum size of 2 bytes, so the glibc function should set ERANGE and return NULL for size of 1 byte.
The master branch has been updated by Siddhesh Poyarekar <siddhesh@sourceware.org>: https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=23e0e8f5f1fb5ed150253d986ecccdc90c2dcd5e commit 23e0e8f5f1fb5ed150253d986ecccdc90c2dcd5e 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) 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>
The release/2.34/master branch has been updated by Siddhesh Poyarekar <siddhesh@sourceware.org>: https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=472e799a5f2102bc0c3206dbd5a801765fceb39c commit 472e799a5f2102bc0c3206dbd5a801765fceb39c 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) 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)
Fixed on trunk and 2.34.
The release/2.34/master branch has been updated by Aurelien Jarno <aurel32@sourceware.org>: https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=1b9cd6a7214db1812a20eb3591cf42f9190a5d1a commit 1b9cd6a7214db1812a20eb3591cf42f9190a5d1a Author: Aurelien Jarno <aurelien@aurel32.net> Date: Mon Jan 24 23:45:03 2022 +0100 NEWS: add bug entry for BZ #28769 and BZ #28770
The release/2.33/master branch has been updated by Aurelien Jarno <aurel32@sourceware.org>: https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=bcdde07537d733eafe1f28cea3084003e140f5ff commit bcdde07537d733eafe1f28cea3084003e140f5ff 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) 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)
The release/2.31/master branch has been updated by Aurelien Jarno <aurel32@sourceware.org>: https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=4ad1659d8c024953b11c2f0fef2219529171da7e commit 4ad1659d8c024953b11c2f0fef2219529171da7e 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) 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)