The patch titled Subject: fs/exec: require argv[0] presence in do_execveat_common() has been added to the -mm tree. Its filename is fs-exec-require-argv-presence-in-do_execveat_common.patch
This patch should soon appear at https://ozlabs.org/~akpm/mmots/broken-out/fs-exec-require-argv-presence-in-d... and later at https://ozlabs.org/~akpm/mmotm/broken-out/fs-exec-require-argv-presence-in-d...
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------------------------------------------------------ From: Ariadne Conill ariadne@dereferenced.org Subject: fs/exec: require argv[0] presence in do_execveat_common()
In several other operating systems, it is a hard requirement that the second argument to execve(2) be the name of a program, thus prohibiting a scenario where argc < 1. POSIX 2017 also recommends this behaviour, but it is not an explicit requirement[0]:
The argument arg0 should point to a filename string that is associated with the process being started by one of the exec functions.
To ensure that execve(2) with argc < 1 is not a useful tool for shellcode to use, we can validate this in do_execveat_common() and fail for this scenario, effectively blocking successful exploitation of CVE-2021-4034 and similar bugs which depend on execve(2) working with argc < 1.
We use -EINVAL for this case, mirroring recent changes to FreeBSD and OpenBSD. -EINVAL is also used by QNX for this, while Solaris uses -EFAULT.
In earlier versions of the patch, it was proposed that we create a fake argv for applications to use when argc < 1, but it was concluded that it would be better to just fail the execve(2) in these cases, as launching a process with an empty or NULL argv[0] was likely to just cause more problems.
Interestingly, Michael Kerrisk opened an issue about this in 2008[1], but there was no consensus to support fixing this issue then. Hopefully now that CVE-2021-4034 shows practical exploitative use[2] of this bug in a shellcode, we can reconsider.
This issue is being tracked in the KSPP issue tracker[3].
There are a few[4][5] minor edge cases (primarily in test suites) that are caught by this, but we plan to work with the projects to fix those edge cases.
[0]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html [1]: https://bugzilla.kernel.org/show_bug.cgi?id=8408 [2]: https://www.qualys.com/2022/01/25/cve-2021-4034/pwnkit.txt [3]: https://github.com/KSPP/linux/issues/176 [4]: https://codesearch.debian.net/search?q=execve%5C+*%5C%28%5B%5E%2C%5D%2B%2C+*... [5]: https://codesearch.debian.net/search?q=execlp%3F%5Cs*%5C%28%5B%5E%2C%5D%2B%2...
Link: https://lkml.kernel.org/r/20220127000724.15106-1-ariadne@dereferenced.org Signed-off-by: Ariadne Conill ariadne@dereferenced.org Reported-by: Michael Kerrisk mtk.manpages@gmail.com Cc: Matthew Wilcox willy@infradead.org Cc: Christian Brauner brauner@kernel.org Cc: Rich Felker dalias@libc.org Cc: Eric Biederman ebiederm@xmission.com Cc: Alexander Viro viro@zeniv.linux.org.uk Cc: Kees Cook keescook@chromium.org Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
fs/exec.c | 4 ++++ 1 file changed, 4 insertions(+)
--- a/fs/exec.c~fs-exec-require-argv-presence-in-do_execveat_common +++ a/fs/exec.c @@ -1897,6 +1897,10 @@ static int do_execveat_common(int fd, st }
retval = count(argv, MAX_ARG_STRINGS); + if (retval == 0) { + pr_warn_once("Attempted to run process '%s' with NULL argv\n", bprm->filename); + retval = -EINVAL; + } if (retval < 0) goto out_free; bprm->argc = retval; _
Patches currently in -mm which might be from ariadne@dereferenced.org are
fs-exec-require-argv-presence-in-do_execveat_common.patch