On Wed, Feb 02, 2022 at 07:50:42AM -0800, Kees Cook wrote:
On February 1, 2022 6:53:25 AM PST, Rich Felker dalias@libc.org wrote:
On Mon, Jan 31, 2022 at 04:09:47PM -0800, Kees Cook wrote:
Quoting[1] Ariadne Conill:
"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[2]:
The argument arg0 should point to a filename string that is associated with the process being started by one of the exec functions..... Interestingly, Michael Kerrisk opened an issue about this in 2008[3], but there was no consensus to support fixing this issue then. Hopefully now that CVE-2021-4034 shows practical exploitative use[4] of this bug in a shellcode, we can reconsider.
This issue is being tracked in the KSPP issue tracker[5]."
While the initial code searches[6][7] turned up what appeared to be mostly corner case tests, trying to that just reject argv == NULL (or an immediately terminated pointer list) quickly started tripping[8] existing userspace programs.
The next best approach is forcing a single empty string into argv and adjusting argc to match. The number of programs depending on argc == 0 seems a smaller set than those calling execve with a NULL argv.
Account for the additional stack space in bprm_stack_limits(). Inject an empty string when argc == 0 (and set argc = 1). Warn about the case so userspace has some notice about the change:
process './argc0' launched './argc0' with NULL argv: empty string addedAdditionally WARN() and reject NULL argv usage for kernel threads.
[1] https://lore.kernel.org/lkml/20220127000724.15106-1-ariadne@dereferenced.org... [2] https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html [3] https://bugzilla.kernel.org/show_bug.cgi?id=8408 [4] https://www.qualys.com/2022/01/25/cve-2021-4034/pwnkit.txt [5] https://github.com/KSPP/linux/issues/176 [6] https://codesearch.debian.net/search?q=execve%5C+*%5C%28%5B%5E%2C%5D%2B%2C+*... [7] https://codesearch.debian.net/search?q=execlp%3F%5Cs*%5C%28%5B%5E%2C%5D%2B%2... [8] https://lore.kernel.org/lkml/20220131144352.GE16385@xsang-OptiPlex-9020/
Reported-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: linux-fsdevel@vger.kernel.org Cc: stable@vger.kernel.org Signed-off-by: Kees Cook keescook@chromium.org
fs/exec.c | 26 +++++++++++++++++++++++++- 1 file changed, 25 insertions(+), 1 deletion(-)
diff --git a/fs/exec.c b/fs/exec.c index 79f2c9483302..bbf3aadf7ce1 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -495,8 +495,14 @@ static int bprm_stack_limits(struct linux_binprm *bprm) * the stack. They aren't stored until much later when we can't * signal to the parent that the child has run out of stack space. * Instead, calculate it here so it's possible to fail gracefully.
** In the case of argc = 0, make sure there is space for adding a* empty string (which will bump argc to 1), to ensure confused* userspace programs don't start processing from argv[1], thinking* argc can never be 0, to keep them from walking envp by accident.* See do_execveat_common().
- ptr_size = (bprm->argc + bprm->envc) * sizeof(void *);
- ptr_size = (min(bprm->argc, 1) + bprm->envc) * sizeof(void *);
From #musl:
<mixi> kees: shouldn't the min(bprm->argc, 1) be max(...) in your patch?
Fix has already been sent, yup.
I'm pretty sure without fixing that, you're introducing a giant vuln here.
I wouldn't say "giant", but yes, it weakened a defense in depth for avoiding high stack utilization.
I thought it was deciding the amount of memory to allocate/reserve for the arg slots, but based on the comment it looks like it's just a way to fail early rather than making the new process image fault later if they don't fit.
I believe this is the second time a patch attempting to fix this non-vuln has proposed adding a new vuln...
Mistakes happen, and that's why there is review and testing. Thank you for being part of the review process! :)
I know, and I'm sorry for being a bit hostile over it, and for jumping the gun about the severity. I just get frustrated when I see a rush to make changes over an incidental part of a popularized vuln, with disproportionate weight on "doing something" and not enough on being careful.
Rich