Hi,
On Wed, 26 Jan 2022, Eric W. Biederman wrote:
Matthew Wilcox <willy@xxxxxxxxxxxxx> writes:
On Wed, Jan 26, 2022 at 11:44:47AM +0000, Ariadne Conill wrote:
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
of this bug in a shellcode, we can reconsider.
[0]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html
[1]: https://bugzilla.kernel.org/show_bug.cgi?id=8408
Having now read 8408 ... if ABI change is a concern (and I really doubt
it is), we could treat calling execve() with a NULL argv as if the
caller had passed an array of length 1 with the first element set to
NULL. Just like we reopen fds 0,1,2 for suid execs if they were
closed.
Where do we reopen fds 0,1,2 for suid execs? I feel silly but I looked
through the code fs/exec.c quickly and I could not see it.
I am attracted to the notion of converting an empty argv array passed
to the kernel into something we can safely pass to userspace.
I think it would need to be having the first entry point to "" instead
of the first entry being NULL. That would maintain the invariant that you
can always dereference a pointer in the argv array.
Yes, I think this is correct, because there's a lot of programs out there
which will try to blindly read from argv[0], assuming it is present.
Ensuring we wind up with {"", NULL} would be the way I would want to
approach this if we go that route.
This approach would solve the problem with pkexec, but I still think there
is some wisdom in denying with -EFAULT outright like other systems do.
Ariadne
