Messages in this thread | | | Date | Wed, 26 Jan 2022 14:23:59 -0600 (CST) | From | Ariadne Conill <> | Subject | Re: [PATCH v2] fs/exec: require argv[0] presence in do_execveat_common() |
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Hi,
On Wed, 26 Jan 2022, Kees Cook wrote:
> On Wed, Jan 26, 2022 at 11:44:47AM +0000, Ariadne Conill wrote: >> In several other operating systems, it is a hard requirement that the >> first 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 gadget 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 this gadget. >> >> The use of -EFAULT for this case is similar to other systems, such >> as FreeBSD, OpenBSD and Solaris. QNX uses -EINVAL for this case. >> >> 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 >> >> Changes from v1: >> - Rework commit message significantly. >> - Make the argv[0] check explicit rather than hijacking the error-check >> for count(). >> >> Signed-off-by: Ariadne Conill <ariadne@dereferenced.org> >> --- >> fs/exec.c | 4 ++++ >> 1 file changed, 4 insertions(+) >> >> diff --git a/fs/exec.c b/fs/exec.c >> index 79f2c9483302..e52c41991aab 100644 >> --- a/fs/exec.c >> +++ b/fs/exec.c >> @@ -1899,6 +1899,10 @@ static int do_execveat_common(int fd, struct filename *filename, >> retval = count(argv, MAX_ARG_STRINGS); >> if (retval < 0) >> goto out_free; >> + if (retval == 0) { >> + retval = -EFAULT; >> + goto out_free; >> + } >> bprm->argc = retval; >> >> retval = count(envp, MAX_ARG_STRINGS); >> -- >> 2.34.1 > > Okay, so, the dangerous condition is userspace iterating through envp > when it thinks it's iterating argv. > > Assuming it is not okay to break valgrind's test suite: > https://sources.debian.org/src/valgrind/1:3.18.1-1/none/tests/execve.c/?hl=22#L22 > we cannot reject a NULL argv (test will fail), and we cannot mutate > argc=0 into argc=1 (test will enter infinite loop). > > Perhaps we need to reject argv=NULL when envp!=NULL, and add a > pr_warn_once() about using a NULL argv?
Sure, I can rework the patch to do it for only the envp != NULL case.
I think we should combine it with the {NULL, NULL} padding patch in this case though, since it appears to work, that way the execve(..., NULL, NULL) case gets some protection.
> I note that glibc already warns about NULL argv: > argc0.c:7:3: warning: null argument where non-null required (argument 2) > [-Wnonnull] > 7 | execve(argv[0], NULL, envp); > | ^~~~~~ > > in the future we could expand this to only looking at argv=NULL?
I don't think musl's headers generate a diagnostic for this, but main(0, {NULL}) is not a supported use-case at least as far as Alpine is concerned. I am sure it is the same with the other musl distributions.
Will send a v3 patch with this logic change and move to EINVAL shortly.
Ariadne
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