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    SubjectRe: [PATCH v2] fs/exec: require argv[0] presence in do_execveat_common()
    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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