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

[Rich Felker <dalias@libc.org>]

On Wed, Jan 26, 2022 at 04:39:47AM +0000, Ariadne Conill wrote:
> The first argument to argv when used with execv family of calls is
> required to be the name of the program being executed, per POSIX.

That's not quite the story. The relevant text is a "should", meaning
that to be "strictly conforming" an application has to follow the
convention, but still can't assume its invoker did. (Note that most
programs do not aim to be "strictly conforming"; it's not just the
word strictly applied as an adjective to conforming, but a definition
of its own imposing very stringent portability conditions beyond what
the standard already imposes.) Moreover, POSIX (following ISO C, after
this was changed from early C drafts) rejected making it a
requirement. This is documented in the RATIONALE for execve:

    Early proposals required that the value of argc passed to main()
    be "one or greater". This was driven by the same requirement in
    drafts of the ISO C standard. In fact, historical implementations
    have passed a value of zero when no arguments are supplied to the
    caller of the exec functions. This requirement was removed from
    the ISO C standard and subsequently removed from this volume of
    POSIX.1-2017 as well. The wording, in particular the use of the
    word should, requires a Strictly Conforming POSIX Application to
    pass at least one argument to the exec function, thus guaranteeing
    that argc be one or greater when invoked by such an application.
    In fact, this is good practice, since many existing applications
    reference argv[0] without first checking the value of argc.

Source: https://pubs.opengroup.org/onlinepubs/9699919799/functions/execve.html

Note that despite citing itself as POSIX.1-2017 above, this is not a
change in the 2017 edition; it's just the way they self-cite. As far
as I can tell, the change goes back to prior to the first publication
of the standard.

> By validating this in do_execveat_common(), we can prevent execution
> of shellcode which invokes execv(2) family syscalls with argc < 1,
> a scenario which is disallowed by POSIX, thus providing a mitigation
> against CVE-2021-4034 and similar bugs in the future.
> 
> The use of -EFAULT for this case is similar to other systems, such
> as FreeBSD and OpenBSD.

I don't like this choice of error, since in principle EFAULT should
never happen when you haven't invoked memory-safety-violating UB.
Something like EINVAL would be more appropriate. But if the existing
practice for systems that do this is to use EFAULT, it's probably best
to do the same thing.

> Interestingly, Michael Kerrisk opened an issue about this in 2008,
> 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.

I'm not really opposed to attempting to change this with consensus
(like, actually proposing it on the Austin Group tracker), but a less
invasive change would be just enforcing it for the case where exec is
a privilege boundary (suid/sgid/caps). There's really no motivation
for changing longstanding standard behavior in a
non-privilege-boundary case.

Rich