From: Kees Cook <keescook@chromium.org>
To: Ariadne Conill <ariadne@dereferenced.org>
Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
Eric Biederman <ebiederm@xmission.com>,
Alexander Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH v2] fs/exec: require argv[0] presence in do_execveat_common()
Date: Wed, 26 Jan 2022 12:09:08 -0800 [thread overview]
Message-ID: <202201261202.EC027EB@keescook> (raw)
In-Reply-To: <20220126114447.25776-1-ariadne@dereferenced.org>
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?
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?
--
Kees Cook
next prev parent reply other threads:[~2022-01-26 20:09 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-26 11:44 [PATCH v2] fs/exec: require argv[0] presence in do_execveat_common() Ariadne Conill
2022-01-26 14:40 ` Matthew Wilcox
2022-01-26 17:41 ` Ariadne Conill
2022-01-26 14:59 ` Matthew Wilcox
2022-01-26 16:40 ` Kees Cook
2022-01-26 16:57 ` Eric W. Biederman
2022-01-26 17:32 ` Ariadne Conill
2022-01-26 18:03 ` Matthew Wilcox
2022-01-26 18:38 ` Ariadne Conill
2022-01-26 20:09 ` Kees Cook [this message]
2022-01-26 20:23 ` Ariadne Conill
2022-01-26 20:56 ` Kees Cook
2022-01-26 21:13 ` Ariadne Conill
2022-01-26 21:25 ` Kees Cook
2022-01-26 21:30 ` Ariadne Conill
2022-01-26 22:49 ` Kees Cook
2022-01-26 23:07 ` Ariadne Conill
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