| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <5e963fab-88d4-2039-1cf4-6661e9bd16b@dereferenced.org>
Date: Wed, 26 Jan 2022 15:13:10 -0600 (CST)
From: Ariadne Conill <ariadne@...eferenced.org>
To: Kees Cook <keescook@...omium.org>
cc: Ariadne Conill <ariadne@...eferenced.org>,
linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
Eric Biederman <ebiederm@...ssion.com>,
Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [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 02:23:59PM -0600, Ariadne Conill wrote:
>> 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@...eferenced.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 don't think the padding will actually work correctly, for the reason
> Jann pointed out. My testing shows that suddenly my envp becomes NULL,
> but libc is just counting argc to find envp to pass into main.
>
>>> 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.
>
> I took a spin too. Refuses execve(..., NULL, !NULL), injects "" argv[0]
> for execve(..., NULL, NULL):
>
>
> diff --git a/fs/exec.c b/fs/exec.c
> index a098c133d8d7..0565089d5f9e 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1917,9 +1917,40 @@ static int do_execveat_common(int fd, struct filename *filename,
> if (retval < 0)
> goto out_free;
>
> - retval = copy_strings(bprm->argc, argv, bprm);
> - if (retval < 0)
> - goto out_free;
> + if (likely(bprm->argc > 0)) {
> + retval = copy_strings(bprm->argc, argv, bprm);
> + if (retval < 0)
> + goto out_free;
> + } else {
> + const char * const argv0 = "";
> +
> + /*
> + * Start making some noise about the argc == NULL case that
> + * POSIX doesn't like and other Unix-like systems refuse.
> + */
> + pr_warn_once("process '%s' used a NULL argv\n", bprm->filename);
> +
> + /*
> + * Refuse to execute when argc == 0 and envc > 0, since this
> + * can lead to userspace iterating envp if it fails to check
> + * for argc == 0.
> + *
> + * i.e. continue to allow: execve(path, NULL, NULL);
> + */
> + if (bprm->envc > 0) {
> + retval = -EINVAL;
> + goto out_free;
> + }
> +
> + /*
> + * Force an argv of {"", NULL} if argc == 0 so that broken
> + * userspace that assumes argc != 0 will not be surprised.
> + */
> + bprm->argc = 1;
> + retval = copy_strings_kernel(bprm->argc, &argv0, bprm);
> + if (retval < 0)
> + goto out_free;
> + }
>
> retval = bprm_execve(bprm, fd, filename, flags);
> out_free:
Looks good to me, but I wonder if we shouldn't set an argv of
{bprm->filename, NULL} instead of {"", NULL}. Discussion in IRC led to
the realization that multicall programs will try to use argv[0] and might
crash in this scenario. If we're going to fake an argv, I guess we should
try to do it right.
Ariadne
Powered by blists - more mailing lists