Messages in this thread | | | Date | Wed, 26 Jan 2022 11:41:05 -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, Matthew Wilcox 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; >> + } > > I don't object to the concept, but it's a more common pattern in Linux > to do this: > > retval = count(argv, MAX_ARG_STRINGS); > + if (retval == 0) > + retval = -EFAULT; > if (retval < 0) > goto out_free;
Yeah, that seems fine. We can of course do it that way, which I will revise the patch to do if we decide to stick with denial over making a "safe" argv instead.
> (aka I like my bikesheds painted in Toasty Eggshell)
Toasty Eggshell is a nice color for a bikeshed :)
Ariadne
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