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@xxxxxxxxxxxxxxxx>
---
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
