The patch titled
Subject: fs/exec: require argv[0] presence in do_execveat_common()
has been added to the -mm tree. Its filename is
fs-exec-require-argv-presence-in-do_execveat_common.patch
This patch should soon appear at
https://ozlabs.org/~akpm/mmots/broken-out/fs-exec-require-argv-presence-in-do_execveat_common.patch
and later at
https://ozlabs.org/~akpm/mmotm/broken-out/fs-exec-require-argv-presence-in-do_execveat_common.patch
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------------------------------------------------------
From: Ariadne Conill <ariadne@xxxxxxxxxxxxxxxx>
Subject: fs/exec: require argv[0] presence in do_execveat_common()
In several other operating systems, it is a hard requirement that the
second 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 tool 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 execve(2) working with argc < 1.
We use -EINVAL for this case, mirroring recent changes to FreeBSD and
OpenBSD. -EINVAL is also used by QNX for this, while Solaris uses
-EFAULT.
In earlier versions of the patch, it was proposed that we create a fake
argv for applications to use when argc < 1, but it was concluded that it
would be better to just fail the execve(2) in these cases, as launching a
process with an empty or NULL argv[0] was likely to just cause more
problems.
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[2] of this bug in a
shellcode, we can reconsider.
This issue is being tracked in the KSPP issue tracker[3].
There are a few[4][5] minor edge cases (primarily in test suites) that are
caught by this, but we plan to work with the projects to fix those edge
cases.
[0]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html
[1]: https://bugzilla.kernel.org/show_bug.cgi?id=8408
[2]: https://www.qualys.com/2022/01/25/cve-2021-4034/pwnkit.txt
[3]: https://github.com/KSPP/linux/issues/176
[4]: https://codesearch.debian.net/search?q=execve%5C+*%5C%28%5B%5E%2C%5D%2B%2C+*NULL&literal=0
[5]: https://codesearch.debian.net/search?q=execlp%3F%5Cs*%5C%28%5B%5E%2C%5D%2B%2C%5Cs*NULL&literal=0
Link: https://lkml.kernel.org/r/20220127000724.15106-1-ariadne@xxxxxxxxxxxxxxxx
Signed-off-by: Ariadne Conill <ariadne@xxxxxxxxxxxxxxxx>
Reported-by: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Rich Felker <dalias@xxxxxxxx>
Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
fs/exec.c | 4 ++++
1 file changed, 4 insertions(+)
--- a/fs/exec.c~fs-exec-require-argv-presence-in-do_execveat_common
+++ a/fs/exec.c
@@ -1897,6 +1897,10 @@ static int do_execveat_common(int fd, st
}
retval = count(argv, MAX_ARG_STRINGS);
+ if (retval == 0) {
+ pr_warn_once("Attempted to run process '%s' with NULL argv\n", bprm->filename);
+ retval = -EINVAL;
+ }
if (retval < 0)
goto out_free;
bprm->argc = retval;
_
Patches currently in -mm which might be from ariadne@xxxxxxxxxxxxxxxx are
fs-exec-require-argv-presence-in-do_execveat_common.patch
