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Message-Id: <20220127000724.15106-1-ariadne@dereferenced.org>
Date: Thu, 27 Jan 2022 00:07:24 +0000
From: Ariadne Conill <ariadne@...eferenced.org>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
Eric Biederman <ebiederm@...ssion.com>,
Kees Cook <keescook@...omium.org>,
Alexander Viro <viro@...iv.linux.org.uk>,
Ariadne Conill <ariadne@...eferenced.org>,
Michael Kerrisk <mtk.manpages@...il.com>,
Matthew Wilcox <willy@...radead.org>,
Christian Brauner <brauner@...nel.org>,
Rich Felker <dalias@...c.org>, linux-mm@...ck.org,
stable@...r.kernel.org
Subject: [PATCH v3] 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
Changes from v2:
- Switch to using -EINVAL as the error code for this.
- Use pr_warn_once() to warn when an execve(2) is rejected due to NULL
argv.
Changes from v1:
- Rework commit message significantly.
- Make the argv[0] check explicit rather than hijacking the error-check
for count().
Reported-by: Michael Kerrisk <mtk.manpages@...il.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Matthew Wilcox <willy@...radead.org>
Cc: Christian Brauner <brauner@...nel.org>
Cc: Rich Felker <dalias@...c.org>
Cc: Eric Biederman <ebiederm@...ssion.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>
Cc: Kees Cook <keescook@...omium.org>
Cc: linux-fsdevel@...r.kernel.org
Cc: linux-mm@...ck.org
Cc: stable@...r.kernel.org
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..982730cfe3b8 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1897,6 +1897,10 @@ static int do_execveat_common(int fd, struct filename *filename,
}
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;
--
2.34.1
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