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From: Ariadne Conill <ariadne@dereferenced.org>
To: linux-kernel@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org,
	Eric Biederman <ebiederm@xmission.com>,
	Kees Cook <keescook@chromium.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Ariadne Conill <ariadne@dereferenced.org>
Subject: [PATCH] fs/exec: require argv[0] presence in do_execveat_common()
Date: Wed, 26 Jan 2022 04:39:47 +0000	[thread overview]
Message-ID: <20220126043947.10058-1-ariadne@dereferenced.org> (raw)

The first argument to argv when used with execv family of calls is
required to be the name of the program being executed, per POSIX.

By validating this in do_execveat_common(), we can prevent execution
of shellcode which invokes execv(2) family syscalls with argc < 1,
a scenario which is disallowed by POSIX, thus providing a mitigation
against CVE-2021-4034 and similar bugs in the future.

The use of -EFAULT for this case is similar to other systems, such
as FreeBSD and OpenBSD.

Interestingly, Michael Kerrisk opened an issue about this in 2008,
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.

Signed-off-by: Ariadne Conill <ariadne@dereferenced.org>
---
 fs/exec.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/exec.c b/fs/exec.c
index 79f2c9483302..de0b832473ed 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1897,8 +1897,10 @@ static int do_execveat_common(int fd, struct filename *filename,
 	}
 
 	retval = count(argv, MAX_ARG_STRINGS);
-	if (retval < 0)
+	if (retval < 1) {
+		retval = -EFAULT;
 		goto out_free;
+	}
 	bprm->argc = retval;
 
 	retval = count(envp, MAX_ARG_STRINGS);
-- 
2.34.1


             reply	other threads:[~2022-01-26  4:48 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-01-26  4:39 Ariadne Conill [this message]
2022-01-26  6:42 ` Kees Cook
2022-01-26  7:28   ` Kees Cook
2022-01-26 11:18     ` Ariadne Conill
2022-01-26 12:33       ` Heikki Kallasjoki
2022-01-26 23:57         ` Kees Cook
2022-01-27  0:20           ` Eric W. Biederman
2022-01-26 16:59     ` David Laight
2022-01-26 13:27 ` Rich Felker
2022-01-26 14:46   ` Christian Brauner
2022-01-26 17:37   ` Ariadne Conill
2022-02-01 20:54   ` hypervis0r
2022-01-26 15:02 Alexey Dobriyan
2022-01-27  0:00 ` Kees Cook

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