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* [PATCH v3] fs/exec: require argv[0] presence in do_execveat_common()
@ 2022-01-27  0:07 Ariadne Conill
  2022-01-27  5:29 ` Kees Cook
  2022-01-31 14:43 ` [fs/exec] 80bd5afdd8: kernel test robot
  0 siblings, 2 replies; 14+ messages in thread
From: Ariadne Conill @ 2022-01-27  0:07 UTC (permalink / raw)
  To: Andrew Morton
  Cc: linux-kernel, linux-fsdevel, Eric Biederman, Kees Cook,
	Alexander Viro, Ariadne Conill, Michael Kerrisk, Matthew Wilcox,
	Christian Brauner, Rich Felker, linux-mm, stable

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

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

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.


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

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 <>
To: Andrew Morton <>
Cc: Matthew Wilcox <>
Cc: Christian Brauner <>
Cc: Rich Felker <>
Cc: Eric Biederman <>
Cc: Alexander Viro <>
Cc: Kees Cook <>
Signed-off-by: Ariadne Conill <>
 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;

^ permalink raw reply related	[flat|nested] 14+ messages in thread

end of thread, other threads:[~2022-02-01 13:28 UTC | newest]

Thread overview: 14+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2022-01-27  0:07 [PATCH v3] fs/exec: require argv[0] presence in do_execveat_common() Ariadne Conill
2022-01-27  5:29 ` Kees Cook
2022-01-27 16:51   ` Eric W. Biederman
2022-01-31 14:43 ` [fs/exec] 80bd5afdd8: kernel test robot
2022-01-31 15:08   ` Christian Brauner
2022-01-31 15:19     ` Matthew Wilcox
2022-01-31 15:37       ` Christian Brauner
2022-01-31 15:51         ` Matthew Wilcox
2022-01-31 16:14           ` Christian Brauner
2022-01-31 17:13             ` Christian Brauner
2022-01-31 21:59               ` Andrew Morton
2022-01-31 22:49                 ` Kees Cook
2022-02-01 13:28                   ` Christian Brauner
2022-02-01 13:28                 ` Christian Brauner

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