Commit d34c7430 authored Jan 31, 2022 by Kees Cook Committed by Srinivasarao Pathipati Aug 16, 2023

exec: Force single empty string when argv is empty

commit dcd46d897adb70d63e025f175a00a89797d31a43 upstream.

Quoting[1] Ariadne Conill:

"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[2]:

    The argument arg0 should point to a filename string that is
    associated with the process being started by one of the exec
    functions.
...
Interestingly, Michael Kerrisk opened an issue about this in 2008[3],
but there was no consensus to support fixing this issue then.
Hopefully now that CVE-2021-4034 shows practical exploitative use[4]
of this bug in a shellcode, we can reconsider.

This issue is being tracked in the KSPP issue tracker[5]."

While the initial code searches[6][7] turned up what appeared to be
mostly corner case tests, trying to that just reject argv == NULL
(or an immediately terminated pointer list) quickly started tripping[8]
existing userspace programs.

The next best approach is forcing a single empty string into argv and
adjusting argc to match. The number of programs depending on argc == 0
seems a smaller set than those calling execve with a NULL argv.

Account for the additional stack space in bprm_stack_limits(). Inject an
empty string when argc == 0 (and set argc = 1). Warn about the case so
userspace has some notice about the change:

    process './argc0' launched './argc0' with NULL argv: empty string added

Additionally WARN() and reject NULL argv usage for kernel threads.

[1] https://lore.kernel.org/lkml/20220127000724.15106-1-ariadne@dereferenced.org/
[2] https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html
[3] https://bugzilla.kernel.org/show_bug.cgi?id=8408
[4] https://www.qualys.com/2022/01/25/cve-2021-4034/pwnkit.txt
[5] https://github.com/KSPP/linux/issues/176
[6] https://codesearch.debian.net/search?q=execve%5C+*%5C%28%5B%5E%2C%5D%2B%2C+*NULL&literal=0
[7] https://codesearch.debian.net/search?q=execlp%3F%5Cs*%5C%28%5B%5E%2C%5D%2B%2C%5Cs*NULL&literal=0
[8] https://lore.kernel.org/lkml/20220131144352.GE16385@xsang-OptiPlex-9020/



Change-Id: Ie940481088d6b5de45f450501144585dba003a5e
Reported-by: Ariadne Conill <ariadne@dereferenced.org>
Reported-by: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Rich Felker <dalias@libc.org>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Christian Brauner <brauner@kernel.org>
Acked-by: Ariadne Conill <ariadne@dereferenced.org>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lore.kernel.org/r/20220201000947.2453721-1-keescook@chromium.org


[vegard: fixed conflicts due to missing
 886d7de631da71e30909980fdbf318f7caade262^- and
 3950e975431bc914f7e81b8f2a2dbdf2064acb0f^- and
 655c16a8ce9c15842547f40ce23fd148aeccc074]
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Git-commit: b50fb8db
Git-repo: https://android.googlesource.com/kernel/common/


Signed-off-by: Srinivasarao Pathipati <quic_c_spathi@quicinc.com>

parent 4eaf155c

fs/exec.c

+17 −0

Original line number	Diff line number	Diff line
		@@ -1805,6 +1805,9 @@ static int __do_execve_file(int fd, struct filename *filename,
		goto out_unmark;

		bprm->argc = count(argv, MAX_ARG_STRINGS);
		if (bprm->argc == 0)
		pr_warn_once("process '%s' launched '%s' with NULL argv: empty string added\n",
		current->comm, bprm->filename);
		if ((retval = bprm->argc) < 0)
		goto out;

		@@ -1829,6 +1832,20 @@ static int __do_execve_file(int fd, struct filename *filename,
		if (retval < 0)
		goto out;

		/*
		* When argv is empty, add an empty string ("") as argv[0] to
		* ensure confused userspace programs that start processing
		* from argv[1] won't end up walking envp. See also
		* bprm_stack_limits().
		*/
		if (bprm->argc == 0) {
		const char *argv[] = { "", NULL };
		retval = copy_strings_kernel(1, argv, bprm);
		if (retval < 0)
		goto out;
		bprm->argc = 1;
		}

		retval = exec_binprm(bprm);
		if (retval < 0)
		goto out;