FAILED: patch "[PATCH] exec: Force single empty string when argv is empty" failed to apply to 4.19-stable tree

PATCH New [email protected] 2 participants 2 messages Apr 2, 2022 – Jun 1, 2022

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2022-04-02 12:43 UTC #0 lore raw

The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From dcd46d897adb70d63e025f175a00a89797d31a43 Mon Sep 17 00:00:00 2001
From: Kees Cook <[email protected]>
Date: Mon, 31 Jan 2022 16:09:47 -0800
Subject: [PATCH] exec: Force single empty string when argv is empty

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/[email protected]/
[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/

Reported-by Ariadne Conill <[email protected]>

Reported-by Michael Kerrisk <[email protected]>

Cc Matthew Wilcox <[email protected]>

Cc Christian Brauner <[email protected]>

Cc Rich Felker <[email protected]>

Cc Eric Biederman <[email protected]>

Cc Alexander Viro <[email protected]>

Cc [email protected]

Signed-off-by Kees Cook <[email protected]>

Acked-by Christian Brauner <[email protected]>

Acked-by Ariadne Conill <[email protected]>

Acked-by Andy Lutomirski <[email protected]>

Link https://lore.kernel.org/r/[email protected] open here

fs/exec.c +25 -1

index 79f2c9483302..40b1008fb0f7 100644
@@ -495,8 +495,14 @@ static int bprm_stack_limits(struct linux_binprm *bprm)
 	 * the stack. They aren't stored until much later when we can't
 	 * signal to the parent that the child has run out of stack space.
 	 * Instead, calculate it here so it's possible to fail gracefully.
+	 *
+	 * In the case of argc = 0, make sure there is space for adding a
+	 * empty string (which will bump argc to 1), to ensure confused
+	 * userspace programs don't start processing from argv[1], thinking
+	 * argc can never be 0, to keep them from walking envp by accident.
+	 * See do_execveat_common().
 	 */
-	ptr_size = (bprm->argc + bprm->envc) * sizeof(void *);
+	ptr_size = (max(bprm->argc, 1) + bprm->envc) * sizeof(void *);
 	if (limit <= ptr_size)
 		return -E2BIG;
 	limit -= ptr_size;

@@ -1897,6 +1903,9 @@ static int do_execveat_common(int fd, struct filename *filename,
 	}
 
 	retval = count(argv, MAX_ARG_STRINGS);
+	if (retval == 0)
+		pr_warn_once("process '%s' launched '%s' with NULL argv: empty string added\n",
+			     current->comm, bprm->filename);
 	if (retval < 0)
 		goto out_free;
 	bprm->argc = retval;

@@ -1923,6 +1932,19 @@ static int do_execveat_common(int fd, struct filename *filename,
 	if (retval < 0)
 		goto out_free;
 
+	/*
+	 * 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) {
+		retval = copy_string_kernel("", bprm);
+		if (retval < 0)
+			goto out_free;
+		bprm->argc = 1;
+	}
+
 	retval = bprm_execve(bprm, fd, filename, flags);
 out_free:
 	free_bprm(bprm);

@@ -1951,6 +1973,8 @@ int kernel_execve(const char *kernel_filename,
 	}
 
 	retval = count_strings_kernel(argv);
+	if (WARN_ON_ONCE(retval == 0))
+		retval = -EINVAL;
 	if (retval < 0)
 		goto out_free;
 	bprm->argc = retval;

Vegard Nossum

2022-06-01 10:11 UTC #1 lore raw

PATCH 1/1 exec: Force single empty string when argv is empty

From: Kees Cook <[email protected]>

commit dcd46d897adb70d63e025f175a00a89797d31a43 upstream.

Quoting[1] Ariadne Conill:

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

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.

Reported-by Ariadne Conill <[email protected]>

Reported-by Michael Kerrisk <[email protected]>

Cc Matthew Wilcox <[email protected]>

Cc Christian Brauner <[email protected]>

Cc Rich Felker <[email protected]>

Cc Eric Biederman <[email protected]>

Cc Alexander Viro <[email protected]>

Cc [email protected]

Signed-off-by Kees Cook <[email protected]>

Acked-by Christian Brauner <[email protected]>

Acked-by Ariadne Conill <[email protected]>

Acked-by Andy Lutomirski <[email protected]>

Link https://lore.kernel.org/r/[email protected] open here

[vegard: fixed conflicts due to missing
886d7de631da71e30909980fdbf318f7caade262^- and
3950e975431bc914f7e81b8f2a2dbdf2064acb0f^- and
655c16a8ce9c15842547f40ce23fd148aeccc074]

Signed-off-by Vegard Nossum <[email protected]>

---
 fs/exec.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

This has been tested in both argc == 0 and argc >= 1 cases, but I would still appreciate a review given the differences with mainline. If it's considered too risky I'm also fine with dropping it -- just wanted to make sure this didn't fall through the cracks, as it does block a real (albeit old by now) exploit.

fs/exec.c +17 -0

index e87e3c020c61e..28e3b5eb2f4a1 100644
@@ -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;

Signature · 1 line

2.35.1.46.g38062e73e0