[PATCH] fs/binfmt_elf: Add padding NULL when argc == 0

[Kees Cook <keescook@chromium.org>]

Quoting Ariadne Conill:

"In several other operating systems, it is a hard requirement that the
first 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[1]:

    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[2],
but there was no consensus to support fixing this issue then.
Hopefully now that CVE-2021-4034 shows practical exploitative use[3]
of this bug in a shellcode, we can reconsider."

An examination of existing[4] users of execve(..., NULL, NULL) shows
mostly test code, or example rootkit code. While rejecting a NULL argv
would be preferred, it looks like the main cause of userspace confusion
is an assumption that argc >= 1, and buggy programs may skip argv[0]
when iterating. To protect against userspace bugs of this nature, insert
an extra NULL pointer in argv when argc == 0, so that argv[1] != envp[0].

Note that this is only done in the argc == 0 case because some userspace
programs expect to find envp at exactly argv[argc]. The overlap of these
two misguided assumptions is believed to be zero.

[1] https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html
[2] https://bugzilla.kernel.org/show_bug.cgi?id=8408
[3] https://www.qualys.com/2022/01/25/cve-2021-4034/pwnkit.txt
[4] https://codesearch.debian.net/search?q=execve%5C+*%5C%28%5B%5E%2C%5D%2B%2C+*NULL&literal=0

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>
---
 fs/binfmt_elf.c | 10 +++++++++-
 fs/exec.c       |  7 ++++++-
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 605017eb9349..e456c48658ad 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -297,7 +297,8 @@ create_elf_tables(struct linux_binprm *bprm, const struct elfhdr *exec,
 	ei_index = elf_info - (elf_addr_t *)mm->saved_auxv;
 	sp = STACK_ADD(p, ei_index);
 
-	items = (argc + 1) + (envc + 1) + 1;
+	/* Make room for extra pointer when argc == 0. See below. */
+	items = (min(argc, 1) + 1) + (envc + 1) + 1;
 	bprm->p = STACK_ROUND(sp, items);
 
 	/* Point sp at the lowest address on the stack */
@@ -326,6 +327,13 @@ create_elf_tables(struct linux_binprm *bprm, const struct elfhdr *exec,
 
 	/* Populate list of argv pointers back to argv strings. */
 	p = mm->arg_end = mm->arg_start;
+	/*
+	 * Include an extra NULL pointer in argv when argc == 0 so
+	 * that argv[1] != envp[0] to help userspace programs from
+	 * mishandling argc == 0. See fs/exec.c bprm_stack_limits().
+	 */
+	if (argc == 0 && put_user(0, sp++))
+		return -EFAULT;
 	while (argc-- > 0) {
 		size_t len;
 		if (put_user((elf_addr_t)p, sp++))
diff --git a/fs/exec.c b/fs/exec.c
index 79f2c9483302..0b36384e55b1 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -495,8 +495,13 @@ 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 < 1, make sure there is a NULL pointer gap
+	 * between argv and envp to ensure confused userspace programs don't
+	 * start processing from argv[1], thinking argc can never be 0,
+	 * to block them from walking envp by accident. See fs/binfmt_elf.c.
 	 */
-	ptr_size = (bprm->argc + bprm->envc) * sizeof(void *);
+	ptr_size = (min(bprm->argc, 1) + bprm->envc) * sizeof(void *);
 	if (limit <= ptr_size)
 		return -E2BIG;
 	limit -= ptr_size;
-- 
2.30.2