From: Kees Cook keescook@chromium.org
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+*... [7] https://codesearch.debian.net/search?q=execlp%3F%5Cs*%5C%28%5B%5E%2C%5D%2B%2... [8] https://lore.kernel.org/lkml/20220131144352.GE16385@xsang-OptiPlex-9020/
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^-] Signed-off-by: Vegard Nossum vegard.nossum@oracle.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- fs/exec.c | 25 ++++++++++++++++++++++++- 1 file changed, 24 insertions(+), 1 deletion(-)
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.
--- a/fs/exec.c +++ b/fs/exec.c @@ -454,6 +454,9 @@ static int prepare_arg_pages(struct linu unsigned long limit, ptr_size;
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 (bprm->argc < 0) return bprm->argc;
@@ -482,8 +485,14 @@ static int prepare_arg_pages(struct linu * 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; @@ -1848,6 +1857,20 @@ static int __do_execve_file(int fd, stru 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;