CVE-2022-49264

In the Linux kernel, the following vulnerability has been resolved:

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/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%5CsNULL&literal=0 [8] https://lore.kernel.org/lkml/20220131144352.GE16385@xsang-OptiPlex-9020/

Affected Software

References

• https://git.kernel.org/stable/c/1290eb4412aa0f0e9f3434b406dc8e255da85f9e
• https://git.kernel.org/stable/c/1fe82bfd9e4ce93399d815ca458b58505191c3e8
• https://git.kernel.org/stable/c/27a6f495b63a1804cc71be45911065db7757a98c
• https://git.kernel.org/stable/c/41f6ea5b9aaa28b740d47ffe995a5013211fdbb0
• https://git.kernel.org/stable/c/98e0c7c702894987732776736c99b85ade6fba45
• https://git.kernel.org/stable/c/a8054d3fa5deb84b215d6be1b910a978f3cb840d
• https://git.kernel.org/stable/c/b50fb8dbc8b81aaa126387de428f4c42a7c72a73
• https://git.kernel.org/stable/c/cfbfff8ce5e3d674947581f1eb9af0a1b1807950
• https://git.kernel.org/stable/c/dcd46d897adb70d63e025f175a00a89797d31a43