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/[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/