Kees Cook <keescook@xxxxxxxxxxxx> writes: > On Thu, Jan 27, 2022 at 12:07:24AM +0000, Ariadne Conill wrote: >> 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[0]: >> >> The argument arg0 should point to a filename string that is >> associated with the process being started by one of the exec >> functions. >> >> To ensure that execve(2) with argc < 1 is not a useful tool for >> shellcode to use, we can validate this in do_execveat_common() and >> fail for this scenario, effectively blocking successful exploitation >> of CVE-2021-4034 and similar bugs which depend on execve(2) working >> with argc < 1. >> >> We use -EINVAL for this case, mirroring recent changes to FreeBSD and >> OpenBSD. -EINVAL is also used by QNX for this, while Solaris uses >> -EFAULT. >> >> In earlier versions of the patch, it was proposed that we create a >> fake argv for applications to use when argc < 1, but it was concluded >> that it would be better to just fail the execve(2) in these cases, as >> launching a process with an empty or NULL argv[0] was likely to just >> cause more problems. > > Let's do it and see what breaks. :) > > I do see at least tools/testing/selftests/exec/recursion-depth.c will > need a fix. And maybe testcases/kernel/syscalls/execveat/execveat.h > in LTP. > > Acked-by: Kees Cook <keescook@xxxxxxxxxxxx> Yes since this only appears to be tests that will break. Acked-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx> Especially since you are signing up to help fix the tests. >> >> Interestingly, Michael Kerrisk opened an issue about this in 2008[1], >> but there was no consensus to support fixing this issue then. >> Hopefully now that CVE-2021-4034 shows practical exploitative use[2] >> of this bug in a shellcode, we can reconsider. >> >> This issue is being tracked in the KSPP issue tracker[3]. >> >> There are a few[4][5] minor edge cases (primarily in test suites) that >> are caught by this, but we plan to work with the projects to fix those >> edge cases. >> >> [0]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html >> [1]: https://bugzilla.kernel.org/show_bug.cgi?id=8408 >> [2]: https://www.qualys.com/2022/01/25/cve-2021-4034/pwnkit.txt >> [3]: https://github.com/KSPP/linux/issues/176 >> [4]: https://codesearch.debian.net/search?q=execve%5C+*%5C%28%5B%5E%2C%5D%2B%2C+*NULL&literal=0 >> [5]: https://codesearch.debian.net/search?q=execlp%3F%5Cs*%5C%28%5B%5E%2C%5D%2B%2C%5Cs*NULL&literal=0 >> >> Changes from v2: >> - Switch to using -EINVAL as the error code for this. >> - Use pr_warn_once() to warn when an execve(2) is rejected due to NULL >> argv. >> >> Changes from v1: >> - Rework commit message significantly. >> - Make the argv[0] check explicit rather than hijacking the error-check >> for count(). >> >> Reported-by: Michael Kerrisk <mtk.manpages@xxxxxxxxx> >> To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> >> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> >> Cc: Christian Brauner <brauner@xxxxxxxxxx> >> Cc: Rich Felker <dalias@xxxxxxxx> >> Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx> >> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx> >> Cc: Kees Cook <keescook@xxxxxxxxxxxx> >> Cc: linux-fsdevel@xxxxxxxxxxxxxxx >> Cc: linux-mm@xxxxxxxxx >> Cc: stable@xxxxxxxxxxxxxxx >> Signed-off-by: Ariadne Conill <ariadne@xxxxxxxxxxxxxxxx> >> --- >> fs/exec.c | 4 ++++ >> 1 file changed, 4 insertions(+) >> >> diff --git a/fs/exec.c b/fs/exec.c >> index 79f2c9483302..982730cfe3b8 100644 >> --- a/fs/exec.c >> +++ b/fs/exec.c >> @@ -1897,6 +1897,10 @@ static int do_execveat_common(int fd, struct filename *filename, >> } >> >> retval = count(argv, MAX_ARG_STRINGS); >> + if (retval == 0) { >> + pr_warn_once("Attempted to run process '%s' with NULL argv\n", bprm->filename); >> + retval = -EINVAL; >> + } >> if (retval < 0) >> goto out_free; >> bprm->argc = retval; >> -- >> 2.34.1 >>
