On Wed, Feb 02, 2022 at 07:50:42AM -0800, Kees Cook wrote: > > > On February 1, 2022 6:53:25 AM PST, Rich Felker <dalias@xxxxxxxx> wrote: > >On Mon, Jan 31, 2022 at 04:09:47PM -0800, Kees Cook wrote: > >> 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@xxxxxxxxxxxxxxxx/ > >> [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/ > >> > >> Reported-by: Ariadne Conill <ariadne@xxxxxxxxxxxxxxxx> > >> Reported-by: Michael Kerrisk <mtk.manpages@xxxxxxxxx> > >> 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: linux-fsdevel@xxxxxxxxxxxxxxx > >> Cc: stable@xxxxxxxxxxxxxxx > >> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> > >> --- > >> fs/exec.c | 26 +++++++++++++++++++++++++- > >> 1 file changed, 25 insertions(+), 1 deletion(-) > >> > >> diff --git a/fs/exec.c b/fs/exec.c > >> index 79f2c9483302..bbf3aadf7ce1 100644 > >> --- a/fs/exec.c > >> +++ b/fs/exec.c > >> @@ -495,8 +495,14 @@ 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 = 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 = (min(bprm->argc, 1) + bprm->envc) * sizeof(void *); > > > >From #musl: > > > ><mixi> kees: shouldn't the min(bprm->argc, 1) be max(...) in your patch? > > Fix has already been sent, yup. > > >I'm pretty sure without fixing that, you're introducing a giant vuln > >here. > > I wouldn't say "giant", but yes, it weakened a defense in depth for > avoiding high stack utilization. I thought it was deciding the amount of memory to allocate/reserve for the arg slots, but based on the comment it looks like it's just a way to fail early rather than making the new process image fault later if they don't fit. > > I believe this is the second time a patch attempting to fix this > >non-vuln has proposed adding a new vuln... > > Mistakes happen, and that's why there is review and testing. Thank > you for being part of the review process! :) I know, and I'm sorry for being a bit hostile over it, and for jumping the gun about the severity. I just get frustrated when I see a rush to make changes over an incidental part of a popularized vuln, with disproportionate weight on "doing something" and not enough on being careful. Rich
