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? I'm pretty sure without fixing that, you're introducing a giant vuln here. I believe this is the second time a patch attempting to fix this non-vuln has proposed adding a new vuln... Rich
