On Wed, Jan 26, 2022 at 11:44:47AM +0000, Ariadne Conill wrote: > In several other operating systems, it is a hard requirement that the > first 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 gadget 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 this gadget. > > The use of -EFAULT for this case is similar to other systems, such > as FreeBSD, OpenBSD and Solaris. QNX uses -EINVAL for this case. > > 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 > of this bug in a shellcode, we can reconsider. > > [0]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html > [1]: https://bugzilla.kernel.org/show_bug.cgi?id=8408 > > Changes from v1: > - Rework commit message significantly. > - Make the argv[0] check explicit rather than hijacking the error-check > for count(). > > 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..e52c41991aab 100644 > --- a/fs/exec.c > +++ b/fs/exec.c > @@ -1899,6 +1899,10 @@ static int do_execveat_common(int fd, struct filename *filename, > retval = count(argv, MAX_ARG_STRINGS); > if (retval < 0) > goto out_free; > + if (retval == 0) { > + retval = -EFAULT; > + goto out_free; > + } > bprm->argc = retval; > > retval = count(envp, MAX_ARG_STRINGS); > -- > 2.34.1 Okay, so, the dangerous condition is userspace iterating through envp when it thinks it's iterating argv. Assuming it is not okay to break valgrind's test suite: https://sources.debian.org/src/valgrind/1:3.18.1-1/none/tests/execve.c/?hl=22#L22 we cannot reject a NULL argv (test will fail), and we cannot mutate argc=0 into argc=1 (test will enter infinite loop). Perhaps we need to reject argv=NULL when envp!=NULL, and add a pr_warn_once() about using a NULL argv? I note that glibc already warns about NULL argv: argc0.c:7:3: warning: null argument where non-null required (argument 2) [-Wnonnull] 7 | execve(argv[0], NULL, envp); | ^~~~~~ in the future we could expand this to only looking at argv=NULL? -- Kees Cook
