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> > > 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 > -- Kees Cook
