On Wed, Jun 01, 2022 at 12:12:26PM +0200, Vegard Nossum wrote: > From: Kees Cook <keescook@xxxxxxxxxxxx> > > commit dcd46d897adb70d63e025f175a00a89797d31a43 upstream. > > 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> > Acked-by: Christian Brauner <brauner@xxxxxxxxxx> > Acked-by: Ariadne Conill <ariadne@xxxxxxxxxxxxxxxx> > Acked-by: Andy Lutomirski <luto@xxxxxxxxxx> > Link: https://lore.kernel.org/r/20220201000947.2453721-1-keescook@xxxxxxxxxxxx > [vegard: fixed conflicts due to missing > 886d7de631da71e30909980fdbf318f7caade262^- and > 3950e975431bc914f7e81b8f2a2dbdf2064acb0f^- and > 655c16a8ce9c15842547f40ce23fd148aeccc074] > Signed-off-by: Vegard Nossum <vegard.nossum@xxxxxxxxxx> > --- > fs/exec.c | 17 +++++++++++++++++ > 1 file changed, 17 insertions(+) > > This has been tested in both argc == 0 and argc >= 1 cases, but I would > still appreciate a review given the differences with mainline. If it's > considered too risky I'm also fine with dropping it -- just wanted to > make sure this didn't fall through the cracks, as it does block a real > (albeit old by now) exploit. > > diff --git a/fs/exec.c b/fs/exec.c > index 482a8b4f41a5b..19f8b075d3b6b 100644 > --- a/fs/exec.c > +++ b/fs/exec.c > @@ -1758,6 +1758,9 @@ static int do_execveat_common(int fd, struct filename *filename, > goto out_unmark; > > bprm->argc = count(argv, MAX_ARG_STRINGS); > + if (bprm->argc == 0) > + pr_warn_once("process '%s' launched '%s' with NULL argv: empty string added\n", > + current->comm, bprm->filename); > if ((retval = bprm->argc) < 0) > goto out; > > @@ -1782,6 +1785,20 @@ static int do_execveat_common(int fd, struct filename *filename, > if (retval < 0) > goto out; > > + /* > + * When argv is empty, add an empty string ("") as argv[0] to > + * ensure confused userspace programs that start processing > + * from argv[1] won't end up walking envp. See also > + * bprm_stack_limits(). > + */ > + if (bprm->argc == 0) { > + const char *argv[] = { "", NULL }; > + retval = copy_strings_kernel(1, argv, bprm); > + if (retval < 0) > + goto out; > + bprm->argc = 1; > + } > + > retval = exec_binprm(bprm); > if (retval < 0) > goto out; > -- > 2.35.1.46.g38062e73e0 > All now queued up, thanks. greg k-h
