From: Kees Cook keescook@chromium.org Subject: exec: avoid RLIMIT_STACK races with prlimit()

While the defense-in-depth RLIMIT_STACK limit on setuid processes was protected against races from other threads calling setrlimit(), I missed protecting it against races from external processes calling prlimit(). This adds locking around the change and makes sure that rlim_max is set too.

Link: http://lkml.kernel.org/r/20171127193457.GA11348@beast Fixes: 64701dee4178e ("exec: Use sane stack rlimit under secureexec") Signed-off-by: Kees Cook keescook@chromium.org Reported-by: Ben Hutchings ben.hutchings@codethink.co.uk Reported-by: Brad Spengler spender@grsecurity.net Acked-by: Serge Hallyn serge@hallyn.com Cc: James Morris james.l.morris@oracle.com Cc: Andy Lutomirski luto@kernel.org Cc: Oleg Nesterov oleg@redhat.com Cc: Jiri Slaby jslaby@suse.cz Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

fs/exec.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-)

diff -puN fs/exec.c~exec-avoid-rlimit_stack-races-with-prlimit fs/exec.c --- a/fs/exec.c~exec-avoid-rlimit_stack-races-with-prlimit +++ a/fs/exec.c @@ -1340,10 +1340,15 @@ void setup_new_exec(struct linux_binprm * avoid bad behavior from the prior rlimits. This has to * happen before arch_pick_mmap_layout(), which examines * RLIMIT_STACK, but after the point of no return to avoid - * needing to clean up the change on failure. + * races from other threads changing the limits. This also + * must be protected from races with prlimit() calls. */ + task_lock(current->group_leader); if (current->signal->rlim[RLIMIT_STACK].rlim_cur > _STK_LIM) current->signal->rlim[RLIMIT_STACK].rlim_cur = _STK_LIM; + if (current->signal->rlim[RLIMIT_STACK].rlim_max > _STK_LIM) + current->signal->rlim[RLIMIT_STACK].rlim_max = _STK_LIM; + task_unlock(current->group_leader); }

arch_pick_mmap_layout(current->mm); _