Re: [RFC][PATCH] exec: Use init rlimits for setuid exec

[Kees Cook <keescook@chromium.org>]

On Thu, Jul 6, 2017 at 10:49 PM, Kees Cook <keescook@chromium.org> wrote:
> On Thu, Jul 6, 2017 at 10:39 PM, Linus Torvalds
>> And I think the credentials switch (which is the point of no return
>> anyway) happens before we start mmap'ing the executable etc. We used
>> to have some odd code there and do it in the completely wrong order
>> (checking that the binary was executable for the *old* user, which
>> makes no sense, iirc)
>
> Yeah, it all happens in setup_new_exec(). The first thing is layout
> selection, then switching credentials. It could be made to take a hint
> from GNU_STACK (which was parsed before setup_new_exec() is called),
> check security_bprm_secureexec() and then make the rlimit changes, all
> before the layout selection.

At Andy's suggestion I'm using security_bprm_secureexec() to test for
setuid-ness. However, this seems to expect the credentials to have
already been installed. And yet ... the following patch still works
correctly when I call it "early". I'm going to look again in the
morning.

diff --git a/fs/exec.c b/fs/exec.c
index b60804216b59..a4d2433a44ec 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1334,9 +1334,20 @@ EXPORT_SYMBOL(would_dump);

 void setup_new_exec(struct linux_binprm * bprm)
 {
+       /* This is the point of no return */
+
+       /*
+        * If this is a setuid execution, reset the stack limit to
+        * a sane default to avoid bad behavior from the prior rlimits.
+        */
+       if (security_bprm_secureexec(bprm)) {
+               struct rlimit default_stack = { _STK_LIM, RLIM_INFINITY };
+
+               current->signal->rlim[RLIMIT_STACK] = default_stack;
+       }
+
        arch_pick_mmap_layout(current->mm);

-       /* This is the point of no return */
        current->sas_ss_sp = current->sas_ss_size = 0;

        if (uid_eq(current_euid(), current_uid()) &&
gid_eq(current_egid(), current_gid()))



-- 
Kees Cook
Pixel Security