Messages in this thread Patch in this message | | | From | Kees Cook <> | Date | Thu, 6 Jul 2017 23:40:30 -0700 | Subject | Re: [RFC][PATCH] exec: Use init rlimits for setuid exec |
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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
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