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

From Kees Cook <keescook@chromium.org>
Newsgroups linux.kernel
Subject Re: [RFC][PATCH] exec: Use init rlimits for setuid exec
Date 2017-07-07 08:50 +0200
Message-ID <u0vfr-5W-3@gated-at.bofh.it> (permalink)
References (5 earlier) <u0tnj-7hv-1@gated-at.bofh.it> <u0tQl-7Je-3@gated-at.bofh.it> <u0tQl-7Je-1@gated-at.bofh.it> <u0u9H-7Qw-3@gated-at.bofh.it> <u0ujn-7Uj-7@gated-at.bofh.it>
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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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Thread

[RFC][PATCH] exec: Use init rlimits for setuid exec Kees Cook <keescook@chromium.org> - 2017-07-06 06:40 +0200
  Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Andy Lutomirski <luto@kernel.org> - 2017-07-06 07:00 +0200
    Re: [RFC][PATCH] exec: Use init rlimits for setuid exec ebiederm@xmission.com (Eric W. Biederman) - 2017-07-06 15:00 +0200
      Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Andy Lutomirski <luto@kernel.org> - 2017-07-06 17:30 +0200
  Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Willy Tarreau <w@1wt.eu> - 2017-07-06 07:50 +0200
  Re: [RFC][PATCH] exec: Use init rlimits for setuid exec ebiederm@xmission.com (Eric W. Biederman) - 2017-07-06 14:50 +0200
    Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Andy Lutomirski <luto@kernel.org> - 2017-07-06 17:40 +0200
  Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Linus Torvalds <torvalds@linux-foundation.org> - 2017-07-06 18:40 +0200
    Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Linus Torvalds <torvalds@linux-foundation.org> - 2017-07-06 19:00 +0200
    Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Kees Cook <keescook@chromium.org> - 2017-07-06 19:30 +0200
      Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Linus Torvalds <torvalds@linux-foundation.org> - 2017-07-06 20:00 +0200
        Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Kees Cook <keescook@chromium.org> - 2017-07-06 21:20 +0200
          Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Andy Lutomirski <luto@kernel.org> - 2017-07-07 06:50 +0200
            Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Linus Torvalds <torvalds@linux-foundation.org> - 2017-07-07 07:10 +0200
            Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Kees Cook <keescook@chromium.org> - 2017-07-07 07:20 +0200
              Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Linus Torvalds <torvalds@linux-foundation.org> - 2017-07-07 07:40 +0200
                Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Kees Cook <keescook@chromium.org> - 2017-07-07 07:50 +0200
                Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Kees Cook <keescook@chromium.org> - 2017-07-07 08:50 +0200
                Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Linus Torvalds <torvalds@linux-foundation.org> - 2017-07-07 18:30 +0200
                Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Kees Cook <keescook@chromium.org> - 2017-07-07 20:30 +0200
              Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Andy Lutomirski <luto@kernel.org> - 2017-07-07 07:40 +0200
                Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Kees Cook <keescook@chromium.org> - 2017-07-07 07:50 +0200
                Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Linus Torvalds <torvalds@linux-foundation.org> - 2017-07-07 08:10 +0200
                Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Kees Cook <keescook@chromium.org> - 2017-07-07 08:20 +0200
                Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Linus Torvalds <torvalds@linux-foundation.org> - 2017-07-07 18:10 +0200
                Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Kees Cook <keescook@chromium.org> - 2017-07-07 20:30 +0200
                Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Andy Lutomirski <luto@kernel.org> - 2017-07-07 17:00 +0200
            Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Kees Cook <keescook@chromium.org> - 2017-07-07 07:20 +0200
          Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Michal Hocko <mhocko@kernel.org> - 2017-07-10 10:50 +0200
            Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Linus Torvalds <torvalds@linux-foundation.org> - 2017-07-10 18:20 +0200
              Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Willy Tarreau <w@1wt.eu> - 2017-07-10 19:00 +0200
            Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Kees Cook <keescook@chromium.org> - 2017-07-10 18:20 +0200
              Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Michal Hocko <mhocko@kernel.org> - 2017-07-10 18:30 +0200
                Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Michal Hocko <mhocko@kernel.org> - 2017-07-10 20:20 +0200
                Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Rik van Riel <riel@redhat.com> - 2017-07-10 20:40 +0200
    Re: [RFC][PATCH] exec: Use init rlimits for setuid exec Alan Cox <gnomes@lxorguk.ukuu.org.uk> - 2017-07-13 02:00 +0200

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