linux-kernel - [PATCH v5 14/15] exec: Use sane stack rlimit under secureexec

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Date:   Tue,  1 Aug 2017 12:16:37 -0700
From:   Kees Cook <keescook@...omium.org>
To:     linux-kernel@...r.kernel.org
Cc:     Kees Cook <keescook@...omium.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        James Morris <james.l.morris@...cle.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Andy Lutomirski <luto@...nel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        John Johansen <john.johansen@...onical.com>,
        Paul Moore <paul@...l-moore.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        David Howells <dhowells@...hat.com>,
        linux-fsdevel@...r.kernel.org,
        linux-security-module@...r.kernel.org
Subject: [PATCH v5 14/15] exec: Use sane stack rlimit under secureexec

For a secureexec, before memory layout selection has happened, reset the
stack rlimit to something sane to avoid the caller having control over
the resulting layouts.

$ ulimit -s
8192
$ ulimit -s unlimited
$ /bin/sh -c 'ulimit -s'
unlimited
$ sudo /bin/sh -c 'ulimit -s'
8192

Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Kees Cook <keescook@...omium.org>
Reviewed-by: James Morris <james.l.morris@...cle.com>
Acked-by: Serge Hallyn <serge@...lyn.com>
---
 fs/exec.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/fs/exec.c b/fs/exec.c
index 3006c1c24304..3235cbd85efa 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1350,6 +1350,18 @@ void setup_new_exec(struct linux_binprm * bprm)
 	 */
 	bprm->secureexec |= bprm->cap_elevated;
 
+	if (bprm->secureexec) {
+		/*
+		 * For secureexec, reset the stack limit to sane default to
+		 * 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.
+		 */
+		if (current->signal->rlim[RLIMIT_STACK].rlim_cur > _STK_LIM)
+			current->signal->rlim[RLIMIT_STACK].rlim_cur = _STK_LIM;
+	}
+
 	arch_pick_mmap_layout(current->mm);
 
 	current->sas_ss_sp = current->sas_ss_size = 0;
-- 
2.7.4