linux-kernel - [PATCH v2 3/8] exec: Use secureexec for setting dumpability

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Date:   Mon, 10 Jul 2017 00:57:26 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...nel.org>,
        David Howells <dhowells@...hat.com>,
        Serge Hallyn <serge@...lyn.com>,
        John Johansen <john.johansen@...onical.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Michal Hocko <mhocko@...nel.org>,
        Ben Hutchings <ben@...adent.org.uk>,
        Hugh Dickins <hughd@...gle.com>,
        Oleg Nesterov <oleg@...hat.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...hat.com>,
        James Morris <james.l.morris@...cle.com>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        Ingo Molnar <mingo@...nel.org>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Paul Moore <paul@...l-moore.com>,
        Vivek Goyal <vgoyal@...hat.com>,
        Mickaël Salaün <mic@...ikod.net>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org, selinux@...ho.nsa.gov
Subject: [PATCH v2 3/8] exec: Use secureexec for setting dumpability

The examination of "current" to decide dumpability is wrong. This was a
check of and euid/uid (or egid/gid) mismatch in the existing process,
not the newly created one. This appears to stretch back into even the
"history.git" tree. Luckily, dumpability is later set in commit_creds().
In earlier kernel versions before creds existed, similar checks also
existed late in the exec flow, covering up the mistake as far back as I
could find.

The commit_creds() check examines differences of euid, uid, egid, gid,
and capabilities between the old and new creds. It would look like
the setup_new_exec() dumpability test could be entirely removed, but
strictly speaking, the secureexec test covers a different set of tests
than what commit_creds() checks for. So, fix this test to use secureexec,
which includes the same logical check (euid != uid || egid != gid),
but checks bprm->cred, not current->cred.

One would wonder if we need a security_commit_creds() LSM hook and to
move the existing checks in commit_creds() into commoncaps.c, which
would allow expanding the logic to all LSMs. Currently this doesn't
seem needed, though.

Signed-off-by: Kees Cook <keescook@...omium.org>
---
 fs/exec.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/exec.c b/fs/exec.c
index b92e37fb53aa..3e519d4f0bd3 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1346,7 +1346,7 @@ void setup_new_exec(struct linux_binprm * bprm)

 	current->sas_ss_sp = current->sas_ss_size = 0;

-	if (uid_eq(current_euid(), current_uid()) && gid_eq(current_egid(), current_gid()))
+	if (!bprm->secureexec)
 		set_dumpable(current->mm, SUID_DUMP_USER);
 	else
 		set_dumpable(current->mm, suid_dumpable);
-- 
2.7.4