linux-kernel - [PATCH 3.16 24/72] Smack: Don't ignore other bprm->unsafe flags if LSM_UNSAFE

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Message-ID: <lsq.1575813165.387139124@decadent.org.uk>
Date:   Sun, 08 Dec 2019 13:53:08 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Casey Schaufler" <casey@...aufler-ca.com>,
        "Jann Horn" <jannh@...gle.com>
Subject: [PATCH 3.16 24/72] Smack: Don't ignore other bprm->unsafe flags
 if LSM_UNSAFE_PTRACE is set

3.16.79-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Jann Horn <jannh@...gle.com>

commit 3675f052b43ba51b99b85b073c7070e083f3e6fb upstream.

There is a logic bug in the current smack_bprm_set_creds():
If LSM_UNSAFE_PTRACE is set, but the ptrace state is deemed to be
acceptable (e.g. because the ptracer detached in the meantime), the other
->unsafe flags aren't checked. As far as I can tell, this means that
something like the following could work (but I haven't tested it):

 - task A: create task B with fork()
 - task B: set NO_NEW_PRIVS
 - task B: install a seccomp filter that makes open() return 0 under some
   conditions
 - task B: replace fd 0 with a malicious library
 - task A: attach to task B with PTRACE_ATTACH
 - task B: execve() a file with an SMACK64EXEC extended attribute
 - task A: while task B is still in the middle of execve(), exit (which
   destroys the ptrace relationship)

Make sure that if any flags other than LSM_UNSAFE_PTRACE are set in
bprm->unsafe, we reject the execve().

Fixes: 5663884caab1 ("Smack: unify all ptrace accesses in the smack")
Signed-off-by: Jann Horn <jannh@...gle.com>
Signed-off-by: Casey Schaufler <casey@...aufler-ca.com>
[bwh: Backported to 3.16: Ignore LSM_UNSAFE_PTRACE_CAP, which is also handled
 by the preceding if-statement.]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 security/smack/smack_lsm.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -553,7 +553,8 @@ static int smack_bprm_set_creds(struct l
 
 		if (rc != 0)
 			return rc;
-	} else if (bprm->unsafe)
+	}
+	if (bprm->unsafe & ~(LSM_UNSAFE_PTRACE | LSM_UNSAFE_PTRACE_CAP))
 		return -EPERM;
 
 	bsp->smk_task = isp->smk_task;