[PATCH 3.16 24/72] Smack: Don't ignore other bprm->unsafe flags if LSM_UNSAFE_PTRACE is set

Ben Hutchings
2019-12-08T13:58:08Z

From: Jann Horn <jannh@google.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@google.com>
Signed-off-by: Casey Schaufler <casey@schaufler-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@decadent.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;