Re: [PATCH] general protection fault in sock_has_perm

[Stephen Smalley]

On Thu, 2018-01-18 at 13:58 -0800, Mark Salyzyn wrote:
> general protection fault: 0000 [#1] PREEMPT SMP KASAN
> CPU: 1 PID: 14233 Comm: syz-executor2 Not tainted 4.4.112-g5f6325b
> #28
> task: ffff8801d1095f00 task.stack: ffff8800b5950000
> RIP: 0010:[<ffffffff81b69b7e>]  [<ffffffff81b69b7e>]
> sock_has_perm+0x1fe/0x3e0 security/selinux/hooks.c:4069
> RSP: 0018:ffff8800b5957ce0  EFLAGS: 00010202
> RAX: dffffc0000000000 RBX: 1ffff10016b2af9f RCX: ffffffff81b69b51
> RDX: 0000000000000002 RSI: 0000000000000000 RDI: 0000000000000010
> RBP: ffff8800b5957de0 R08: 0000000000000001 R09: 0000000000000001
> R10: 0000000000000000 R11: 1ffff10016b2af68 R12: ffff8800b5957db8
> R13: 0000000000000000 R14: ffff8800b7259f40 R15: 00000000000000d7
> FS:  00007f72f5ae2700(0000) GS:ffff8801db300000(0000)
> knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000a2fa38 CR3: 00000001d7980000 CR4: 0000000000160670
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Stack:
>  ffffffff81b69a1f ffff8800b5957d58 00008000b5957d30 0000000041b58ab3
>  ffffffff83fc82f2 ffffffff81b69980 0000000000000246 ffff8801d1096770
>  ffff8801d3165668 ffffffff8157844b ffff8801d1095f00
>  ffff880000000001
> Call Trace:
> [<ffffffff81b6a19d>] selinux_socket_setsockopt+0x4d/0x80
> security/selinux/hooks.c:4338
> [<ffffffff81b4873d>] security_socket_setsockopt+0x7d/0xb0
> security/security.c:1257
> [<ffffffff82df1ac8>] SYSC_setsockopt net/socket.c:1757 [inline]
> [<ffffffff82df1ac8>] SyS_setsockopt+0xe8/0x250 net/socket.c:1746
> [<ffffffff83776499>] entry_SYSCALL_64_fastpath+0x16/0x92
> Code: c2 42 9b b6 81 be 01 00 00 00 48 c7 c7 a0 cb 2b 84 e8
> f7 2f 6d ff 49 8d 7d 10 48 b8 00 00 00 00 00 fc ff df 48 89
> fa 48 c1 ea 03 <0f> b6 04 02 84 c0 74 08 3c 03 0f 8e 83 01 00
> 00 41 8b 75 10 31
> RIP  [<ffffffff81b69b7e>] sock_has_perm+0x1fe/0x3e0
> security/selinux/hooks.c:4069
> RSP <ffff8800b5957ce0>
> ---[ end trace 7b5aaf788fef6174 ]---
> 
> In the absence of commit a4298e4522d6 ("net: add SOCK_RCU_FREE socket
> flag") and all the associated infrastructure changes to take
> advantage
> of a RCU grace period before freeing, there is a heightened
> possibility that a security check is performed while an ill-timed
> setsockopt call races in from user space.  It then is prudent to null
> check sk_security, and if the case, reject the permissions.
> 
> This adjustment is orthogonal to infrastructure improvements that may
> nullify the needed check, but should be added as good code hygiene.
> 
> Signed-off-by: Mark Salyzyn <salyzyn@xxxxxxxxxxx>
> Cc: Paul Moore <paul@xxxxxxxxxxxxxx>
> Cc: Stephen Smalley <sds@xxxxxxxxxxxxx>
> Cc: Eric Paris <eparis@xxxxxxxxxxxxxx>
> Cc: James Morris <james.l.morris@xxxxxxxxxx>
> Cc: "Serge E. Hallyn" <serge@xxxxxxxxxx>
> Cc: selinux@xxxxxxxxxxxxx
> Cc: linux-security-module@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx
> ---
> This patch should be applied to all stable trees (author wants
> minimum of 3.18, 4.4, 4.9 and 4.14)
> 
>  security/selinux/hooks.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 8644d864e3c1..95d7c8143373 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -4342,7 +4342,7 @@ static int sock_has_perm(struct sock *sk, u32
> perms)
>  	struct common_audit_data ad;
>  	struct lsm_network_audit net = {0,};
>  
> -	if (sksec->sid == SECINITSID_KERNEL)
> +	if (!sksec || sksec->sid == SECINITSID_KERNEL)
>  		return 0;

The patch description says "null check the sk_security, and if the
case, reject the permissions."  The patch code instead has it return
0/success, i.e. permission granted.  Which one is correct?  If we
return -EACCES, then we might break userspace; if we return 0, we might
be allowing an operation that should have been denied.  Both seem like
losing propositions.

Could we instead have selinux_sk_free_security() defer freeing of the
sock security blob to a call_rcu(), like we did for
inode_free_security, or change the caller of it to not free it until
the sock is truly freed?

>  
>  	ad.type = LSM_AUDIT_DATA_NET;