[PATCH] apparmor: Fix null pointer deref when receiving skb during sock creation

[Xiao Liang]

The panic below is observed when receiving ICMP packets with secmark set
while an ICMP raw socket is being created. SK_CTX(sk)->label is updated
in apparmor_socket_post_create(), but the packet is delivered to the
socket before that, causing the null pointer dereference.
Drop the packet if label context is not set.

    BUG: kernel NULL pointer dereference, address: 000000000000004c
    #PF: supervisor read access in kernel mode
    #PF: error_code(0x0000) - not-present page
    PGD 0 P4D 0
    Oops: 0000 [#1] PREEMPT SMP NOPTI
    CPU: 0 PID: 407 Comm: a.out Not tainted 6.4.12-arch1-1 #1 3e6fa2753a2d75925c34ecb78e22e85a65d083df
    Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 05/28/2020
    RIP: 0010:aa_label_next_confined+0xb/0x40
    Code: 00 00 48 89 ef e8 d5 25 0c 00 e9 66 ff ff ff 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 66 0f 1f 00 0f 1f 44 00 00 89 f0 <8b> 77 4c 39 c6 7e 1f 48 63 d0 48 8d 14 d7 eb 0b 83 c0 01 48 83 c2
    RSP: 0018:ffffa92940003b08 EFLAGS: 00010246
    RAX: 0000000000000000 RBX: 0000000000000000 RCX: 000000000000000e
    RDX: ffffa92940003be8 RSI: 0000000000000000 RDI: 0000000000000000
    RBP: ffff8b57471e7800 R08: ffff8b574c642400 R09: 0000000000000002
    R10: ffffffffbd820eeb R11: ffffffffbeb7ff00 R12: ffff8b574c642400
    R13: 0000000000000001 R14: 0000000000000001 R15: 0000000000000000
    FS:  00007fb092ea7640(0000) GS:ffff8b577bc00000(0000) knlGS:0000000000000000
    CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    CR2: 000000000000004c CR3: 00000001020f2005 CR4: 00000000007706f0
    PKRU: 55555554
    Call Trace:
     <IRQ>
     ? __die+0x23/0x70
     ? page_fault_oops+0x171/0x4e0
     ? exc_page_fault+0x7f/0x180
     ? asm_exc_page_fault+0x26/0x30
     ? aa_label_next_confined+0xb/0x40
     apparmor_secmark_check+0xec/0x330
     security_sock_rcv_skb+0x35/0x50
     sk_filter_trim_cap+0x47/0x250
     sock_queue_rcv_skb_reason+0x20/0x60
     raw_rcv+0x13c/0x210
     raw_local_deliver+0x1f3/0x250
     ip_protocol_deliver_rcu+0x4f/0x2f0
     ip_local_deliver_finish+0x76/0xa0
     __netif_receive_skb_one_core+0x89/0xa0
     netif_receive_skb+0x119/0x170
     ? __netdev_alloc_skb+0x3d/0x140
     vmxnet3_rq_rx_complete+0xb23/0x1010 [vmxnet3 56a84f9c97178c57a43a24ec073b45a9d6f01f3a]
     vmxnet3_poll_rx_only+0x36/0xb0 [vmxnet3 56a84f9c97178c57a43a24ec073b45a9d6f01f3a]
     __napi_poll+0x28/0x1b0
     net_rx_action+0x2a4/0x380
     __do_softirq+0xd1/0x2c8
     __irq_exit_rcu+0xbb/0xf0
     common_interrupt+0x86/0xa0
     </IRQ>
     <TASK>
     asm_common_interrupt+0x26/0x40
    RIP: 0010:apparmor_socket_post_create+0xb/0x200
    Code: 08 48 85 ff 75 a1 eb b1 0f 1f 80 00 00 00 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa 0f 1f 44 00 00 41 54 <55> 48 89 fd 53 45 85 c0 0f 84 b2 00 00 00 48 8b 1d 80 56 3f 02 48
    RSP: 0018:ffffa92940ce7e50 EFLAGS: 00000286
    RAX: ffffffffbc756440 RBX: 0000000000000000 RCX: 0000000000000001
    RDX: 0000000000000003 RSI: 0000000000000002 RDI: ffff8b574eaab740
    RBP: 0000000000000001 R08: 0000000000000000 R09: 0000000000000000
    R10: ffff8b57444cec70 R11: 0000000000000000 R12: 0000000000000003
    R13: 0000000000000002 R14: ffff8b574eaab740 R15: ffffffffbd8e4748
     ? __pfx_apparmor_socket_post_create+0x10/0x10
     security_socket_post_create+0x4b/0x80
     __sock_create+0x176/0x1f0
     __sys_socket+0x89/0x100
     __x64_sys_socket+0x17/0x20
     do_syscall_64+0x5d/0x90
     ? do_syscall_64+0x6c/0x90
     ? do_syscall_64+0x6c/0x90
     ? do_syscall_64+0x6c/0x90
     entry_SYSCALL_64_after_hwframe+0x72/0xdc

Fixes: ab9f2115081a ("apparmor: Allow filtering based on secmark policy")
Signed-off-by: Xiao Liang <shaw.leon at gmail.com>
---
 security/apparmor/lsm.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
index 108eccc5ada5..0dbff677ac2e 100644
--- a/security/apparmor/lsm.c
+++ b/security/apparmor/lsm.c
@@ -1097,6 +1097,13 @@ static int apparmor_socket_sock_rcv_skb(struct sock *sk, struct sk_buff *skb)
 	if (!skb->secmark)
 		return 0;
 
+	/*
+	 * If reach here before socket_post_create hook is called, in which
+	 * case label is null, drop the packet.
+	 */
+	if (!ctx->label)
+		return -EACCES;
+
 	return apparmor_secmark_check(ctx->label, OP_RECVMSG, AA_MAY_RECEIVE,
 				      skb->secmark, sk);
 }
-- 
2.42.0