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Date: Tue, 25 Feb 2014 16:52:26 +0200
From: Johan Hedberg <johan.hedberg@gmail.com>
To: Andrzej Kaczmarek <andrzej.kaczmarek@tieto.com>
Cc: linux-bluetooth@vger.kernel.org
Subject: Re: [PATCH] Bluetooth: Fix NULL pointer dereference when sending data
Message-ID: <20140225145226.GA13119@localhost.P-661HNU-F1>
References: <1393323371-13232-1-git-send-email-andrzej.kaczmarek@tieto.com>
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Hi Andrzej,

On Tue, Feb 25, 2014, Andrzej Kaczmarek wrote:
> When trying to allocate skb for new PDU, l2cap_chan is unlocked so we
> can sleep waiting for memory as otherwise there's possible deadlock as
> fixed in e454c84464. However, in a6a5568c03 lock was moved from socket
> to channel level and it's no longer safe to just unlock and lock again
> without checking l2cap_chan state since channel can be disconnected
> when lock is not held.
> 
> This patch adds missing checks for l2cap_chan state when returning from
> call which allocates skb.
> 
> Scenario is easily reproducible by running rfcomm-tester in a loop.
> 
> BUG: unable to handle kernel NULL pointer dereference at         (null)
> IP: [<ffffffffa0442169>] l2cap_do_send+0x29/0x120 [bluetooth]
> PGD 0
> Oops: 0000 [#1] SMP
> Modules linked in:
> CPU: 7 PID: 4038 Comm: krfcommd Not tainted 3.14.0-rc2+ #15
> Hardware name: Dell Inc. OptiPlex 790/0HY9JP, BIOS A10 11/24/2011
> task: ffff8802bdd731c0 ti: ffff8801ec986000 task.ti: ffff8801ec986000
> RIP: 0010:[<ffffffffa0442169>]  [<ffffffffa0442169>] l2cap_do_send+0x29/0x120
> RSP: 0018:ffff8801ec987ad8  EFLAGS: 00010202
> RAX: 0000000000000000 RBX: ffff8800c5796800 RCX: 0000000000000000
> RDX: ffff880410e7a800 RSI: ffff8802b6c1da00 RDI: ffff8800c5796800
> RBP: ffff8801ec987af8 R08: 00000000000000c0 R09: 0000000000000300
> R10: 000000000000573b R11: 000000000000573a R12: ffff8802b6c1da00
> R13: 0000000000000000 R14: ffff8802b6c1da00 R15: 0000000000000000
> FS:  0000000000000000(0000) GS:ffff88042dce0000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000000 CR3: 000000041257c000 CR4: 00000000000407e0
> Stack:
>  ffff8801ec987d78 ffff8800c5796800 ffff8801ec987d78 0000000000000000
>  ffff8801ec987ba8 ffffffffa0449e37 0000000000000004 ffff8801ec987af0
>  ffff8801ec987d40 0000000000000282 0000000000000000 ffffffff00000004
> Call Trace:
>  [<ffffffffa0449e37>] l2cap_chan_send+0xaa7/0x1120 [bluetooth]
>  [<ffffffff81770100>] ? _raw_spin_unlock_bh+0x20/0x40
>  [<ffffffffa045188b>] l2cap_sock_sendmsg+0xcb/0x110 [bluetooth]
>  [<ffffffff81652b0f>] sock_sendmsg+0xaf/0xc0
>  [<ffffffff810a8381>] ? update_curr+0x141/0x200
>  [<ffffffff810a8961>] ? dequeue_entity+0x181/0x520
>  [<ffffffff81652b60>] kernel_sendmsg+0x40/0x60
>  [<ffffffffa04a8505>] rfcomm_send_frame+0x45/0x70 [rfcomm]
>  [<ffffffff810766f0>] ? internal_add_timer+0x20/0x50
>  [<ffffffffa04a8564>] rfcomm_send_cmd+0x34/0x60 [rfcomm]
>  [<ffffffffa04a8605>] rfcomm_send_disc+0x75/0xa0 [rfcomm]
>  [<ffffffffa04aacec>] rfcomm_run+0x8cc/0x1a30 [rfcomm]
>  [<ffffffffa04aa420>] ? rfcomm_check_accept+0xc0/0xc0 [rfcomm]
>  [<ffffffff8108e3a9>] kthread+0xc9/0xe0
>  [<ffffffff8108e2e0>] ? flush_kthread_worker+0xb0/0xb0
>  [<ffffffff817795fc>] ret_from_fork+0x7c/0xb0
>  [<ffffffff8108e2e0>] ? flush_kthread_worker+0xb0/0xb0
> Code: 00 00 66 66 66 66 90 55 48 89 e5 48 83 ec 20 f6 05 d6 a3 02 00 04
> RIP  [<ffffffffa0442169>] l2cap_do_send+0x29/0x120 [bluetooth]
>  RSP <ffff8801ec987ad8>
> CR2: 0000000000000000
> 
> Signed-off-by: Andrzej Kaczmarek <andrzej.kaczmarek@tieto.com>
> ---
>  net/bluetooth/l2cap_core.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
> index 6ace116..cfc7c69 100644
> --- a/net/bluetooth/l2cap_core.c
> +++ b/net/bluetooth/l2cap_core.c
> @@ -2434,6 +2434,11 @@ int l2cap_chan_send(struct l2cap_chan *chan, struct msghdr *msg, size_t len,
>  		if (IS_ERR(skb))
>  			return PTR_ERR(skb);
>  
> +		if (chan->state != BT_CONNECTED) {
> +			kfree_skb(skb);
> +			return -ENOTCONN;
> +		}
> +
>  		l2cap_do_send(chan, skb);
>  		return len;
>  	}
> @@ -2483,6 +2488,11 @@ int l2cap_chan_send(struct l2cap_chan *chan, struct msghdr *msg, size_t len,
>  		if (IS_ERR(skb))
>  			return PTR_ERR(skb);
>  
> +		if (chan->state != BT_CONNECTED) {
> +			kfree_skb(skb);
> +			return -ENOTCONN;
> +		}
> +
>  		l2cap_do_send(chan, skb);
>  		err = len;
>  		break;

Acked-by: Johan Hedberg <johan.hedberg@intel.com>

However, I'd consider adding code comment explaining the lock was
released and reacquired and the state needs to therefore be rechecked.

Johan