Messages in this thread | | | Date | Mon, 27 Nov 2023 11:38:16 +0100 | Subject | Re: [PATCH 2/4] nfc: Protect access to nfc_dev in an llcp_sock with a rwlock | From | Krzysztof Kozlowski <> |
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On 25/11/2023 21:26, Siddh Raman Pant wrote: > llcp_sock_sendmsg() calls nfc_llcp_send_ui_frame(), which accesses the > nfc_dev from the llcp_sock for getting the headroom and tailroom needed > for skb allocation.
This path should have reference to nfc device: nfc_get_device(). Why is this not sufficient?
> > Parallely, the nfc_dev can be freed via the nfc_unregister_device() > codepath (nfc_release() being called due to the class unregister in > nfc_exit()), leading to the UAF reported by Syzkaller. > > We have the following call tree before freeing: > > nfc_unregister_device() > -> nfc_llcp_unregister_device() > -> local_cleanup() > -> nfc_llcp_socket_release() > > nfc_llcp_socket_release() sets the state of sockets to LLCP_CLOSED, > and this is encountered necessarily before any freeing of nfc_dev.
Sorry, I don't understand. What is encountered before freeing?
> > Thus, add a rwlock in struct llcp_sock to synchronize access to > nfc_dev. nfc_dev in an llcp_sock will be NULLed in a write critical > section when socket state has been set to closed. Thus, we can avoid > the UAF by bailing out from a read critical section upon seeing NULL. > > Since this is repeated multiple times in nfc_llcp_socket_release(), > extract the behaviour into a new function. > > Reported-and-tested-by: syzbot+bbe84a4010eeea00982d@syzkaller.appspotmail.com > Closes: https://syzkaller.appspot.com/bug?extid=bbe84a4010eeea00982d > Signed-off-by: Siddh Raman Pant <code@siddh.me> > --- > net/nfc/llcp.h | 1 + > net/nfc/llcp_commands.c | 27 ++++++++++++++++++++++++--- > net/nfc/llcp_core.c | 31 +++++++++++++++++++------------ > net/nfc/llcp_sock.c | 2 ++ > 4 files changed, 46 insertions(+), 15 deletions(-) > > diff --git a/net/nfc/llcp.h b/net/nfc/llcp.h > index d8345ed57c95..800cbe8e3d6b 100644 > --- a/net/nfc/llcp.h > +++ b/net/nfc/llcp.h > @@ -102,6 +102,7 @@ struct nfc_llcp_local { > struct nfc_llcp_sock { > struct sock sk; > struct nfc_dev *dev; > + rwlock_t rw_dev_lock;
I dislike the idea of introducing the third (!!!) lock here. It looks like a bandaid for this one particular problem.
> struct nfc_llcp_local *local; > u32 target_idx; > u32 nfc_protocol; > diff --git a/net/nfc/llcp_commands.c b/net/nfc/llcp_commands.c > index 39c7c59bbf66..b132830bc206 100644 > --- a/net/nfc/llcp_commands.c > +++ b/net/nfc/llcp_commands.c > @@ -315,13 +315,24 @@ static struct sk_buff *llcp_allocate_pdu(struct nfc_llcp_sock *sock, > { > struct sk_buff *skb; > int err, headroom, tailroom; > + unsigned long irq_flags; > > if (sock->ssap == 0) > return NULL; > > + read_lock_irqsave(&sock->rw_dev_lock, irq_flags); > + > + if (!sock->dev) { > + read_unlock_irqrestore(&sock->rw_dev_lock, irq_flags); > + pr_err("NFC device does not exit\n");
exist?
Best regards, Krzysztof
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