[PATCH] fuse: fix use after free issue in fuse_dev_do_read()

[Sahitya Tummala <stummala@xxxxxxxxxxxxxx>]

There is a potential race between fuse_dev_do_write()
and request_wait_answer() contexts as shown below:

TASK 1:
__fuse_request_send():
  |--spin_lock(&fiq->waitq.lock);
  |--queue_request();
  |--spin_unlock(&fiq->waitq.lock);
  |--request_wait_answer():
       |--if (test_bit(FR_SENT, &req->flags))
       <gets pre-empted after it is validated true>  TASK 2:
       						     fuse_dev_do_write():
						       |--clears bit FR_SENT,
						       |--request_end():
						          |--sets bit FR_FINISHED
							  |--spin_lock(&fiq->waitq.lock);
							  |--list_del_init(&req->intr_entry);
							  |--spin_unlock(&fiq->waitq.lock);
							  |--fuse_put_request();
       |--queue_interrupt();
       <request gets queued to interrupts list>
            |--wake_up_locked(&fiq->waitq);
       |--wait_event_freezable();
       <as FR_FINISHED is set, it returns and then
       the caller frees this request>

Now, the next fuse_dev_do_read(), see interrupts list is not empty
and then calls fuse_read_interrupt() which tries to access the request
which is already free'd and gets the below crash:

[11432.401266] Unable to handle kernel paging request at virtual address
6b6b6b6b6b6b6b6b
...
[11432.418518] Kernel BUG at ffffff80083720e0
[11432.456168] PC is at __list_del_entry+0x6c/0xc4
[11432.463573] LR is at fuse_dev_do_read+0x1ac/0x474
...
[11432.679999] [<ffffff80083720e0>] __list_del_entry+0x6c/0xc4
[11432.687794] [<ffffff80082c65e0>] fuse_dev_do_read+0x1ac/0x474
[11432.693180] [<ffffff80082c6b14>] fuse_dev_read+0x6c/0x78
[11432.699082] [<ffffff80081d5638>] __vfs_read+0xc0/0xe8
[11432.704459] [<ffffff80081d5efc>] vfs_read+0x90/0x108
[11432.709406] [<ffffff80081d67f0>] SyS_read+0x58/0x94

As FR_FINISHED bit is set before deleting the intr_entry with input
queue lock in request completion path, do the testing of this flag and
queueing atomically with the same lock in queue_interrupt().

Signed-off-by: Sahitya Tummala <stummala@xxxxxxxxxxxxxx>
---
 fs/fuse/dev.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 4e06a27..b656e18 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -399,6 +399,10 @@ static void request_end(struct fuse_conn *fc, struct fuse_req *req)
 static void queue_interrupt(struct fuse_iqueue *fiq, struct fuse_req *req)
 {
 	spin_lock(&fiq->waitq.lock);
+	if (test_bit(FR_FINISHED, &req->flags)) {
+		spin_unlock(&fiq->waitq.lock);
+		return;
+	}
 	if (list_empty(&req->intr_entry)) {
 		list_add_tail(&req->intr_entry, &fiq->interrupts);
 		wake_up_locked(&fiq->waitq);
-- 
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