4.9-stable review patch. If anyone has any objections, please let me know. ------------------ From: Sahitya Tummala <stummala@xxxxxxxxxxxxxx> commit 6ba4d2722d06960102c981322035239cd66f7316 upstream. 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> Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx> Fixes: fd22d62ed0c3 ("fuse: no fc->lock for iqueue parts") Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- fs/fuse/dev.c | 4 ++++ 1 file changed, 4 insertions(+) --- a/fs/fuse/dev.c +++ b/fs/fuse/dev.c @@ -399,6 +399,10 @@ static void request_end(struct fuse_conn 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);