5.4-stable review patch. If anyone has any objections, please let me know.
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From: David Howells dhowells@redhat.com
[ Upstream commit 52bf9f6c09fca8c74388cd41cc24e5d1bff812a9 ]
If an AFS cell that has an unreachable (eg. ENETUNREACH) server listed (VL server or fileserver), an asynchronous probe to one of its addresses may fail immediately because sendmsg() returns an error. When this happens, a refcount underflow can happen if certain events hit a very small window.
The way this occurs is:
(1) There are two levels of "call" object, the afs_call and the rxrpc_call. Each of them can be transitioned to a "completed" state in the event of success or failure.
(2) Asynchronous afs_calls are self-referential whilst they are active to prevent them from evaporating when they're not being processed. This reference is disposed of when the afs_call is completed.
Note that an afs_call may only be completed once; once completed completing it again will do nothing.
(3) When a call transmission is made, the app-side rxrpc code queues a Tx buffer for the rxrpc I/O thread to transmit. The I/O thread invokes sendmsg() to transmit it - and in the case of failure, it transitions the rxrpc_call to the completed state.
(4) When an rxrpc_call is completed, the app layer is notified. In this case, the app is kafs and it schedules a work item to process events pertaining to an afs_call.
(5) When the afs_call event processor is run, it goes down through the RPC-specific handler to afs_extract_data() to retrieve data from rxrpc - and, in this case, it picks up the error from the rxrpc_call and returns it.
The error is then propagated to the afs_call and that is completed too. At this point the self-reference is released.
(6) If the rxrpc I/O thread manages to complete the rxrpc_call within the window between rxrpc_send_data() queuing the request packet and checking for call completion on the way out, then rxrpc_kernel_send_data() will return the error from sendmsg() to the app.
(7) Then afs_make_call() will see an error and will jump to the error handling path which will attempt to clean up the afs_call.
(8) The problem comes when the error handling path in afs_make_call() tries to unconditionally drop an async afs_call's self-reference. This self-reference, however, may already have been dropped by afs_extract_data() completing the afs_call
(9) The refcount underflows when we return to afs_do_probe_vlserver() and that tries to drop its reference on the afs_call.
Fix this by making afs_make_call() attempt to complete the afs_call rather than unconditionally putting it. That way, if afs_extract_data() manages to complete the call first, afs_make_call() won't do anything.
The bug can be forced by making do_udp_sendmsg() return -ENETUNREACH and sticking an msleep() in rxrpc_send_data() after the 'success:' label to widen the race window.
The error message looks something like:
refcount_t: underflow; use-after-free. WARNING: CPU: 3 PID: 720 at lib/refcount.c:28 refcount_warn_saturate+0xba/0x110 ... RIP: 0010:refcount_warn_saturate+0xba/0x110 ... afs_put_call+0x1dc/0x1f0 [kafs] afs_fs_get_capabilities+0x8b/0xe0 [kafs] afs_fs_probe_fileserver+0x188/0x1e0 [kafs] afs_lookup_server+0x3bf/0x3f0 [kafs] afs_alloc_server_list+0x130/0x2e0 [kafs] afs_create_volume+0x162/0x400 [kafs] afs_get_tree+0x266/0x410 [kafs] vfs_get_tree+0x25/0xc0 fc_mount+0xe/0x40 afs_d_automount+0x1b3/0x390 [kafs] __traverse_mounts+0x8f/0x210 step_into+0x340/0x760 path_openat+0x13a/0x1260 do_filp_open+0xaf/0x160 do_sys_openat2+0xaf/0x170
or something like:
refcount_t: underflow; use-after-free. ... RIP: 0010:refcount_warn_saturate+0x99/0xda ... afs_put_call+0x4a/0x175 afs_send_vl_probes+0x108/0x172 afs_select_vlserver+0xd6/0x311 afs_do_cell_detect_alias+0x5e/0x1e9 afs_cell_detect_alias+0x44/0x92 afs_validate_fc+0x9d/0x134 afs_get_tree+0x20/0x2e6 vfs_get_tree+0x1d/0xc9 fc_mount+0xe/0x33 afs_d_automount+0x48/0x9d __traverse_mounts+0xe0/0x166 step_into+0x140/0x274 open_last_lookups+0x1c1/0x1df path_openat+0x138/0x1c3 do_filp_open+0x55/0xb4 do_sys_openat2+0x6c/0xb6
Fixes: 34fa47612bfe ("afs: Fix race in async call refcounting") Reported-by: Bill MacAllister bill@ca-zephyr.org Closes: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1052304 Suggested-by: Jeffrey E Altman jaltman@auristor.com Signed-off-by: David Howells dhowells@redhat.com Reviewed-by: Jeffrey Altman jaltman@auristor.com cc: Marc Dionne marc.dionne@auristor.com cc: linux-afs@lists.infradead.org Link: https://lore.kernel.org/r/2633992.1702073229@warthog.procyon.org.uk/ # v1 Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Sasha Levin sashal@kernel.org --- fs/afs/rxrpc.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/afs/rxrpc.c b/fs/afs/rxrpc.c index 49fcce6529a60..30952f073fb41 100644 --- a/fs/afs/rxrpc.c +++ b/fs/afs/rxrpc.c @@ -490,7 +490,7 @@ void afs_make_call(struct afs_addr_cursor *ac, struct afs_call *call, gfp_t gfp) if (call->async) { if (cancel_work_sync(&call->async_work)) afs_put_call(call); - afs_put_call(call); + afs_set_call_complete(call, ret, 0); }
ac->error = ret;