[PATCH 0/2] fuse: fix request lifetime races in the resend path

[Jun Yang <littleddfu@gmail.com>]

Two fixes for races introduced together with FUSE_NOTIFY_RESEND
(760eac73f9f6, v6.9), where fuse_chan_resend() moves in-flight requests
from fpq->processing back onto fiq->pending.

Both concern the same invariant. FR_PENDING means "queued on fiq->pending,
protected by fiq->lock", and it is what fuse_remove_pending_req() relies on
to unlink a request and drop the queue's reference. A request on
fiq->pending can therefore be released without going through
fuse_request_end(), so anything that sets FR_PENDING, or that leaves a
request linked elsewhere while FR_PENDING is set, has to be done under
fiq->lock.

Patch 1 sets FR_PENDING under fiq->lock. fuse_chan_resend() currently
publishes the bit while the requests are reachable only through a
stack-local list, so a concurrent waiter can unlink and release a request
that fuse_chan_resend() is still iterating over.

Patch 2 re-checks FR_SENT under fiq->lock in fuse_dev_queue_interrupt().
Its callers sample FR_SENT unlocked and fuse_chan_resend() clears it under
fiq->lock, so a request can end up queued on fiq->pending and linked on
fiq->interrupts at the same time.

Dependency between the two patches
==================================

They are independent and neither supersedes the other, so please apply them
together rather than picking one. Patch 2 does not affect the unlocked
FR_PENDING publish that patch 1 fixes. Patch 1 cannot catch an intr_entry
that is linked after its locked walk has already run, because that link
happens once fuse_chan_resend() has released fiq->lock. Patch 1 on its own
also makes the condition patch 2 fixes easier to hit, since requests that
would previously have been torn out of the resend list now survive to be
re-queued.

Both were found by code audit and confirmed on v7.2-rc6 (075b74841bd0),
where the series was built and tested; the resend and interrupt paths were
verified to still be exercised with the series applied.

A KASAN reproducer for this issue is available if requested.

Reported-by: TencentOS Corvus AI <corvus@tencent.com>
Signed-off-by: Jun Yang <junvyyang@tencent.com>

Jun Yang (2):
  fuse: set FR_PENDING under fiq->lock in fuse_chan_resend()
  fuse: don't queue an interrupt for a request that is back on
    fiq->pending

 fs/fuse/dev.c | 29 +++++++++++++++--------------
 1 file changed, 15 insertions(+), 14 deletions(-)

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2.43.7