eventfd: prevent underflow for eventfd semaphores

For eventfd with flag EFD_SEMAPHORE, when its ctx->count is 0, calling
eventfd_ctx_do_read will cause ctx->count to overflow to ULLONG_MAX.

An underflow can happen with EFD_SEMAPHORE eventfds in at least the
following three subsystems:

(1) virt/kvm/eventfd.c
(2) drivers/vfio/virqfd.c
(3) drivers/virt/acrn/irqfd.c

where (2) and (3) are just modeled after (1). An eventfd must be
specified for use with the KVM_IRQFD ioctl(). This can also be an
EFD_SEMAPHORE eventfd. When the eventfd count is zero or has been
decremented to zero an underflow can be triggered when the irqfd is shut
down by raising the KVM_IRQFD_FLAG_DEASSIGN flag in the KVM_IRQFD
ioctl():

        // ctx->count == 0
        kvm_vm_ioctl()
        -> kvm_irqfd()
           -> kvm_irqfd_deassign()
              -> irqfd_deactivate()
                 -> irqfd_shutdown()
                    -> eventfd_ctx_remove_wait_queue(&cnt)
                       -> eventfd_ctx_do_read(&cnt)

Userspace polling on the eventfd wouldn't notice the underflow because 1
is always returned as the value from eventfd_read() while ctx->count
would've underflowed. It's not a huge deal because this should only be
happening when the irqfd is shutdown but we should still fix it and
avoid the spurious wakeup.

Fixes: cb289d6244a3 ("eventfd - allow atomic read and waitqueue remove")
Signed-off-by: Wen Yang <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Dylan Yudaken <[email protected]>
Cc: David Woodhouse <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: [email protected]
Cc: [email protected]
Message-Id: <[email protected]>
[brauner: rewrite commit message and add explanation how this underflow can happen]
Signed-off-by: Christian Brauner <[email protected]>

0 files changed, 0 insertions, 0 deletions