5.4-stable review patch. If anyone has any objections, please let me know. ------------------ From: Wen Yang <wenyang.linux@xxxxxxxxxxx> [ Upstream commit 758b492047816a3158d027e9fca660bc5bcf20bf ] 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 <wenyang.linux@xxxxxxxxxxx> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx> Cc: Jens Axboe <axboe@xxxxxxxxx> Cc: Christian Brauner <brauner@xxxxxxxxxx> Cc: Christoph Hellwig <hch@xxxxxx> Cc: Dylan Yudaken <dylany@xxxxxx> Cc: David Woodhouse <dwmw@xxxxxxxxxxxx> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> Cc: linux-fsdevel@xxxxxxxxxxxxxxx Cc: linux-kernel@xxxxxxxxxxxxxxx Message-Id: <tencent_7588DFD1F365950A757310D764517A14B306@xxxxxx> [brauner: rewrite commit message and add explanation how this underflow can happen] Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> --- fs/eventfd.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/fs/eventfd.c b/fs/eventfd.c index 26b3d821e9168..e144094c831df 100644 --- a/fs/eventfd.c +++ b/fs/eventfd.c @@ -185,7 +185,7 @@ void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt) { lockdep_assert_held(&ctx->wqh.lock); - *cnt = (ctx->flags & EFD_SEMAPHORE) ? 1 : ctx->count; + *cnt = ((ctx->flags & EFD_SEMAPHORE) && ctx->count) ? 1 : ctx->count; ctx->count -= *cnt; } EXPORT_SYMBOL_GPL(eventfd_ctx_do_read); -- 2.40.1