5.4-stable review patch. If anyone has any objections, please let me know.
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From: Wen Yang wenyang.linux@foxmail.com
[ 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@foxmail.com Cc: Alexander Viro viro@zeniv.linux.org.uk Cc: Jens Axboe axboe@kernel.dk Cc: Christian Brauner brauner@kernel.org Cc: Christoph Hellwig hch@lst.de Cc: Dylan Yudaken dylany@fb.com Cc: David Woodhouse dwmw@amazon.co.uk Cc: Matthew Wilcox willy@infradead.org Cc: linux-fsdevel@vger.kernel.org Cc: linux-kernel@vger.kernel.org Message-Id: tencent_7588DFD1F365950A757310D764517A14B306@qq.com [brauner: rewrite commit message and add explanation how this underflow can happen] Signed-off-by: Christian Brauner brauner@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- 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);