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authorWen Yang <wenyang.linux@foxmail.com>2023-07-09 08:54:51 +0200
committerChristian Brauner <brauner@kernel.org>2023-07-11 11:41:34 +0200
commit758b492047816a3158d027e9fca660bc5bcf20bf (patch)
tree9206e4ec7a35669c327b3c8d8e2d210b7c5602a1 /fs/eventfd.c
parentdnotify: Pass argument of fcntl_dirnotify as int (diff)
downloadlinux-758b492047816a3158d027e9fca660bc5bcf20bf.tar.xz
linux-758b492047816a3158d027e9fca660bc5bcf20bf.zip
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 <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>
Diffstat (limited to '')
-rw-r--r--fs/eventfd.c2
1 files changed, 1 insertions, 1 deletions
diff --git a/fs/eventfd.c b/fs/eventfd.c
index 8aa36cd37351..33a918f9566c 100644
--- a/fs/eventfd.c
+++ b/fs/eventfd.c
@@ -189,7 +189,7 @@ 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);