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authorWen Yang <[email protected]>2023-07-09 14:54:51 +0800
committerChristian Brauner <[email protected]>2023-07-11 11:41:34 +0200
commit758b492047816a3158d027e9fca660bc5bcf20bf (patch)
tree9206e4ec7a35669c327b3c8d8e2d210b7c5602a1
parentf4ae4081e5a871b4c4e2f802ccf73f8d69566073 (diff)
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]>
-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);