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author | Wen Yang <wenyang.linux@foxmail.com> | 2023-07-09 14:54:51 +0800 |
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committer | Christian Brauner <brauner@kernel.org> | 2023-07-11 11:41:34 +0200 |
commit | 758b492047816a3158d027e9fca660bc5bcf20bf (patch) | |
tree | 9206e4ec7a35669c327b3c8d8e2d210b7c5602a1 /fs/eventfd.c | |
parent | f4ae4081e5a871b4c4e2f802ccf73f8d69566073 (diff) | |
download | linux-stable-758b492047816a3158d027e9fca660bc5bcf20bf.tar.gz linux-stable-758b492047816a3158d027e9fca660bc5bcf20bf.tar.bz2 linux-stable-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 'fs/eventfd.c')
-rw-r--r-- | fs/eventfd.c | 2 |
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); |