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Message-ID: <20230920112847.780028922@linuxfoundation.org>
Date: Wed, 20 Sep 2023 13:28:04 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: stable@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
patches@...ts.linux.dev, Wen Yang <wenyang.linux@...mail.com>,
Alexander Viro <viro@...iv.linux.org.uk>,
Jens Axboe <axboe@...nel.dk>,
Christian Brauner <brauner@...nel.org>,
Christoph Hellwig <hch@....de>, Dylan Yudaken <dylany@...com>,
David Woodhouse <dwmw@...zon.co.uk>,
Matthew Wilcox <willy@...radead.org>,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
Sasha Levin <sashal@...nel.org>
Subject: [PATCH 4.19 044/273] eventfd: prevent underflow for eventfd semaphores
4.19-stable review patch. If anyone has any objections, please let me know.
------------------
From: Wen Yang <wenyang.linux@...mail.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@...mail.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>
Cc: Jens Axboe <axboe@...nel.dk>
Cc: Christian Brauner <brauner@...nel.org>
Cc: Christoph Hellwig <hch@....de>
Cc: Dylan Yudaken <dylany@...com>
Cc: David Woodhouse <dwmw@...zon.co.uk>
Cc: Matthew Wilcox <willy@...radead.org>
Cc: linux-fsdevel@...r.kernel.org
Cc: linux-kernel@...r.kernel.org
Message-Id: <tencent_7588DFD1F365950A757310D764517A14B306@...com>
[brauner: rewrite commit message and add explanation how this underflow can happen]
Signed-off-by: Christian Brauner <brauner@...nel.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
fs/eventfd.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/eventfd.c b/fs/eventfd.c
index a96de1f0377bc..66864100b823c 100644
--- a/fs/eventfd.c
+++ b/fs/eventfd.c
@@ -178,7 +178,7 @@ 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
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