linux-kernel - [PATCH 6.4 101/737] eventfd: prevent underflow for eventfd semaphores

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Date:   Mon, 11 Sep 2023 15:39:20 +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 6.4 101/737] eventfd: prevent underflow for eventfd semaphores

6.4-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 95850a13ce8d0..1ffbf7c1cd16d 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);
-- 
2.40.1