Re: [PATCH] eventfd: avoid overflow to ULLONG_MAX when ctx->count is 0
From: Wen Yang
Date: Mon Jul 10 2023 - 11:02:59 EST
On 2023/7/10 22:12, Christian Brauner wrote:
On Sun, Jul 09, 2023 at 02:54:51PM +0800, wenyang.linux@xxxxxxxxxxx wrote:
From: Wen Yang <wenyang.linux@xxxxxxxxxxx>
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.
Fixes: cb289d6244a3 ("eventfd - allow atomic read and waitqueue remove")
Signed-off-by: Wen Yang <wenyang.linux@xxxxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Dylan Yudaken <dylany@xxxxxx>
Cc: David Woodhouse <dwmw@xxxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
---
So this looks ok but I would like to see an analysis how the overflow
can happen. I'm looking at the callers and it seems that once ctx->count
hits 0 eventfd_read() won't call eventfd_ctx_do_read() anymore. So is
there a caller that can call directly or indirectly
eventfd_ctx_do_read() on a ctx->count == 0?
eventfd_read() ensures that ctx->count is not 0 before calling
eventfd_ctx_do_read() and it is correct.
But it is not appropriate for eventfd_ctx_remove_wait_queue() to call
eventfd_ctx_do_read() unconditionally,
as it may not only causes ctx->count to overflow, but also unnecessarily
calls wake_up_locked_poll().
I am sorry for just adding the following string in the patch:
Fixes: cb289d6244a3 ("eventfd - allow atomic read and waitqueue remove")
Looking forward to your suggestions.
--
Best wishes,
Wen
I'm just slightly skeptical about patches that fix issues without an
analysis how this can happen.
fs/eventfd.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/fs/eventfd.c b/fs/eventfd.c
index 8aa36cd37351..10a101df19cd 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);
@@ -269,6 +269,8 @@ static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t c
return -EFAULT;
if (ucnt == ULLONG_MAX)
return -EINVAL;
+ if ((ctx->flags & EFD_SEMAPHORE) && !ucnt)
+ return -EINVAL;
spin_lock_irq(&ctx->wqh.lock);
res = -EAGAIN;
if (ULLONG_MAX - ctx->count > ucnt)
--
2.25.1