Re: [PATCH] eventfd: avoid overflow to ULLONG_MAX when ctx->count is 0

[Christian Brauner <brauner@kernel.org>]

On Sun, Jul 09, 2023 at 02:54:51PM +0800, wenyang.linux@foxmail.com wrote:
> From: Wen Yang <wenyang.linux@foxmail.com>
> 
> 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@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
> ---

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?

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
>