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Date:   Mon, 10 Jul 2023 23:02:33 +0800
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Subject: Re: [PATCH] eventfd: avoid overflow to ULLONG_MAX when ctx->count is
 0
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To:     Christian Brauner <brauner@kernel.org>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Jens Axboe <axboe@kernel.dk>, Christoph Hellwig <hch@lst.de>,
        Dylan Yudaken <dylany@fb.com>,
        David Woodhouse <dwmw@amazon.co.uk>,
        Matthew Wilcox <willy@infradead.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
References: <tencent_7588DFD1F365950A757310D764517A14B306@qq.com>
 <20230710-fahrbahn-flocken-03818a6b2e91@brauner>
From:   Wen Yang <wenyang.linux@foxmail.com>
In-Reply-To: <20230710-fahrbahn-flocken-03818a6b2e91@brauner>
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On 2023/7/10 22:12, Christian Brauner wrote:
> 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?
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
>>