netdev - Re: [PATCH net 1/4] inet: frags: avoid theoretical race in ip_frag

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Message-ID: <CANn89iKJXNksYB1nOnsQAXgsrYYaVa78JLeSuG_y3b9QaXnoMg@mail.gmail.com>
Date: Mon, 8 Dec 2025 07:18:13 -0800
From: Eric Dumazet <edumazet@...gle.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: davem@...emloft.net, netdev@...r.kernel.org, pabeni@...hat.com, 
	andrew+netdev@...n.ch, horms@...nel.org, pablo@...filter.org, fw@...len.de, 
	netfilter-devel@...r.kernel.org, willemdebruijn.kernel@...il.com, 
	kuniyu@...gle.com
Subject: Re: [PATCH net 1/4] inet: frags: avoid theoretical race in ip_frag_reinit()

On Sat, Dec 6, 2025 at 5:10 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> In ip_frag_reinit() we want to move the frag timeout timer into
> the future. If the timer fires in the meantime we inadvertently
> scheduled it again, and since the timer assumes a ref on frag_queue
> we need to acquire one to balance things out.
>
> This is technically racy, we should have acquired the reference
> _before_ we touch the timer, it may fire again before we take the ref.
> Avoid this entire dance by using mod_timer_pending() which only modifies
> the timer if its pending (and which exists since Linux v2.6.30)
>
> Note that this was the only place we ever took a ref on frag_queue
> since Eric's conversion to RCU. So we could potentially replace
> the whole refcnt field with an atomic flag and a bit more RCU.
>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Jakub Kicinski <kuba@...nel.org>
> ---

Reviewed-by: Eric Dumazet <edumazet@...gle.com>