netdev - Re: [PATCH v2] skbuff: Fix a race between coalescing and releasing SKBs

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Message-ID: <ed4b1f1bf72ea1234a283a26d88e00658e9e4311.camel@gmail.com>
Date:   Thu, 06 Apr 2023 08:25:51 -0700
From:   Alexander H Duyck <alexander.duyck@...il.com>
To:     Liang Chen <liangchen.linux@...il.com>, kuba@...nel.org,
        ilias.apalodimas@...aro.org, hawk@...nel.org
Cc:     davem@...emloft.net, edumazet@...gle.com, pabeni@...hat.com,
        netdev@...r.kernel.org
Subject: Re: [PATCH v2] skbuff: Fix a race between coalescing and releasing
 SKBs

On Thu, 2023-04-06 at 19:48 +0800, Liang Chen wrote:
> Commit 1effe8ca4e34 ("skbuff: fix coalescing for page_pool fragment
> recycling") allowed coalescing to proceed with non page pool page and
> page pool page when @from is cloned, i.e.
> 
> to->pp_recycle    --> false
> from->pp_recycle  --> true
> skb_cloned(from)  --> true
> 
> However, it actually requires skb_cloned(@from) to hold true until
> coalescing finishes in this situation. If the other cloned SKB is
> released while the merging is in process, from_shinfo->nr_frags will be
> set to 0 towards the end of the function, causing the increment of frag
> page _refcount to be unexpectedly skipped resulting in inconsistent
> reference counts. Later when SKB(@to) is released, it frees the page
> directly even though the page pool page is still in use, leading to
> use-after-free or double-free errors.
> 
> So it needs to be specially handled at where the ref count may get lost.
> 
> The double-free error message below prompted us to investigate:
> BUG: Bad page state in process swapper/1  pfn:0e0d1
> page:00000000c6548b28 refcount:-1 mapcount:0 mapping:0000000000000000
> index:0x2 pfn:0xe0d1
> flags: 0xfffffc0000000(node=0|zone=1|lastcpupid=0x1fffff)
> raw: 000fffffc0000000 0000000000000000 ffffffff00000101 0000000000000000
> raw: 0000000000000002 0000000000000000 ffffffffffffffff 0000000000000000
> page dumped because: nonzero _refcount
> 
> CPU: 1 PID: 0 Comm: swapper/1 Tainted: G            E      6.2.0+
> Call Trace:
>  <IRQ>
> dump_stack_lvl+0x32/0x50
> bad_page+0x69/0xf0
> free_pcp_prepare+0x260/0x2f0
> free_unref_page+0x20/0x1c0
> skb_release_data+0x10b/0x1a0
> napi_consume_skb+0x56/0x150
> net_rx_action+0xf0/0x350
> ? __napi_schedule+0x79/0x90
> __do_softirq+0xc8/0x2b1
> __irq_exit_rcu+0xb9/0xf0
> common_interrupt+0x82/0xa0
> </IRQ>
> <TASK>
> asm_common_interrupt+0x22/0x40
> RIP: 0010:default_idle+0xb/0x20
> 
> Signed-off-by: Liang Chen <liangchen.linux@...il.com>
> ---
> Changes from v1:
> - deal with the ref count problem instead of return back to give more opportunities to coalesce skbs.
> ---
>  net/core/skbuff.c | 22 ++++++++++++++++++++--
>  1 file changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 050a875d09c5..77da8ce74a1e 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -5643,7 +5643,19 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
>  
>  		skb_fill_page_desc(to, to_shinfo->nr_frags,
>  				   page, offset, skb_headlen(from));
> -		*fragstolen = true;
> +
> +		/* When @from is pp_recycle and @to isn't, coalescing is
> +		 * allowed to proceed if @from is cloned. However if the
> +		 * execution reaches this point, @from is already transitioned
> +		 * into non-cloned because the other cloned skb is released
> +		 * somewhere else concurrently. In this case, we need to make
> +		 * sure the ref count is incremented, not directly stealing
> +		 * from page pool.
> +		 */
> +		if (to->pp_recycle != from->pp_recycle)
> +			get_page(page);
> +		else
> +			*fragstolen = true;
>  	} else {
>  		if (to_shinfo->nr_frags +
>  		    from_shinfo->nr_frags > MAX_SKB_FRAGS)
> @@ -5659,7 +5671,13 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
>  	       from_shinfo->nr_frags * sizeof(skb_frag_t));
>  	to_shinfo->nr_frags += from_shinfo->nr_frags;
>  
> -	if (!skb_cloned(from))
> +	/* Same situation as above where head data presents. When @from is
> +	 * pp_recycle and @to isn't, coalescing is allowed to proceed if @from
> +	 * is cloned. However @from can be transitioned into non-cloned
> +	 * concurrently by this point. If it does happen, we need to make sure
> +	 * the ref count is properly incremented.
> +	 */
> +	if (to->pp_recycle == from->pp_recycle && !skb_cloned(from))
>  		from_shinfo->nr_frags = 0;
>  
>  	/* if the skb is not cloned this does nothing

So looking this over I believe this should resolve the issue you
pointed out while maintaining current functionality.

Reviewed-by: Alexander Duyck <alexanderduyck@...com>

One follow-on that we may want to do with this would be to look at
consolidating the 3 spots where we are checking for our combination of
pp_recycle comparison and skb_cloned and maybe pass one boolean flag
indicating that we have to transfer everything by taking page
references.

Also I think we can actually increase the number of cases where we
support coalescing if we were to take apart the skb_head_is_locked call
and move the skb_cloned check from it into your recycle check in the
portion where we are stealing from the header.