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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.
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