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Message-Id: <20230406114825.18597-1-liangchen.linux@gmail.com> Date: Thu, 6 Apr 2023 19:48:25 +0800 From: Liang Chen <liangchen.linux@...il.com> To: kuba@...nel.org, alexander.duyck@...il.com, ilias.apalodimas@...aro.org, hawk@...nel.org Cc: davem@...emloft.net, edumazet@...gle.com, pabeni@...hat.com, netdev@...r.kernel.org, liangchen.linux@...il.com Subject: [PATCH v2] skbuff: Fix a race between coalescing and releasing SKBs 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 -- 2.18.2
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