skbuff: Fix a race between coalescing and releasing SKBs

LiangChen77 · gregkh · commit 906a6689bb01 · 2023-04-20T12:13:54.000+02:00
[ Upstream commit 0646dc3 ] Commit 1effe8c ("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 toward 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 should be prohibited. 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 Fixes: 53e0961 ("page_pool: add frag page recycling support in page pool") Signed-off-by: Liang Chen <liangchen.linux@gmail.com> Reviewed-by: Eric Dumazet <edumazet@google.com> Link: https://lore.kernel.org/r/20230413090353.14448-1-liangchen.linux@gmail.com Signed-off-by: Jakub Kicinski <kuba@kernel.org> Signed-off-by: Sasha Levin <sashal@kernel.org>
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
@@ -5397,18 +5397,18 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
 	if (skb_cloned(to))
 		return false;
 
-	/* In general, avoid mixing slab allocated and page_pool allocated
-	 * pages within the same SKB. However when @to is not pp_recycle and
-	 * @from is cloned, we can transition frag pages from page_pool to
-	 * reference counted.
-	 *
-	 * On the other hand, don't allow coalescing two pp_recycle SKBs if
-	 * @from is cloned, in case the SKB is using page_pool fragment
+	/* In general, avoid mixing page_pool and non-page_pool allocated
+	 * pages within the same SKB. Additionally avoid dealing with clones
+	 * with page_pool pages, in case the SKB is using page_pool fragment
 	 * references (PP_FLAG_PAGE_FRAG). Since we only take full page
 	 * references for cloned SKBs at the moment that would result in
 	 * inconsistent reference counts.
+	 * In theory we could take full references if @from is cloned and
+	 * !@to->pp_recycle but its tricky (due to potential race with
+	 * the clone disappearing) and rare, so not worth dealing with.
 	 */
-	if (to->pp_recycle != (from->pp_recycle && !skb_cloned(from)))
+	if (to->pp_recycle != from->pp_recycle ||
+	    (from->pp_recycle && skb_cloned(from)))
 		return false;
 
 	if (len <= skb_tailroom(to)) {
