Patch "skbuff: Fix a race between coalescing and releasing SKBs" has been added to the 5.15-stable tree

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This is a note to let you know that I've just added the patch titled

    skbuff: Fix a race between coalescing and releasing SKBs

to the 5.15-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     skbuff-fix-a-race-between-coalescing-and-releasing-s.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 5a5c6517194ba30599087ac5a090d9771b94d495
Author: Liang Chen <liangchen.linux@xxxxxxxxx>
Date:   Thu Apr 13 17:03:53 2023 +0800

    skbuff: Fix a race between coalescing and releasing SKBs
    
    [ Upstream commit 0646dc31ca886693274df5749cd0c8c1eaaeb5ca ]
    
    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 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: 53e0961da1c7 ("page_pool: add frag page recycling support in page pool")
    Signed-off-by: Liang Chen <liangchen.linux@xxxxxxxxx>
    Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
    Link: https://lore.kernel.org/r/20230413090353.14448-1-liangchen.linux@xxxxxxxxx
    Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 2d3f82b622366..46cc3a7632f79 100644
--- 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)) {



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