Re: [PATCH net v2] page_pool: keep frag_offset aligned for odd-sized requests

[Jesper Dangaard Brouer <hawk@kernel.org>]

On 28/08/2026 08.08, Florian Schauer wrote:
> page_pool_alloc_frag_netmem() rounds the requested fragment size with
> 
> 	size = ALIGN(size, dma_get_cache_alignment());
> 
> dma_get_cache_alignment() returns 1 unless the architecture defines
> ARCH_DMA_MINALIGN, which DMA-coherent architectures such as x86 do not.
> There the ALIGN() is a no-op and pool->frag_offset advances by the raw,
> unrounded size.
> 
> A single caller asking for an odd size then leaves frag_offset misaligned
> for every fragment carved out of that page afterwards.  The pool is shared,
> so the damage is not confined to the caller that caused it.
> 
> The per-cpu system_page_pool used by generic XDP hits this.
> skb_pp_cow_data() allocates its fragments with the raw packet length:
> 
> 	size = min_t(u32, len, PAGE_SIZE);
> 	truesize = size;
> 	page = page_pool_dev_alloc(pool, &page_off, &truesize);
> 
> leaving frag_offset odd for whatever is carved out of that page next.  Its
> own head allocation is already aligned -- SKB_HEAD_ALIGN(size) plus the
> XDP_PACKET_HEADROOM its callers pass -- so it is a later user of the shared
> pool that pays: page_pool_dev_alloc_va() returns a misaligned buffer,
> napi_build_skb() installs it as skb->head, and skb_shinfo(skb) ==
> skb->head + skb->end is misaligned with it.
> 
> skb_shinfo()->dataref is a 4-byte atomic_t at offset 0x20, so the
> atomic_inc() in __skb_clone() straddles a cache line.  On x86 with split
> lock detection -- fatal for kernel split locks by default -- this panics
> the machine:
> 
>    Oops: Split lock detected
>    RIP: 0010:skb_clone+0x154/0x1e0
>    Call Trace:
>     <IRQ>
>     raw_local_deliver+0x1ed/0x2c0
>     ip_protocol_deliver_rcu+0x54/0x1c0
>     ip_local_deliver_finish+0x85/0x100
>     ip_local_deliver+0x67/0x100
>     __netif_receive_skb_one_core+0x85/0xa0
>     process_backlog+0x87/0x130
> 
> Reproduced by attaching any generic-mode XDP program to loopback and
> opening a RAW IPPROTO_UDP socket, which makes raw_local_deliver() clone
> every locally delivered UDP packet; ordinary DNS traffic then triggers it,
> roughly once per 2500 clones.  Observed on 6.12.101 and 7.1.8.
> 
> Tracing page_pool_alloc_frag_netmem() over one such run shows the
> amplification -- two odd-sized requests, nine misaligned offsets:
> 
>    requested size & 7:   0: 17035    5: 1    7: 1
>    frag_offset & 7:      0: 17028    3: 1    4: 1    5: 1    6: 1    7: 5
> 
> and skb_pp_cow_data() returning heads that were aligned on entry:
> 
>    head 0xffff8f4c86aeac00 -> 0xffff8f4c53a9a9c4 (&7=4)
>    head 0xffff8f4d6a8a42c0 -> 0xffff8f4c4f7b7a45 (&7=5)
> 
> Round the fragment size up to at least the alignment struct skb_shared_info
> requires, so fragments are always suitably aligned for the objects callers
> build on them.  Architectures needing a larger DMA alignment keep it.
> 
> This also makes the remainder computed in page_pool_alloc_netmem(),
> 
> 	*size = max_size - *offset;
> 
> aligned, since max_size is a power of two -- which fixes the matching
> misalignment of skb->end.
> 
> Verified with a controlled A/B under QEMU/KVM: same tree, same config,
> same compiler, same rootfs and identical traffic, differing only by this
> patch.  A SEC("xdp.frags") XDP_PASS program on lo plus UDP datagrams
> larger than max_head_size drives skb_pp_cow_data()'s fragment loop, which
> passes raw packet lengths to the pool.  Measured at the return of
> skb_pp_cow_data():
> 
>                            unpatched   patched
>    skb_pp_cow_data calls       40800     40800
>    misaligned skb->head         1120         0
>    dataref at line offset >60     80         0
> 
> The last row counts the accesses that actually fault:
> skb_shinfo()->dataref sits at head+end+0x20 and is a 4-byte atomic, so
> `lock incl` splits a 64-byte cache line only when that address lands at
> offset 61..63.  All 80 occurrences were at offset 61; the panic reported
> above was at offset 62.  Eliminating the misalignment removes every one
> of them.
> 
> Same class of bug as commit 3bed3cc4156e ("net: Do not allocate page
> fragments that are not skb aligned"), which fixed the older
> netdev_alloc_frag()/napi_alloc_frag() allocators.
> 
> Fixes: 53e0961da1c7 ("page_pool: add frag page recycling support in page pool")
> Cc: stable@vger.kernel.org
> Signed-off-by: Florian Schauer <florian@schauer.to>
> ---
> v2:
>    - express the minimum alignment as __alignof__(struct skb_shared_info)
>      instead of sizeof(long), and drop the explanatory comment the previous
>      version carried, since the expression now states the requirement
>      directly (Eric Dumazet)
>    - no functional change vs v1 on 64-bit: the emitted code is identical
>    - Cc the maintainers and the blamed author that v1 missed
>    - correct two statements in the commit message: dataref is at offset 0x20
>      in struct skb_shared_info, not its first member, and skb_pp_cow_data()
>      allocates its head before the fragment loop, so the misaligned head
>      comes from an earlier user of the shared pool
> v1: https://lore.kernel.org/netdev/20260826135252.3091193-1-florian@schauer.to/
> 
>   net/core/page_pool.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index 8f8956fb0..08d7f35cf 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -1073,7 +1073,8 @@ netmem_ref page_pool_alloc_frag_netmem(struct page_pool *pool,
>   	if (WARN_ON(size > max_size))
>   		return 0;
>   
> -	size = ALIGN(size, dma_get_cache_alignment());
> +	size = ALIGN(size, max_t(unsigned int, dma_get_cache_alignment(),
> +				 __alignof__(struct skb_shared_info)));
>   	*offset = pool->frag_offset;
>   
>   	if (netmem && *offset + size > max_size) {

On x86_64 __alignof__(struct skb_shared_info) returns 8. So, this should 
work, to avoid getting the lock in skb_shared_info split across cache-lines.

Acked-by: Jesper Dangaard Brouer <hawk@kernel.org>

Thanks for finding and fixing this bug!
--Jesper