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

[Eric Dumazet]

On Mon, Aug 31, 2026 at 11:00 AM Jesper Dangaard Brouer <hawk@xxxxxxxxxx> wrote:
>
>
>
> 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@xxxxxxxxxxxxxxx
> > Signed-off-by: Florian Schauer <florian@xxxxxxxxxx>

Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Thanks.