page_pool: keep frag_offset aligned for odd-sized requests

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-99127
StatePUBLISHED
AssignerLinux
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-10-28 11:17:32 UTC
Updated2026-10-30 13:21:46 UTC
DescriptionIn the Linux kernel, the following vulnerability has been resolved: page_pool: keep frag_offset aligned for odd-sized requests 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 t ---truncated---

Risk And Classification

EPSS: 0.001730000 probability, percentile 0.067370000 (date 2026-11-01)

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Linux Linux affected 53e0961da1c7bbdabd1abebb20de403ec237ec09 8ab058f49e801803c4a755b09fe4a920d194da7f git Not specified
CNA Linux Linux affected 53e0961da1c7bbdabd1abebb20de403ec237ec09 31e1a71995f23446c9d9e6ba170a6b16b110d243 git Not specified
CNA Linux Linux affected 53e0961da1c7bbdabd1abebb20de403ec237ec09 ccc28e498ef1c6eaaa16e61c6edb52c32a4f14bb git Not specified
CNA Linux Linux affected 53e0961da1c7bbdabd1abebb20de403ec237ec09 22b67b0f59f5a7f4fa1e6043a1987e167c5d06d2 git Not specified
CNA Linux Linux affected 53e0961da1c7bbdabd1abebb20de403ec237ec09 813a72beaa5d3f4bb135e0def9b3f07a3511ca0e git Not specified
CNA Linux Linux affected 53e0961da1c7bbdabd1abebb20de403ec237ec09 dca1b51693c81eaf77e50980be373e01ae857394 git Not specified
CNA Linux Linux affected 53e0961da1c7bbdabd1abebb20de403ec237ec09 dc0df5a0c62ccea1d0e08d39a4dc9064de81d411 git Not specified
CNA Linux Linux affected 5.15 Not specified
CNA Linux Linux unaffected 5.15 semver Not specified
CNA Linux Linux unaffected 5.15.224 5.15.* semver Not specified
CNA Linux Linux unaffected 6.1.191 6.1.* semver Not specified
CNA Linux Linux unaffected 6.6.161 6.6.* semver Not specified
CNA Linux Linux unaffected 6.12.115 6.12.* semver Not specified
CNA Linux Linux unaffected 6.18.59 6.18.* semver Not specified
CNA Linux Linux unaffected 7.2.13 7.2.* semver Not specified
CNA Linux Linux unaffected 7.3 * original_commit_for_fix Not specified

References

ReferenceSourceLinkTags
git.kernel.org/stable/c/813a72beaa5d3f4bb135e0def9b3f07a3511ca0e 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/dc0df5a0c62ccea1d0e08d39a4dc9064de81d411 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/dca1b51693c81eaf77e50980be373e01ae857394 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/22b67b0f59f5a7f4fa1e6043a1987e167c5d06d2 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/8ab058f49e801803c4a755b09fe4a920d194da7f 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/31e1a71995f23446c9d9e6ba170a6b16b110d243 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/ccc28e498ef1c6eaaa16e61c6edb52c32a4f14bb 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
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