page_pool: keep frag_offset aligned for odd-sized requests

In 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
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