[BUG] mt7925e/MT7927: RX page_pool buffers stamped every 16 bytes, corrupting skb_shared_info->frag_list -> #SS in kfree_skb_list_reason

[Vladislav Rysin <vrysin@gmail.com>]

Hi,

I am hitting a reproducible kernel panic with an MT7927 (Filogic 380) card on
mt7925e. Four panics over five days, all with byte-identical registers. I have
analysed two vmcores and believe I have the mechanism, though not the culprit.

Hardware / software
-------------------
Dell Precision 5820 Tower, BIOS 2.48.0, 250 GB RAM
MEDIATEK MT7927 802.11be 2x2 [14c3:7927], subsystem 105b:e104
  behind a dedicated root port (b2:00.0 -> b3:00.0), PCIe 8GT/s x1
  (LnkSta matches LnkCap; UESta clean, no AER events)
driver: mt7925e, MT7927 raw CHIPID=0x0000, forcing chip=0x7927
firmware: linux-firmware-20260810
  HW/SW Version 0x8a108a10, Build Time 20260414133750a
  WM Firmware Version ____000000, Build Time 20260414134255
link at time of crashes: VHT80, 5745 MHz, 866.7 Mbit/s, VHT-NSS 2 (not EHT)

Reproduces on:
  7.2.3 vanilla (in-tree mt7925e, kernel Not tainted)   <- crashes 3 and 4
  7.1.12 and 7.1.10 with the mediatek-mt7927-dkms 2.14 backport <- crashes 1, 2

Uptime at panic: 22h34m, 27h55m, 4h53m, 8h50m. Traffic-dependent, not timed.

The panic
---------
Oops: stack segment: 0000 [#1] SMP NOPTI
CPU: 6 UID: 0 PID: 808 Comm: napi/phy0-0 Kdump: loaded Not tainted
     7.2.3-300.vanilla.fc44.x86_64
RIP: 0010:kfree_skb_list_reason+0xce/0x260
RBX: 0080a62400000000 RDI: 0080a62400000000 RBP: 0080a62400000000
Call Trace:
 skb_release_data+0x184/0x220
 napi_consume_skb+0x7a/0x160
 skb_defer_free_flush+0x81/0xc0
 napi_threaded_poll_loop+0x14e/0x290
 napi_threaded_poll+0x42/0xb0

A second panic took the same fault from softirq context instead, during the
driver's own recovery, right after an MCU timeout:

 mt7925e 0000:b3:00.0: Message 00020016 (seq 4) timeout
 Workqueue: mt76 mt7925_mac_reset_work [mt7925_common]
 mt7925e_mac_reset+0x20e/0x400 -> __local_bh_enable_ip -> do_softirq
 -> net_rx_action -> skb_defer_free_flush -> kfree_skb_list_reason

So the flushing context is incidental; the skb is already corrupt on the
per-CPU defer list. The corruption happens at RX time.

What the vmcore shows
---------------------
Analysed with drgn 0.2.0 (crash 9.0.1 cannot open a 7.2 dump:
"invalid structure member offset: kmem_cache_s_num").

The RX data page carrying the skb is stamped: every 16 bytes, the dword at
offset +12 is overwritten with 24 a6 80 00 (LE 0x0080a624). 256 of 256
16-byte records in the page, and several other page_pool pages are fully
stamped as well (scattered, not contiguous).

The unstamped bytes are genuine payload -- local MAC, AP MAC and the local
IPv4 address are all plainly visible in the same page:

  +0e80  00 00 66 ac f7 29 1b 46 f8 1a 2b 1a  24 a6 80 00
  +0ea0  c0 a8 56 22 01 bb e3 fc 94 66 fb f5  24 a6 80 00

Why the register value is always identical:

  - skb_shared_info sits at the buffer tail, page offset 0xec0 (fixed
    q->buf_size); identical in both dumps
  - frag_list is at shinfo+8
  - the 16-byte-stride stamp lands on shinfo+12, i.e. the *upper* four bytes
    of the frag_list pointer
  - the lower half stays 0, so frag_list becomes 0x0080a624_00000000
  - non-NULL, so skb_release_data() calls kfree_skb_list_reason() on it, which
    does mov 0x0(%rbp),%rbp on a non-canonical address -> #SS

That is why four panics on two kernel versions, two driver builds and two
different call paths produced byte-identical registers. It is not random
memory corruption; it is one field, stamped by one writer, every time.

Interpretation (this part is inference)
---------------------------------------
A 16-byte stride touching only DW3 matches
struct mt76_desc { __le32 buf0, ctrl, buf1, info; } -- info is DW3 at
offset 12. That would mean descriptor-shaped writes are landing on
page_pool RX data pages. The stride, the offset and the constant are
measured; the field identity is a guess and I would not want it taken as
more than that.

I could not identify what 0x0080a624 decodes to as a descriptor word.

Ruled out
---------
 - Out-of-tree driver: reproduces on in-tree 7.2.3 with the kernel Not tainted
 - Hardware / PCIe link: link trains at full capability, UESta clean, no AER,
   EDAC CE/UE both zero. A marginal link does not reproduce a fixed value at a
   fixed offset four times.
 - Wi-Fi power save: disabled at runtime and persistently; panicked anyway
 - ASPM: already disabled by the driver at probe (LnkCtl: ASPM Disabled)
 - GRO fraglist: rx-gro-list is off, so nothing should be building a frag_list
   at all -- consistent with shinfo being overwritten rather than populated

I have two vmcores (1.4 GB and 1.5 GB, 7.2.3, makedumpfile -l -d 31) and can
run any drgn query against them, or provide dumps if useful. Happy to test
patches -- the box reproduces this within a day of normal use.

Note the dumps were taken with -d 31, which excluded the sk_buff slab page
itself; only the data page survived. I am lowering the filter for future
dumps.

Thanks,
Vladislav Rysin