iavf: fix off-by-one issues in iavf_config_rss_reg()
CVE: CVE-2025-71087
Component: kernel
CWE: CWE-787
Severity: Moderate
CVSS v3: 6.7/AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H
Release date: 2025-01-01T00:00:00+00:00

In the Linux kernel, the following vulnerability has been resolved:

iavf: fix off-by-one issues in iavf_config_rss_reg()

There are off-by-one bugs when configuring RSS hash key and lookup
table, causing out-of-bounds reads to memory [1] and out-of-bounds
writes to device registers.

Before commit 43a3d9ba34c9 ("i40evf: Allow PF driver to configure RSS"),
the loop upper bounds were:
    i <= I40E_VFQF_{HKEY,HLUT}_MAX_INDEX
which is safe since the value is the last valid index.

That commit changed the bounds to:
    i <= adapter->rss_{key,lut}_size / 4
where `rss_{key,lut}_size / 4` is the number of dwords, so the last
valid index is `(rss_{key,lut}_size / 4) - 1`. Therefore, using `<=`
accesses one element past the end.

Fix the issues by using `<` instead of `<=`, ensuring we do not exceed
the bounds.

[1] KASAN splat about rss_key_size off-by-one
  BUG: KASAN: slab-out-of-bounds in iavf_config_rss+0x619/0x800
  Read of size 4 at addr ffff888102c50134 by task kworker/u8:6/63

  CPU: 0 UID: 0 PID: 63 Comm: kworker/u8:6 Not tainted 6.18.0-rc2-enjuk-tnguy-00378-g3005f5b77652-dirty #156 PREEMPT(voluntary)
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
  Workqueue: iavf iavf_watchdog_task
  Call Trace:
   <TASK>
   dump_stack_lvl+0x6f/0xb0
   print_report+0x170/0x4f3
   kasan_report+0xe1/0x1a0
   iavf_config_rss+0x619/0x800
   iavf_watchdog_task+0x2be7/0x3230
   process_one_work+0x7fd/0x1420
   worker_thread+0x4d1/0xd40
   kthread+0x344/0x660
   ret_from_fork+0x249/0x320
   ret_from_fork_asm+0x1a/0x30
   </TASK>

  Allocated by task 63:
   kasan_save_stack+0x30/0x50
   kasan_save_track+0x14/0x30
   __kasan_kmalloc+0x7f/0x90
   __kmalloc_noprof+0x246/0x6f0
   iavf_watchdog_task+0x28fc/0x3230
   process_one_work+0x7fd/0x1420
   worker_thread+0x4d1/0xd40
   kthread+0x344/0x660
   ret_from_fork+0x249/0x320
   ret_from_fork_asm+0x1a/0x30

  The buggy address belongs to the object at ffff888102c50100
   which belongs to the cache kmalloc-64 of size 64
  The buggy address is located 0 bytes to the right of
   allocated 52-byte region [ffff888102c50100, ffff888102c50134)

  The buggy address belongs to the physical page:
  page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x102c50
  flags: 0x200000000000000(node=0|zone=2)
  page_type: f5(slab)
  raw: 0200000000000000 ffff8881000418c0 dead000000000122 0000000000000000
  raw: 0000000000000000 0000000080200020 00000000f5000000 0000000000000000
  page dumped because: kasan: bad access detected

  Memory state around the buggy address:
   ffff888102c50000: 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc fc
   ffff888102c50080: 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc fc
  >ffff888102c50100: 00 00 00 00 00 00 04 fc fc fc fc fc fc fc fc fc
                                       ^
   ffff888102c50180: 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc
   ffff888102c50200: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc

Statement: An off-by-one error in iavf_config_rss_reg() causes out-of-bounds reads from the RSS key/LUT arrays and out-of-bounds writes to device registers. The loop upper bound used the number of dwords instead of the last valid index, leading to one extra iteration. This can result in kernel crashes (DoS) and potentially unstable device behavior.

Remediation: