NFS: write_completion: dereference loop-local req, not hdr->req

NFS: write_completion: dereference loop-local req, not hdr->req

5d3869a41f36 ("NFS: fix writeback in presence of errors") introduced
a dereference of hdr->req->wb_lock_context in nfs_write_completion's
per-request loop.  hdr->req is set once at nfs_pgheader_init() time
and is not refcount-protected for the lifetime of the loop; when hdr
aggregates requests from multiple page groups (common under heavy
NFSv3 writeback), a parallel COMMIT on hdr->req's group can drop the
last reference and free it while the outer loop is still iterating
requests from other groups.  KASAN catches this as an 8-byte read at
offset +24 of a freed nfs_page slab object (wb_lock_context).

All requests in a given pgio share the same open_context, so reading
the loop-local req's wb_lock_context yields the same value and is
safe -- req is still on hdr->pages and holds its writeback kref
through the commit branch.

Caught with kasan:

BUG: KASAN: slab-use-after-free in nfs_write_completion+0x8f8/0xa50 [nfs]
Read of size 8 at addr ffff888118af2058 by task kworker/u16:16/122062
CPU: 2 UID: 0 PID: 122062 Comm: kworker/u16:16 Kdump: loaded Not tainted 7.1.0-rc4+ #ge05a759574b2 PREEMPT
Workqueue: nfsiod rpc_async_release
Call Trace:
 <TASK>
 dump_stack_lvl+0xaf/0x100
 ? nfs_write_completion+0x8f8/0xa50 [nfs]
 print_report+0x157/0x4a1
 ? __virt_addr_valid+0x1fb/0x400
 ? nfs_write_completion+0x8f8/0xa50 [nfs]
 kasan_report+0xc2/0x190
 ? nfs_write_completion+0x8f8/0xa50 [nfs]
 nfs_write_completion+0x8f8/0xa50 [nfs]
 ? nfs_commit_release_pages+0xbd0/0xbd0 [nfs]
 ? lock_acquire+0x182/0x2e0
 ? process_one_work+0x937/0x1890
 ? nfs_pgio_header_alloc+0xd0/0xd0 [nfs]
 rpc_free_task+0xee/0x160
 rpc_async_release+0x5d/0xb0
 process_one_work+0x9b0/0x1890
 ? pwq_dec_nr_in_flight+0xed0/0xed0
 ? rpc_final_put_task+0x140/0x140
 worker_thread+0x75a/0x10a0
 ? process_one_work+0x1890/0x1890
 ? kthread+0x1af/0x4d0
 ? process_one_work+0x1890/0x1890
 kthread+0x3d3/0x4d0
 ? kthread_affine_node+0x2c0/0x2c0
 ret_from_fork+0x669/0xa50
 ? native_tss_update_io_bitmap+0x660/0x660
 ? __switch_to+0x9dd/0x1310
 ? kthread_affine_node+0x2c0/0x2c0
 ret_from_fork_asm+0x11/0x20
 </TASK>

Allocated by task 121997 on cpu 3 at 31643.290294s:
 kasan_save_stack+0x1e/0x40
 kasan_save_track+0x13/0x60
 __kasan_slab_alloc+0x62/0x70
 kmem_cache_alloc_noprof+0x1ab/0x4e0
 nfs_page_create+0x152/0x460 [nfs]
 nfs_page_create_from_folio+0x7e/0x210 [nfs]
 nfs_update_folio+0x7a9/0x32a0 [nfs]
 nfs_write_end+0x290/0xc60 [nfs]
 generic_perform_write+0x4ce/0x990
 nfs_file_write+0x6b3/0xce0 [nfs]
 vfs_write+0x63c/0xfa0
 ksys_write+0x122/0x240
 do_syscall_64+0xc3/0x13f0
 entry_SYSCALL_64_after_hwframe+0x4b/0x53

Freed by task 122046 on cpu 0 at 31647.037964s:
 kasan_save_stack+0x1e/0x40
 kasan_save_track+0x13/0x60
 kasan_save_free_info+0x37/0x60
 __kasan_slab_free+0x3b/0x60
 kmem_cache_free+0x11b/0x5a0
 nfs_page_group_destroy+0x13a/0x210 [nfs]
 nfs_unlock_and_release_request+0x64/0x90 [nfs]
 nfs_commit_release_pages+0x339/0xbd0 [nfs]
 nfs_commit_release+0x51/0xb0 [nfs]
 rpc_free_task+0xee/0x160
 rpc_async_release+0x5d/0xb0
 process_one_work+0x9b0/0x1890
 worker_thread+0x75a/0x10a0
 kthread+0x3d3/0x4d0
 ret_from_fork+0x669/0xa50
 ret_from_fork_asm+0x11/0x20

The buggy address belongs to the object at ffff888118af2040\x0a which belongs to the cache nfs_page of size 96
The buggy address is located 24 bytes inside of\x0a freed 96-byte region [ffff888118af2040, ffff888118af20a0)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x118af2
head: order:1 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
flags: 0x4000000000000040(head|zone=2)
page_type: f5(slab)
raw: 4000000000000040 ffff88818cf2c4c0 ffffea000e61b990 ffffea0004e7d110
raw: 0000000000000000 0000000800190019 00000000f5000000 0000000000000000
head: 4000000000000040 ffff88818cf2c4c0 ffffea000e61b990 ffffea0004e7d110
head: 0000000000000000 0000000800190019 00000000f5000000 0000000000000000
head: 4000000000000001 ffffffffffffff81 00000000ffffffff 00000000ffffffff
head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000002
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 1, migratetype Unmovable, gfp_mask 0xd20c0(__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 121997, tgid 121997 (rsync), ts 31643290274577, free_ts 31642154777182
 post_alloc_hook+0xd1/0x100
 get_page_from_freelist+0xbad/0x2910
 __alloc_frozen_pages_noprof+0x1c6/0x4a0
 allocate_slab+0x330/0x620
 ___slab_alloc+0xe9/0x930
 kmem_cache_alloc_noprof+0x35b/0x4e0
 nfs_page_create+0x152/0x460 [nfs]
 nfs_page_create_from_folio+0x7e/0x210 [nfs]
 nfs_update_folio+0x7a9/0x32a0 [nfs]
 nfs_write_end+0x290/0xc60 [nfs]
 generic_perform_write+0x4ce/0x990
 nfs_file_write+0x6b3/0xce0 [nfs]
 vfs_write+0x63c/0xfa0
 ksys_write+0x122/0x240
 do_syscall_64+0xc3/0x13f0
 entry_SYSCALL_64_after_hwframe+0x4b/0x53
page last free pid 122202 tgid 122202 stack trace:
 __free_frozen_pages+0x6da/0xf30
 qlist_free_all+0x53/0x130
 kasan_quarantine_reduce+0x198/0x1f0
 __kasan_slab_alloc+0x46/0x70
 kmem_cache_alloc_noprof+0x1ab/0x4e0
 __alloc_object+0x2f/0x230
 __create_object+0x22/0x80
 kmem_cache_alloc_node_noprof+0x416/0x4d0
 __alloc_skb+0x146/0x6e0
 tcp_stream_alloc_skb+0x35/0x660
 tcp_sendmsg_locked+0x1746/0x4260
 tcp_sendmsg+0x2f/0x40
 inet_sendmsg+0x9e/0xe0
 __sock_sendmsg+0xd9/0x180
 sock_sendmsg+0x122/0x200
 xprt_sock_sendmsg+0x4ff/0x9a0

Memory state around the buggy address:
 ffff888118af1f00: 00 00 00 00 00 00 00 00 00 00 00 00 00 fc fc fc
 ffff888118af1f80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff888118af2000: fc fc fc fc fc fc fc fc fa fb fb fb fb fb fb fb
                                                    ^
 ffff888118af2080: fb fb fb fb fc fc fc fc fc fc fc fc fc fc fc fc
 ffff888118af2100: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================

Reviewed-by Jeff Layton <jlayton@kernel.org>

Fixes: 5d3869a41f36 ("NFS: fix writeback in presence of errors")
Cc: Olga Kornievskaia <okorniev@redhat.com>
Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: Anna Schumaker <anna@kernel.org>
Cc: linux-nfs@vger.kernel.org
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Dave Jones <davej@codemonkey.org.uk>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>