[syzbot] KASAN: use-after-free Read in ucma_destroy_private_ctx

[syzbot] KASAN: use-after-free Read in ucma_destroy_private_ctx

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    a8ad9a2434dc Merge tag 'efi-urgent-for-v5.16-2' of git://g..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=10cf5253b00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=1a86c22260afac2f
dashboard link: https://syzkaller.appspot.com/bug?extid=e3f96c43d19782dd14a7
compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2

Unfortunately, I don't have any reproducer for this issue yet.

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+e3f96c43d19782dd14a7@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: use-after-free in ucma_cleanup_multicast drivers/infiniband/core/ucma.c:491 [inline]
BUG: KASAN: use-after-free in ucma_destroy_private_ctx+0x914/0xb70 drivers/infiniband/core/ucma.c:579
Read of size 8 at addr ffff88801bb74b00 by task syz-executor.1/25529

CPU: 0 PID: 25529 Comm: syz-executor.1 Not tainted 5.16.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
 print_address_description.constprop.0.cold+0x8d/0x320 mm/kasan/report.c:247
 __kasan_report mm/kasan/report.c:433 [inline]
 kasan_report.cold+0x83/0xdf mm/kasan/report.c:450
 ucma_cleanup_multicast drivers/infiniband/core/ucma.c:491 [inline]
 ucma_destroy_private_ctx+0x914/0xb70 drivers/infiniband/core/ucma.c:579
 ucma_destroy_id+0x1e6/0x280 drivers/infiniband/core/ucma.c:614
 ucma_write+0x25c/0x350 drivers/infiniband/core/ucma.c:1732
 vfs_write+0x28e/0xae0 fs/read_write.c:588
 ksys_write+0x1ee/0x250 fs/read_write.c:643
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x7f2fcd207e99
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 bc ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f2fcbb7d168 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f2fcd31af60 RCX: 00007f2fcd207e99
RDX: 0000000000000018 RSI: 00000000200000c0 RDI: 0000000000000004
RBP: 00007f2fcd261ff1 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fff66a135bf R14: 00007f2fcbb7d300 R15: 0000000000022000
 </TASK>

Allocated by task 25524:
 kasan_save_stack+0x1e/0x50 mm/kasan/common.c:38
 kasan_set_track mm/kasan/common.c:46 [inline]
 set_alloc_info mm/kasan/common.c:434 [inline]
 ____kasan_kmalloc mm/kasan/common.c:513 [inline]
 ____kasan_kmalloc mm/kasan/common.c:472 [inline]
 __kasan_kmalloc+0xa9/0xd0 mm/kasan/common.c:522
 kmalloc include/linux/slab.h:590 [inline]
 kzalloc include/linux/slab.h:724 [inline]
 ucma_process_join+0x22a/0x6f0 drivers/infiniband/core/ucma.c:1461
 ucma_join_multicast+0xd5/0x140 drivers/infiniband/core/ucma.c:1546
 ucma_write+0x25c/0x350 drivers/infiniband/core/ucma.c:1732
 vfs_write+0x28e/0xae0 fs/read_write.c:588
 ksys_write+0x1ee/0x250 fs/read_write.c:643
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Freed by task 25521:
 kasan_save_stack+0x1e/0x50 mm/kasan/common.c:38
 kasan_set_track+0x21/0x30 mm/kasan/common.c:46
 kasan_set_free_info+0x20/0x30 mm/kasan/generic.c:370
 ____kasan_slab_free mm/kasan/common.c:366 [inline]
 ____kasan_slab_free mm/kasan/common.c:328 [inline]
 __kasan_slab_free+0xff/0x130 mm/kasan/common.c:374
 kasan_slab_free include/linux/kasan.h:235 [inline]
 slab_free_hook mm/slub.c:1723 [inline]
 slab_free_freelist_hook+0x8b/0x1c0 mm/slub.c:1749
 slab_free mm/slub.c:3513 [inline]
 kfree+0xf6/0x560 mm/slub.c:4561
 ucma_cleanup_multicast drivers/infiniband/core/ucma.c:498 [inline]
 ucma_destroy_private_ctx+0x7a3/0xb70 drivers/infiniband/core/ucma.c:579
 ucma_close+0x10a/0x180 drivers/infiniband/core/ucma.c:1797
 __fput+0x286/0x9f0 fs/file_table.c:280
 task_work_run+0xdd/0x1a0 kernel/task_work.c:164
 tracehook_notify_resume include/linux/tracehook.h:189 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:175 [inline]
 exit_to_user_mode_prepare+0x27e/0x290 kernel/entry/common.c:207
 __syscall_exit_to_user_mode_work kernel/entry/common.c:289 [inline]
 syscall_exit_to_user_mode+0x19/0x60 kernel/entry/common.c:300
 do_syscall_64+0x42/0xb0 arch/x86/entry/common.c:86
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Last potentially related work creation:
 kasan_save_stack+0x1e/0x50 mm/kasan/common.c:38
 __kasan_record_aux_stack+0xf5/0x120 mm/kasan/generic.c:348
 insert_work+0x48/0x370 kernel/workqueue.c:1354
 __queue_work+0x5ca/0xee0 kernel/workqueue.c:1520
 queue_work_on+0xee/0x110 kernel/workqueue.c:1547
 cma_resolve_loopback drivers/infiniband/core/cma.c:3330 [inline]
 rdma_resolve_addr drivers/infiniband/core/cma.c:3433 [inline]
 rdma_resolve_addr+0x86d/0x2300 drivers/infiniband/core/cma.c:3421
 ucma_resolve_ip+0x14e/0x200 drivers/infiniband/core/ucma.c:692
 ucma_write+0x25c/0x350 drivers/infiniband/core/ucma.c:1732
 vfs_write+0x28e/0xae0 fs/read_write.c:588
 ksys_write+0x1ee/0x250 fs/read_write.c:643
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Second to last potentially related work creation:
 kasan_save_stack+0x1e/0x50 mm/kasan/common.c:38
 __kasan_record_aux_stack+0xf5/0x120 mm/kasan/generic.c:348
 insert_work+0x48/0x370 kernel/workqueue.c:1354
 __queue_work+0x5ca/0xee0 kernel/workqueue.c:1520
 queue_work_on+0xee/0x110 kernel/workqueue.c:1547
 cma_resolve_loopback drivers/infiniband/core/cma.c:3330 [inline]
 rdma_resolve_addr drivers/infiniband/core/cma.c:3433 [inline]
 rdma_resolve_addr+0x86d/0x2300 drivers/infiniband/core/cma.c:3421
 ucma_resolve_ip+0x14e/0x200 drivers/infiniband/core/ucma.c:692
 ucma_write+0x25c/0x350 drivers/infiniband/core/ucma.c:1732
 vfs_write+0x28e/0xae0 fs/read_write.c:588
 ksys_write+0x1ee/0x250 fs/read_write.c:643
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

The buggy address belongs to the object at ffff88801bb74b00
 which belongs to the cache kmalloc-192 of size 192
The buggy address is located 0 bytes inside of
 192-byte region [ffff88801bb74b00, ffff88801bb74bc0)
The buggy address belongs to the page:
page:ffffea00006edd00 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x1bb74
flags: 0xfff00000000200(slab|node=0|zone=1|lastcpupid=0x7ff)
raw: 00fff00000000200 dead000000000100 dead000000000122 ffff888010c41a00
raw: 0000000000000000 0000000000100010 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 0, migratetype Unmovable, gfp_mask 0x12c00(GFP_NOIO|__GFP_NOWARN|__GFP_NORETRY), pid 50, ts 7738663409, free_ts 7737390927
 prep_new_page mm/page_alloc.c:2418 [inline]
 get_page_from_freelist+0xa72/0x2f50 mm/page_alloc.c:4149
 __alloc_pages+0x1b2/0x500 mm/page_alloc.c:5369
 __alloc_pages_node include/linux/gfp.h:570 [inline]
 alloc_slab_page mm/slub.c:1795 [inline]
 allocate_slab mm/slub.c:1930 [inline]
 new_slab+0xab/0x4a0 mm/slub.c:1993
 ___slab_alloc+0x918/0xfe0 mm/slub.c:3022
 __slab_alloc.constprop.0+0x4d/0xa0 mm/slub.c:3109
 slab_alloc_node mm/slub.c:3200 [inline]
 __kmalloc_node+0x2cb/0x390 mm/slub.c:4467
 kmalloc_array_node include/linux/slab.h:686 [inline]
 kcalloc_node include/linux/slab.h:691 [inline]
 sbitmap_init_node+0x1c3/0x6f0 lib/sbitmap.c:114
 blk_mq_alloc_hctx block/blk-mq.c:3214 [inline]
 blk_mq_alloc_and_init_hctx block/blk-mq.c:3620 [inline]
 blk_mq_realloc_hw_ctxs+0xc20/0x1530 block/blk-mq.c:3672
 blk_mq_init_allocated_queue+0x324/0x12c0 block/blk-mq.c:3731
 blk_mq_init_queue_data block/blk-mq.c:3568 [inline]
 blk_mq_init_queue+0x75/0xd0 block/blk-mq.c:3578
 scsi_alloc_sdev+0x814/0xd60 drivers/scsi/scsi_scan.c:288
 scsi_probe_and_add_lun+0x200b/0x3590 drivers/scsi/scsi_scan.c:1122
 __scsi_scan_target+0x21f/0xdb0 drivers/scsi/scsi_scan.c:1604
 scsi_scan_channel drivers/scsi/scsi_scan.c:1692 [inline]
 scsi_scan_channel+0x148/0x1e0 drivers/scsi/scsi_scan.c:1668
 scsi_scan_host_selected+0x2df/0x3b0 drivers/scsi/scsi_scan.c:1721
 do_scsi_scan_host+0x1e8/0x260 drivers/scsi/scsi_scan.c:1860
page last free stack trace:
 reset_page_owner include/linux/page_owner.h:24 [inline]
 free_pages_prepare mm/page_alloc.c:1338 [inline]
 free_pcp_prepare+0x374/0x870 mm/page_alloc.c:1389
 free_unref_page_prepare mm/page_alloc.c:3309 [inline]
 free_unref_page+0x19/0x690 mm/page_alloc.c:3388
 __vunmap+0x781/0xb70 mm/vmalloc.c:2632
 free_work+0x58/0x70 mm/vmalloc.c:95
 process_one_work+0x9b2/0x1660 kernel/workqueue.c:2298
 worker_thread+0x65d/0x1130 kernel/workqueue.c:2445
 kthread+0x405/0x4f0 kernel/kthread.c:327
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

Memory state around the buggy address:
 ffff88801bb74a00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff88801bb74a80: 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc fc
>ffff88801bb74b00: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                   ^
 ffff88801bb74b80: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
 ffff88801bb74c00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
==================================================================


---
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Re: [syzbot] KASAN: use-after-free Read in ucma_destroy_private_ctx

[Leon Romanovsky]

On Mon, Jan 03, 2022 at 09:05:16AM -0800, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    a8ad9a2434dc Merge tag 'efi-urgent-for-v5.16-2' of git://g..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=10cf5253b00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=1a86c22260afac2f
> dashboard link: https://syzkaller.appspot.com/bug?extid=e3f96c43d19782dd14a7
> compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+e3f96c43d19782dd14a7@syzkaller.appspotmail.com
> 
> ==================================================================
> BUG: KASAN: use-after-free in ucma_cleanup_multicast drivers/infiniband/core/ucma.c:491 [inline]
> BUG: KASAN: use-after-free in ucma_destroy_private_ctx+0x914/0xb70 drivers/infiniband/core/ucma.c:579
> Read of size 8 at addr ffff88801bb74b00 by task syz-executor.1/25529

Jason,

Can it be race between ucma_process_join() and "if (refcount_read(&ctx->ref))"
check in ucma_destroy_private_ctx()?

The ucma_process_join() grabbed ctx, but released it in error path,
while ucma_destroy_private_ctx() was called without holding any locks?

Thanks

Re: [syzbot] KASAN: use-after-free Read in ucma_destroy_private_ctx

[Leon Romanovsky]

On Wed, Jan 05, 2022 at 07:45:21PM +0800, Hillf Danton wrote:
> On Mon, 3 Jan 2022 20:47:02 +0200 Leon Romanovsky wrote:
> > On Mon, Jan 03, 2022 at 09:05:16AM -0800, syzbot wrote:
> > > Hello,
> > > 
> > > syzbot found the following issue on:
> > > 
> > > HEAD commit:    a8ad9a2434dc Merge tag 'efi-urgent-for-v5.16-2' of git://g..
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=10cf5253b00000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=1a86c22260afac2f
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=e3f96c43d19782dd14a7
> > > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > > 
> > > Unfortunately, I don't have any reproducer for this issue yet.
> > > 
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+e3f96c43d19782dd14a7@syzkaller.appspotmail.com
> > > 
> > > ==================================================================
> > > BUG: KASAN: use-after-free in ucma_cleanup_multicast drivers/infiniband/core/ucma.c:491 [inline]
> > > BUG: KASAN: use-after-free in ucma_destroy_private_ctx+0x914/0xb70 drivers/infiniband/core/ucma.c:579
> > > Read of size 8 at addr ffff88801bb74b00 by task syz-executor.1/25529
> > 
> > Jason,
> > 
> > Can it be race between ucma_process_join() and "if (refcount_read(&ctx->ref))"
> > check in ucma_destroy_private_ctx()?
> 
> Given cmpxchg in both ucma_close() and ucma_destroy_id(),
> ucma_destroy_private_ctx() can not run more than once, in addition to what
> is more weird is that the ucma_fops.release either is running in parallel
> to a writer or completes with a writer left behind. Light on if that weirdness
> is down to anything other than syzbot is highly appreciated.

Unless someone can shed the light, the repro will help here/

> 
> 	Hillf
> > 
> > The ucma_process_join() grabbed ctx, but released it in error path,
> > while ucma_destroy_private_ctx() was called without holding any locks?
> > 
> > Thanks
> > 

Re: [syzbot] KASAN: use-after-free Read in ucma_destroy_private_ctx

[Jason Gunthorpe]

On Wed, Jan 05, 2022 at 07:45:21PM +0800, Hillf Danton wrote:
> On Mon, 3 Jan 2022 20:47:02 +0200 Leon Romanovsky wrote:
> > On Mon, Jan 03, 2022 at 09:05:16AM -0800, syzbot wrote:
> > > Hello,
> > > 
> > > syzbot found the following issue on:
> > > 
> > > HEAD commit:    a8ad9a2434dc Merge tag 'efi-urgent-for-v5.16-2' of git://g..
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=10cf5253b00000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=1a86c22260afac2f
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=e3f96c43d19782dd14a7
> > > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > > 
> > > Unfortunately, I don't have any reproducer for this issue yet.
> > > 
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+e3f96c43d19782dd14a7@syzkaller.appspotmail.com
> > > 
> > > ==================================================================
> > > BUG: KASAN: use-after-free in ucma_cleanup_multicast drivers/infiniband/core/ucma.c:491 [inline]
> > > BUG: KASAN: use-after-free in ucma_destroy_private_ctx+0x914/0xb70 drivers/infiniband/core/ucma.c:579
> > > Read of size 8 at addr ffff88801bb74b00 by task syz-executor.1/25529
> > 
> > Jason,
> > 
> > Can it be race between ucma_process_join() and "if (refcount_read(&ctx->ref))"
> > check in ucma_destroy_private_ctx()?
> 
> Given cmpxchg in both ucma_close() and ucma_destroy_id(),
> ucma_destroy_private_ctx() can not run more than once, in addition to what
> is more weird is that the ucma_fops.release either is running in parallel
> to a writer or completes with a writer left behind. Light on if that weirdness
> is down to anything other than syzbot is highly appreciated.

It is a stupid mistake, it is because

	xa_for_each(&multicast_table, index, mc) {
		if (mc->ctx != ctx)
                   ^^^^^^^

Nothing is locking mc here, this needed to hold the xa_lock to be
correct.

It is caused by this:

commit 95fe51096b7adf1d1e7315c49c75e2f75f162584
Author: Jason Gunthorpe <jgg@ziepe.ca>
Date:   Tue Aug 18 15:05:17 2020 +0300

    RDMA/ucma: Remove mc_list and rely on xarray
    
    It is not really necessary to keep a linked list of mcs associated with
    each context when we can just scan the xarray to find the right things.
    
    The removes another overloading of file->mut by relying on the xarray
    locking for mc instead.
    
    Link: https://lore.kernel.org/r/20200818120526.702120-6-leon@kernel.org
    Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
    Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>


So it is solved either by putting the linked list back, but locking it
using the xa_lock, or by holding the xa_lock when doing the
xa_for_each()

A list is probably the better choice.

Jason

Re: [syzbot] KASAN: use-after-free Read in ucma_destroy_private_ctx

[Leon Romanovsky]

On Fri, Jan 07, 2022 at 08:10:33PM -0400, Jason Gunthorpe wrote:
> On Wed, Jan 05, 2022 at 07:45:21PM +0800, Hillf Danton wrote:
> > On Mon, 3 Jan 2022 20:47:02 +0200 Leon Romanovsky wrote:
> > > On Mon, Jan 03, 2022 at 09:05:16AM -0800, syzbot wrote:
> > > > Hello,
> > > > 
> > > > syzbot found the following issue on:
> > > > 
> > > > HEAD commit:    a8ad9a2434dc Merge tag 'efi-urgent-for-v5.16-2' of git://g..
> > > > git tree:       upstream
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=10cf5253b00000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=1a86c22260afac2f
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=e3f96c43d19782dd14a7
> > > > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > > > 
> > > > Unfortunately, I don't have any reproducer for this issue yet.
> > > > 
> > > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > > Reported-by: syzbot+e3f96c43d19782dd14a7@syzkaller.appspotmail.com
> > > > 
> > > > ==================================================================
> > > > BUG: KASAN: use-after-free in ucma_cleanup_multicast drivers/infiniband/core/ucma.c:491 [inline]
> > > > BUG: KASAN: use-after-free in ucma_destroy_private_ctx+0x914/0xb70 drivers/infiniband/core/ucma.c:579
> > > > Read of size 8 at addr ffff88801bb74b00 by task syz-executor.1/25529
> > > 
> > > Jason,
> > > 
> > > Can it be race between ucma_process_join() and "if (refcount_read(&ctx->ref))"
> > > check in ucma_destroy_private_ctx()?
> > 
> > Given cmpxchg in both ucma_close() and ucma_destroy_id(),
> > ucma_destroy_private_ctx() can not run more than once, in addition to what
> > is more weird is that the ucma_fops.release either is running in parallel
> > to a writer or completes with a writer left behind. Light on if that weirdness
> > is down to anything other than syzbot is highly appreciated.
> 
> It is a stupid mistake, it is because
> 
> 	xa_for_each(&multicast_table, index, mc) {
> 		if (mc->ctx != ctx)
>                    ^^^^^^^
> 
> Nothing is locking mc here, this needed to hold the xa_lock to be
> correct.
> 
> It is caused by this:
> 
> commit 95fe51096b7adf1d1e7315c49c75e2f75f162584
> Author: Jason Gunthorpe <jgg@ziepe.ca>
> Date:   Tue Aug 18 15:05:17 2020 +0300
> 
>     RDMA/ucma: Remove mc_list and rely on xarray
>     
>     It is not really necessary to keep a linked list of mcs associated with
>     each context when we can just scan the xarray to find the right things.
>     
>     The removes another overloading of file->mut by relying on the xarray
>     locking for mc instead.
>     
>     Link: https://lore.kernel.org/r/20200818120526.702120-6-leon@kernel.org
>     Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
>     Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> 
> 
> So it is solved either by putting the linked list back, but locking it
> using the xa_lock, or by holding the xa_lock when doing the
> xa_for_each()
> 
> A list is probably the better choice.

I'll submit the fix after internal review complete.

Thanks

> 
> Jason