Re: [syzbot] [kvm?] [kvm-x86?] KASAN: use-after-free Read in kvm_xen_shared_info_init

Re: [syzbot] [kvm?] [kvm-x86?] KASAN: use-after-free Read in kvm_xen_shared_info_init

[syzbot]

syzbot has found a reproducer for the following issue on:

HEAD commit:    c21bb4193868 Merge tag 'for_linus' of git://git.kernel.org..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=12ceecc6580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=145fa60d73086782
dashboard link: https://syzkaller.appspot.com/bug?extid=0948c82180d475ad24e2
compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16080e49580000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+0948c82180d475ad24e2@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: use-after-free in kvm_xen_shared_info_init+0x3c6/0x440 arch/x86/kvm/xen.c:90
Read of size 4 at addr ffff8880719c4900 by task syz.4.159/7379

CPU: 0 UID: 0 PID: 7379 Comm: syz.4.159 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
 print_address_description mm/kasan/report.c:378 [inline]
 print_report+0x13d/0x4b0 mm/kasan/report.c:482
 kasan_report+0xdf/0x1c0 mm/kasan/report.c:595
 kvm_xen_shared_info_init+0x3c6/0x440 arch/x86/kvm/xen.c:90
 kvm_xen_hvm_set_attr+0xcef/0x16a0 arch/x86/kvm/xen.c:806
 kvm_arch_vm_ioctl+0x2a0/0x18d0 arch/x86/kvm/x86.c:7514
 kvm_vm_ioctl+0x1560/0x4180 virt/kvm/kvm_main.c:5381
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl fs/ioctl.c:583 [inline]
 __x64_sys_ioctl+0x18e/0x210 fs/ioctl.c:583
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x115/0x870 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f11c5d9e019
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 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 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f11c6d15028 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f11c6026090 RCX: 00007f11c5d9e019
RDX: 0000200000000840 RSI: 000000004048aec9 RDI: 0000000000000004
RBP: 00007f11c5e3500c R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f11c6026128 R14: 00007f11c6026090 R15: 00007ffd79c7b7b8
 </TASK>

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0xe pfn:0x719c4
flags: 0xfff00000000000(node=0|zone=1|lastcpupid=0x7ff)
raw: 00fff00000000000 ffffea0001865f08 ffff8880b84414b0 0000000000000000
raw: 000000000000000e 0000000000000000 00000000ffffffff 0000000000000000
page dumped because: kasan: bad access detected
page_owner tracks the page as freed
page last allocated via order 0, migratetype Movable, gfp_mask 0x140cca(GFP_HIGHUSER_MOVABLE|__GFP_COMP), pid 7379, tgid 7374 (syz.4.159), ts 201002456742, free_ts 201021576054
 set_page_owner include/linux/page_owner.h:32 [inline]
 post_alloc_hook+0xfd/0x120 mm/page_alloc.c:1859
 prep_new_page mm/page_alloc.c:1867 [inline]
 get_page_from_freelist+0xf48/0x3530 mm/page_alloc.c:3946
 __alloc_frozen_pages_noprof+0x299/0x2dc0 mm/page_alloc.c:5304
 alloc_pages_mpol+0x1fb/0x540 mm/mempolicy.c:2490
 folio_alloc_mpol_noprof+0x36/0x260 mm/mempolicy.c:2509
 shmem_alloc_folio+0x135/0x160 mm/shmem.c:1917
 shmem_alloc_and_add_folio+0x371/0xd40 mm/shmem.c:1959
 shmem_get_folio_gfp+0x6ad/0x1910 mm/shmem.c:2512
 shmem_fault+0x1f9/0xa20 mm/shmem.c:2713
 __do_fault+0x10b/0x440 mm/memory.c:5425
 do_shared_fault mm/memory.c:5924 [inline]
 do_fault+0x2db/0x1750 mm/memory.c:5998
 do_pte_missing mm/memory.c:4566 [inline]
 handle_pte_fault mm/memory.c:6379 [inline]
 __handle_mm_fault+0x187d/0x2a00 mm/memory.c:6517
 handle_mm_fault+0x37b/0xa30 mm/memory.c:6686
 faultin_page mm/gup.c:1126 [inline]
 __get_user_pages+0x1178/0x32a0 mm/gup.c:1428
 __get_user_pages_locked mm/gup.c:1752 [inline]
 get_user_pages_unlocked+0x30a/0x7d0 mm/gup.c:2681
 hva_to_pfn_slow virt/kvm/kvm_main.c:2903 [inline]
 hva_to_pfn+0x871/0xd60 virt/kvm/kvm_main.c:2999
page last free pid 7379 tgid 7374 stack trace:
 reset_page_owner include/linux/page_owner.h:25 [inline]
 __free_pages_prepare mm/page_alloc.c:1406 [inline]
 __free_frozen_pages+0x79f/0x1090 mm/page_alloc.c:2950
 __folio_put+0x3b4/0x5f0 mm/swap.c:112
 folio_put include/linux/mm.h:2124 [inline]
 put_page include/linux/mm.h:2193 [inline]
 kvm_release_page_clean virt/kvm/kvm_main.c:2813 [inline]
 kvm_release_page_clean+0x1dc/0x250 virt/kvm/kvm_main.c:2807
 hva_to_pfn_retry virt/kvm/pfncache.c:246 [inline]
 __kvm_gpc_refresh+0x1a63/0x22d0 virt/kvm/pfncache.c:330
 __kvm_gpc_activate+0x2ab/0x490 virt/kvm/pfncache.c:424
 kvm_gpc_activate_hva+0x73/0xa0 virt/kvm/pfncache.c:444
 kvm_xen_hvm_set_attr+0x395/0x16a0 arch/x86/kvm/xen.c:798
 kvm_arch_vm_ioctl+0x2a0/0x18d0 arch/x86/kvm/x86.c:7514
 kvm_vm_ioctl+0x1560/0x4180 virt/kvm/kvm_main.c:5381
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl fs/ioctl.c:583 [inline]
 __x64_sys_ioctl+0x18e/0x210 fs/ioctl.c:583
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x115/0x870 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Memory state around the buggy address:
 ffff8880719c4800: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff8880719c4880: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>ffff8880719c4900: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                   ^
 ffff8880719c4980: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff8880719c4a00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
==================================================================


---
If you want syzbot to run the reproducer, reply with:
#syz test: git://repo/address.git branch-or-commit-hash
If you attach or paste a git patch, syzbot will apply it before testing.

Re: [syzbot] [kvm?] [kvm-x86?] KASAN: use-after-free Read in kvm_xen_shared_info_init

[David Woodhouse]

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On Wed, 2026-08-05 at 07:01 -0700, syzbot wrote:
> syzbot has found a reproducer for the following issue on:
> 
> HEAD commit:    c21bb4193868 Merge tag 'for_linus' of git://git.kernel.org..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=12ceecc6580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=145fa60d73086782
> dashboard link: https://syzkaller.appspot.com/bug?extid=0948c82180d475ad24e2
> compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16080e49580000
> 

#syz test: git://git.infradead.org/users/dwmw2/linux.git pfncache-fixes

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Re: [syzbot] [kvm?] [kvm-x86?] KASAN: use-after-free Read in kvm_xen_shared_info_init

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+0948c82180d475ad24e2@syzkaller.appspotmail.com
Tested-by: syzbot+0948c82180d475ad24e2@syzkaller.appspotmail.com

Tested on:

commit:         bd2b92fd [NOT-FOR-UPSTREAM] repro: pfncache HVA-invali..
git tree:       git://git.infradead.org/users/dwmw2/linux.git pfncache-fixes
console output: https://syzkaller.appspot.com/x/log.txt?x=10362bb9580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=145fa60d73086782
dashboard link: https://syzkaller.appspot.com/bug?extid=0948c82180d475ad24e2
compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44

Note: no patches were applied.
Note: testing is done by a robot and is best-effort only.

Re: [syzbot] [kvm?] [kvm-x86?] KASAN: use-after-free Read in kvm_xen_shared_info_init

[David Woodhouse]

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On Wed, 2026-08-05 at 08:46 -0700, syzbot wrote:
> Hello,
> 
> syzbot has tested the proposed patch and the reproducer did not trigger any issue:
> 
> Reported-by: syzbot+0948c82180d475ad24e2@syzkaller.appspotmail.com
> Tested-by: syzbot+0948c82180d475ad24e2@syzkaller.appspotmail.com
> 
> Tested on:
> 
> commit:         bd2b92fd [NOT-FOR-UPSTREAM] repro: pfncache HVA-invali..
> git tree:       git://git.infradead.org/users/dwmw2/linux.git pfncache-fixes
> console output: https://syzkaller.appspot.com/x/log.txt?x=10362bb9580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=145fa60d73086782
> dashboard link: https://syzkaller.appspot.com/bug?extid=0948c82180d475ad24e2
> compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
> 
> Note: no patches were applied.
> Note: testing is done by a robot and is best-effort only.

#syz test: git://git.infradead.org/users/dwmw2/linux.git pfncache-rcu

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Re: [syzbot] [kvm?] [kvm-x86?] KASAN: use-after-free Read in kvm_xen_shared_info_init

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+0948c82180d475ad24e2@syzkaller.appspotmail.com
Tested-by: syzbot+0948c82180d475ad24e2@syzkaller.appspotmail.com

Tested on:

commit:         b020b491 KVM: x86: Use gfn_to_pfn_cache for steal time..
git tree:       git://git.infradead.org/users/dwmw2/linux.git pfncache-rcu
console output: https://syzkaller.appspot.com/x/log.txt?x=14be3bb9580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=145fa60d73086782
dashboard link: https://syzkaller.appspot.com/bug?extid=0948c82180d475ad24e2
compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44

Note: no patches were applied.
Note: testing is done by a robot and is best-effort only.