[syzbot] [kvm?] KASAN: slab-use-after-free Write in kvm_gmem_release

[syzbot] [kvm?] KASAN: slab-use-after-free Write in kvm_gmem_release

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    aaa9c3550b60 Add linux-next specific files for 20251022
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=1021c3e2580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=ad8f5526e5acd067
dashboard link: https://syzkaller.appspot.com/bug?extid=2479e53d0db9b32ae2aa
compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=10dd0c92580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1152c614580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/2fe0a8f92a64/disk-aaa9c355.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/199d2e804802/vmlinux-aaa9c355.xz
kernel image: https://storage.googleapis.com/syzbot-assets/9371d55e359e/bzImage-aaa9c355.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+2479e53d0db9b32ae2aa@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: slab-use-after-free in kvm_gmem_release+0x176/0x440 virt/kvm/guest_memfd.c:353
Write of size 8 at addr ffff88807befa508 by task syz.0.17/6022

CPU: 0 UID: 0 PID: 6022 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
Call Trace:
 <TASK>
 dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
 print_address_description mm/kasan/report.c:378 [inline]
 print_report+0xca/0x240 mm/kasan/report.c:482
 kasan_report+0x118/0x150 mm/kasan/report.c:595
 kvm_gmem_release+0x176/0x440 virt/kvm/guest_memfd.c:353
 __fput+0x44c/0xa70 fs/file_table.c:468
 task_work_run+0x1d4/0x260 kernel/task_work.c:227
 resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
 exit_to_user_mode_loop+0xe9/0x130 kernel/entry/common.c:43
 exit_to_user_mode_prepare include/linux/irq-entry-common.h:225 [inline]
 syscall_exit_to_user_mode_work include/linux/entry-common.h:175 [inline]
 syscall_exit_to_user_mode include/linux/entry-common.h:210 [inline]
 do_syscall_64+0x2bd/0xfa0 arch/x86/entry/syscall_64.c:100
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fbeeff8efc9
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 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffffdf1e2c8 EFLAGS: 00000246 ORIG_RAX: 00000000000001b4
RAX: 0000000000000000 RBX: 00007fbef01e7da0 RCX: 00007fbeeff8efc9
RDX: 0000000000000000 RSI: 000000000000001e RDI: 0000000000000003
RBP: 00007fbef01e7da0 R08: 0000000000000000 R09: 00000004fdf1e5bf
R10: 00007fbef01e7cb0 R11: 0000000000000246 R12: 0000000000019ff7
R13: 00007ffffdf1e3c0 R14: ffffffffffffffff R15: 00007ffffdf1e3e0
 </TASK>

Allocated by task 6023:
 kasan_save_stack mm/kasan/common.c:56 [inline]
 kasan_save_track+0x3e/0x80 mm/kasan/common.c:77
 poison_kmalloc_redzone mm/kasan/common.c:397 [inline]
 __kasan_kmalloc+0x93/0xb0 mm/kasan/common.c:414
 kasan_kmalloc include/linux/kasan.h:262 [inline]
 __kmalloc_cache_noprof+0x3e2/0x700 mm/slub.c:5758
 kmalloc_noprof include/linux/slab.h:957 [inline]
 kzalloc_noprof include/linux/slab.h:1094 [inline]
 kvm_set_memory_region+0x747/0xb90 virt/kvm/kvm_main.c:2104
 kvm_vm_ioctl_set_memory_region+0x6f/0xd0 virt/kvm/kvm_main.c:2154
 kvm_vm_ioctl+0x957/0xc60 virt/kvm/kvm_main.c:5201
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Freed by task 6023:
 kasan_save_stack mm/kasan/common.c:56 [inline]
 kasan_save_track+0x3e/0x80 mm/kasan/common.c:77
 kasan_save_free_info+0x46/0x50 mm/kasan/generic.c:584
 poison_slab_object mm/kasan/common.c:252 [inline]
 __kasan_slab_free+0x5c/0x80 mm/kasan/common.c:284
 kasan_slab_free include/linux/kasan.h:234 [inline]
 slab_free_hook mm/slub.c:2533 [inline]
 slab_free mm/slub.c:6622 [inline]
 kfree+0x19a/0x6d0 mm/slub.c:6829
 kvm_set_memory_region+0x9c4/0xb90 virt/kvm/kvm_main.c:2130
 kvm_vm_ioctl_set_memory_region+0x6f/0xd0 virt/kvm/kvm_main.c:2154
 kvm_vm_ioctl+0x957/0xc60 virt/kvm/kvm_main.c:5201
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

The buggy address belongs to the object at ffff88807befa400
 which belongs to the cache kmalloc-cg-512 of size 512
The buggy address is located 264 bytes inside of
 freed 512-byte region [ffff88807befa400, ffff88807befa600)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x7bef8
head: order:2 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
memcg:ffff888077513701
anon flags: 0xfff00000000040(head|node=0|zone=1|lastcpupid=0x7ff)
page_type: f5(slab)
raw: 00fff00000000040 ffff88813fe30140 0000000000000000 dead000000000001
raw: 0000000000000000 0000000000100010 00000000f5000000 ffff888077513701
head: 00fff00000000040 ffff88813fe30140 0000000000000000 dead000000000001
head: 0000000000000000 0000000000100010 00000000f5000000 ffff888077513701
head: 00fff00000000002 ffffea0001efbe01 00000000ffffffff 00000000ffffffff
head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000004
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 2, migratetype Unmovable, gfp_mask 0xd20c0(__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 5203, tgid 5203 (udevd), ts 35020479428, free_ts 30283368313
 set_page_owner include/linux/page_owner.h:32 [inline]
 post_alloc_hook+0x240/0x2a0 mm/page_alloc.c:1851
 prep_new_page mm/page_alloc.c:1859 [inline]
 get_page_from_freelist+0x2365/0x2440 mm/page_alloc.c:3920
 __alloc_frozen_pages_noprof+0x181/0x370 mm/page_alloc.c:5214
 alloc_pages_mpol+0x232/0x4a0 mm/mempolicy.c:2418
 alloc_slab_page mm/slub.c:3049 [inline]
 allocate_slab+0x96/0x3a0 mm/slub.c:3222
 new_slab mm/slub.c:3276 [inline]
 ___slab_alloc+0xe94/0x18a0 mm/slub.c:4646
 __slab_alloc+0x65/0x100 mm/slub.c:4765
 __slab_alloc_node mm/slub.c:4841 [inline]
 slab_alloc_node mm/slub.c:5263 [inline]
 __do_kmalloc_node mm/slub.c:5636 [inline]
 __kmalloc_node_track_caller_noprof+0x5d0/0x810 mm/slub.c:5746
 kmalloc_reserve+0x136/0x290 net/core/skbuff.c:604
 __alloc_skb+0x142/0x2d0 net/core/skbuff.c:673
 alloc_skb include/linux/skbuff.h:1383 [inline]
 alloc_skb_with_frags+0xca/0x890 net/core/skbuff.c:6685
 sock_alloc_send_pskb+0x84d/0x980 net/core/sock.c:2987
 unix_dgram_sendmsg+0x461/0x1850 net/unix/af_unix.c:2153
 sock_sendmsg_nosec net/socket.c:727 [inline]
 __sock_sendmsg+0x21c/0x270 net/socket.c:742
 sock_write_iter+0x279/0x360 net/socket.c:1195
 new_sync_write fs/read_write.c:593 [inline]
 vfs_write+0x5c9/0xb30 fs/read_write.c:686
page last free pid 1 tgid 1 stack trace:
 reset_page_owner include/linux/page_owner.h:25 [inline]
 free_pages_prepare mm/page_alloc.c:1395 [inline]
 __free_frozen_pages+0xbc8/0xd30 mm/page_alloc.c:2948
 __free_pages mm/page_alloc.c:5333 [inline]
 free_contig_range+0x1bd/0x4a0 mm/page_alloc.c:7176
 destroy_args+0x69/0x660 mm/debug_vm_pgtable.c:958
 debug_vm_pgtable+0x39f/0x3b0 mm/debug_vm_pgtable.c:1345
 do_one_initcall+0x236/0x820 init/main.c:1283
 do_initcall_level+0x104/0x190 init/main.c:1345
 do_initcalls+0x59/0xa0 init/main.c:1361
 kernel_init_freeable+0x334/0x4b0 init/main.c:1593
 kernel_init+0x1d/0x1d0 init/main.c:1483
 ret_from_fork+0x4bc/0x870 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

Memory state around the buggy address:
 ffff88807befa400: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff88807befa480: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff88807befa500: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                      ^
 ffff88807befa580: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff88807befa600: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
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If the report is already addressed, let syzbot know by replying with:
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If you want syzbot to run the reproducer, reply with:
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#syz undup

Re: [syzbot] [kvm?] KASAN: slab-use-after-free Write in kvm_gmem_release

[syzbot]

syzbot has bisected this issue to:

commit d1e54dd08f163a9021433020d16a8f8f70ddc41c
Author: Fuad Tabba <tabba@google.com>
Date:   Tue Jul 29 22:54:40 2025 +0000

    KVM: x86: Enable KVM_GUEST_MEMFD for all 64-bit builds

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=12a663cd980000
start commit:   43e9ad0c55a3 Merge tag 'scsi-fixes' of git://git.kernel.or..
git tree:       upstream
final oops:     https://syzkaller.appspot.com/x/report.txt?x=11a663cd980000
console output: https://syzkaller.appspot.com/x/log.txt?x=16a663cd980000
kernel config:  https://syzkaller.appspot.com/x/.config?x=67b63a24f3c26fca
dashboard link: https://syzkaller.appspot.com/bug?extid=2479e53d0db9b32ae2aa
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=173ecd2f980000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14bc2be2580000

Reported-by: syzbot+2479e53d0db9b32ae2aa@syzkaller.appspotmail.com
Fixes: d1e54dd08f16 ("KVM: x86: Enable KVM_GUEST_MEMFD for all 64-bit builds")

For information about bisection process see: https://goo.gl/tpsmEJ#bisection

Re: [syzbot] [kvm?] KASAN: slab-use-after-free Write in kvm_gmem_release

[Sean Christopherson]

On Thu, Oct 23, 2025, syzbot wrote:
> syzbot has bisected this issue to:
> 
> commit d1e54dd08f163a9021433020d16a8f8f70ddc41c
> Author: Fuad Tabba <tabba@google.com>
> Date:   Tue Jul 29 22:54:40 2025 +0000
> 
>     KVM: x86: Enable KVM_GUEST_MEMFD for all 64-bit builds
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=12a663cd980000
> start commit:   43e9ad0c55a3 Merge tag 'scsi-fixes' of git://git.kernel.or..
> git tree:       upstream
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=11a663cd980000
> console output: https://syzkaller.appspot.com/x/log.txt?x=16a663cd980000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=67b63a24f3c26fca
> dashboard link: https://syzkaller.appspot.com/bug?extid=2479e53d0db9b32ae2aa
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=173ecd2f980000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14bc2be2580000
> 
> Reported-by: syzbot+2479e53d0db9b32ae2aa@syzkaller.appspotmail.com
> Fixes: d1e54dd08f16 ("KVM: x86: Enable KVM_GUEST_MEMFD for all 64-bit builds")

LOL, I was about to say that I suspected that this bug existed since guest_memfd
was first added, but that syzbot only now found the issue due to us enabling
KVM_GUEST_MEMFD broadly.  syzbot beat me to the punch.

#syz test

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 427c0acee9d7..3741ea23e330 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -708,23 +708,11 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
        return r;
 }
 
-void kvm_gmem_unbind(struct kvm_memory_slot *slot)
+static void __kvm_gmem_unbind(struct kvm_memory_slot *slot, struct gmem_file *f)
 {
        unsigned long start = slot->gmem.pgoff;
        unsigned long end = start + slot->npages;
-       struct gmem_file *f;
 
-       /*
-        * Nothing to do if the underlying file was already closed (or is being
-        * closed right now), kvm_gmem_release() invalidates all bindings.
-        */
-       CLASS(gmem_get_file, file)(slot);
-       if (!file)
-               return;
-
-       f = file->private_data;
-
-       filemap_invalidate_lock(file->f_mapping);
        xa_store_range(&f->bindings, start, end - 1, NULL, GFP_KERNEL);
 
        /*
@@ -732,6 +720,35 @@ void kvm_gmem_unbind(struct kvm_memory_slot *slot)
         * cannot see this memslot.
         */
        WRITE_ONCE(slot->gmem.file, NULL);
+}
+
+void kvm_gmem_unbind(struct kvm_memory_slot *slot)
+{
+       /*
+        * Nothing to do if the underlying file was _already_ closed, as
+        * kvm_gmem_release() invalidates and nullifies all bindings.
+        */
+       if (!slot->gmem.file)
+               return;
+
+       CLASS(gmem_get_file, file)(slot);
+
+       /*
+        * However, if the file is _being_ closed, then the bindings need to be
+        * removed as kvm_gmem_release() might not run until after the memslot
+        * is freed.  Note, modifying the bindings is safe even though the file
+        * is dying as kvm_gmem_release() nullifies slot->gmem.file under
+        * slots_lock, and only puts its reference to KVM after destroying all
+        * bindings.  I.e. reaching this point means kvm_gmem_release() can't
+        * concurrently destroy the bindings or free the gmem_file.
+        */
+       if (!file) {
+               __kvm_gmem_unbind(slot, slot->gmem.file->private_data);
+               return;
+       }
+
+       filemap_invalidate_lock(file->f_mapping);
+       __kvm_gmem_unbind(slot, file->private_data);
        filemap_invalidate_unlock(file->f_mapping);
 }

Re: [syzbot] [kvm?] KASAN: slab-use-after-free Write in kvm_gmem_release

[syzbot]

Hello,

syzbot tried to test the proposed patch but the build/boot failed:

failed to apply patch:
checking file virt/kvm/guest_memfd.c
Hunk #1 FAILED at 708.
Hunk #2 succeeded at 648 with fuzz 2 (offset -84 lines).
1 out of 2 hunks FAILED



Tested on:

commit:         2e590d67 Merge tag 'devicetree-fixes-for-6.18-2' of gi..
git tree:       upstream
kernel config:  https://syzkaller.appspot.com/x/.config?x=67b63a24f3c26fca
dashboard link: https://syzkaller.appspot.com/bug?extid=2479e53d0db9b32ae2aa
compiler:       
patch:          https://syzkaller.appspot.com/x/patch.diff?x=15343b04580000

Re: [syzbot] [kvm?] KASAN: slab-use-after-free Write in kvm_gmem_release

[Hillf Danton]

On Thu, Oct 23, 2025, syzbot wrote:
> syzbot has bisected this issue to:
> 
> commit d1e54dd08f163a9021433020d16a8f8f70ddc41c
> Author: Fuad Tabba <tabba@google.com>
> Date:   Tue Jul 29 22:54:40 2025 +0000
> 
>     KVM: x86: Enable KVM_GUEST_MEMFD for all 64-bit builds
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=12a663cd980000
> start commit:   43e9ad0c55a3 Merge tag 'scsi-fixes' of git://git.kernel.or..
> git tree:       upstream
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=11a663cd980000
> console output: https://syzkaller.appspot.com/x/log.txt?x=16a663cd980000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=67b63a24f3c26fca
> dashboard link: https://syzkaller.appspot.com/bug?extid=2479e53d0db9b32ae2aa
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=173ecd2f980000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14bc2be2580000

Test Sean's fix.

#syz test

--- x/virt/kvm/guest_memfd.c
+++ y/virt/kvm/guest_memfd.c
@@ -708,23 +708,11 @@ err:
 	return r;
 }
 
-void kvm_gmem_unbind(struct kvm_memory_slot *slot)
+static void __kvm_gmem_unbind(struct kvm_memory_slot *slot, struct gmem_file *f)
 {
 	unsigned long start = slot->gmem.pgoff;
 	unsigned long end = start + slot->npages;
-	struct gmem_file *f;
-
-	/*
-	 * Nothing to do if the underlying file was already closed (or is being
-	 * closed right now), kvm_gmem_release() invalidates all bindings.
-	 */
-	CLASS(gmem_get_file, file)(slot);
-	if (!file)
-		return;
-
-	f = file->private_data;
 
-	filemap_invalidate_lock(file->f_mapping);
 	xa_store_range(&f->bindings, start, end - 1, NULL, GFP_KERNEL);
 
 	/*
@@ -732,7 +720,36 @@ void kvm_gmem_unbind(struct kvm_memory_s
 	 * cannot see this memslot.
 	 */
 	WRITE_ONCE(slot->gmem.file, NULL);
-	filemap_invalidate_unlock(file->f_mapping);
+}
+
+void kvm_gmem_unbind(struct kvm_memory_slot *slot)
+{
+       /*
+        * Nothing to do if the underlying file was _already_ closed, as
+        * kvm_gmem_release() invalidates and nullifies all bindings.
+        */
+       if (!slot->gmem.file)
+               return;
+
+       CLASS(gmem_get_file, file)(slot);
+
+       /*
+        * However, if the file is _being_ closed, then the bindings need to be
+        * removed as kvm_gmem_release() might not run until after the memslot
+        * is freed.  Note, modifying the bindings is safe even though the file
+        * is dying as kvm_gmem_release() nullifies slot->gmem.file under
+        * slots_lock, and only puts its reference to KVM after destroying all
+        * bindings.  I.e. reaching this point means kvm_gmem_release() can't
+        * concurrently destroy the bindings or free the gmem_file.
+        */
+       if (!file) {
+               __kvm_gmem_unbind(slot, slot->gmem.file->private_data);
+               return;
+       }
+
+       filemap_invalidate_lock(file->f_mapping);
+       __kvm_gmem_unbind(slot, file->private_data);
+       filemap_invalidate_unlock(file->f_mapping);
 }
 
 /* Returns a locked folio on success.  */
--

Re: [syzbot] [kvm?] KASAN: slab-use-after-free Write in kvm_gmem_release

[syzbot]

Hello,

syzbot tried to test the proposed patch but the build/boot failed:

failed to apply patch:
checking file virt/kvm/guest_memfd.c
Hunk #1 FAILED at 708.
Hunk #2 FAILED at 732.
2 out of 2 hunks FAILED



Tested on:

commit:         d2818517 Merge tag 'block-6.18-20251023' of git://git...
git tree:       upstream
kernel config:  https://syzkaller.appspot.com/x/.config?x=67b63a24f3c26fca
dashboard link: https://syzkaller.appspot.com/bug?extid=2479e53d0db9b32ae2aa
compiler:       
patch:          https://syzkaller.appspot.com/x/patch.diff?x=14426e7c580000

Re: [syzbot] [kvm?] KASAN: slab-use-after-free Write in kvm_gmem_release

[Hillf Danton]

On Thu, Oct 23, 2025, syzbot wrote:
> syzbot has bisected this issue to:
> 
> commit d1e54dd08f163a9021433020d16a8f8f70ddc41c
> Author: Fuad Tabba <tabba@google.com>
> Date:   Tue Jul 29 22:54:40 2025 +0000
> 
>     KVM: x86: Enable KVM_GUEST_MEMFD for all 64-bit builds
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=12a663cd980000
> start commit:   43e9ad0c55a3 Merge tag 'scsi-fixes' of git://git.kernel.or..
> git tree:       upstream
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=11a663cd980000
> console output: https://syzkaller.appspot.com/x/log.txt?x=16a663cd980000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=67b63a24f3c26fca
> dashboard link: https://syzkaller.appspot.com/bug?extid=2479e53d0db9b32ae2aa
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=173ecd2f980000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14bc2be2580000

Test Sean's fix.

#syz test linux-next master 

--- x/virt/kvm/guest_memfd.c
+++ y/virt/kvm/guest_memfd.c
@@ -708,23 +708,11 @@ err:
 	return r;
 }
 
-void kvm_gmem_unbind(struct kvm_memory_slot *slot)
+static void __kvm_gmem_unbind(struct kvm_memory_slot *slot, struct gmem_file *f)
 {
 	unsigned long start = slot->gmem.pgoff;
 	unsigned long end = start + slot->npages;
-	struct gmem_file *f;
-
-	/*
-	 * Nothing to do if the underlying file was already closed (or is being
-	 * closed right now), kvm_gmem_release() invalidates all bindings.
-	 */
-	CLASS(gmem_get_file, file)(slot);
-	if (!file)
-		return;
-
-	f = file->private_data;
 
-	filemap_invalidate_lock(file->f_mapping);
 	xa_store_range(&f->bindings, start, end - 1, NULL, GFP_KERNEL);
 
 	/*
@@ -732,7 +720,36 @@ void kvm_gmem_unbind(struct kvm_memory_s
 	 * cannot see this memslot.
 	 */
 	WRITE_ONCE(slot->gmem.file, NULL);
-	filemap_invalidate_unlock(file->f_mapping);
+}
+
+void kvm_gmem_unbind(struct kvm_memory_slot *slot)
+{
+       /*
+        * Nothing to do if the underlying file was _already_ closed, as
+        * kvm_gmem_release() invalidates and nullifies all bindings.
+        */
+       if (!slot->gmem.file)
+               return;
+
+       CLASS(gmem_get_file, file)(slot);
+
+       /*
+        * However, if the file is _being_ closed, then the bindings need to be
+        * removed as kvm_gmem_release() might not run until after the memslot
+        * is freed.  Note, modifying the bindings is safe even though the file
+        * is dying as kvm_gmem_release() nullifies slot->gmem.file under
+        * slots_lock, and only puts its reference to KVM after destroying all
+        * bindings.  I.e. reaching this point means kvm_gmem_release() can't
+        * concurrently destroy the bindings or free the gmem_file.
+        */
+       if (!file) {
+               __kvm_gmem_unbind(slot, slot->gmem.file->private_data);
+               return;
+       }
+
+       filemap_invalidate_lock(file->f_mapping);
+       __kvm_gmem_unbind(slot, file->private_data);
+       filemap_invalidate_unlock(file->f_mapping);
 }
 
 /* Returns a locked folio on success.  */
--

Re: [syzbot] [kvm?] KASAN: slab-use-after-free Write in kvm_gmem_release

[syzbot]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
WARNING in get_data

------------[ cut here ]------------
WARNING: kernel/printk/printk_ringbuffer.c:1278 at get_data+0x48a/0x840 kernel/printk/printk_ringbuffer.c:1278, CPU#1: udevd/5199
Modules linked in:
CPU: 1 UID: 0 PID: 5199 Comm: udevd Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
RIP: 0010:get_data+0x48a/0x840 kernel/printk/printk_ringbuffer.c:1278
Code: 83 c4 f8 48 b8 00 00 00 00 00 fc ff df 41 0f b6 04 07 84 c0 0f 85 ee 01 00 00 44 89 65 00 49 83 c5 08 eb 13 e8 57 cd 1e 00 90 <0f> 0b 90 eb 05 e8 4c cd 1e 00 45 31 ed 4c 89 e8 48 83 c4 28 5b 41
RSP: 0018:ffffc90000a08560 EFLAGS: 00010006
RAX: ffffffff81a16c59 RBX: 00003fffffffffff RCX: ffff88807e471e40
RDX: 0000000000010000 RSI: 00003fffffffffff RDI: 0000000000000000
RBP: 0000000000000012 R08: 0000000000001005 R09: 0000002047c57766
R10: 0000002047c57766 R11: 0000196a8200002e R12: 0000000000000012
R13: 0000000000000000 R14: ffffc90000a086a8 R15: 1ffffffff1bcaa96
FS:  00007fa682a1a880(0000) GS:ffff888126022000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b30c63fff CR3: 000000007e4be000 CR4: 00000000003526f0
Call Trace:
 <IRQ>
 copy_data kernel/printk/printk_ringbuffer.c:1857 [inline]
 prb_read kernel/printk/printk_ringbuffer.c:1966 [inline]
 _prb_read_valid+0x672/0xa90 kernel/printk/printk_ringbuffer.c:2143
 prb_read_valid+0x3c/0x60 kernel/printk/printk_ringbuffer.c:2215
 printk_get_next_message+0x15c/0x7b0 kernel/printk/printk.c:2978
 console_emit_next_record kernel/printk/printk.c:3065 [inline]
 console_flush_one_record kernel/printk/printk.c:3197 [inline]
 console_flush_all+0x4cc/0xb10 kernel/printk/printk.c:3271
 __console_flush_and_unlock kernel/printk/printk.c:3301 [inline]
 console_unlock+0xbb/0x190 kernel/printk/printk.c:3341
 wake_up_klogd_work_func+0xa8/0x130 kernel/printk/printk.c:4550
 irq_work_single+0xe1/0x240 kernel/irq_work.c:221
 irq_work_run_list kernel/irq_work.c:252 [inline]
 irq_work_tick+0x2c2/0x360 kernel/irq_work.c:277
 update_process_times+0x264/0x2f0 kernel/time/timer.c:2476
 tick_sched_handle kernel/time/tick-sched.c:276 [inline]
 tick_nohz_handler+0x39a/0x520 kernel/time/tick-sched.c:297
 __run_hrtimer kernel/time/hrtimer.c:1777 [inline]
 __hrtimer_run_queues+0x4e0/0xc60 kernel/time/hrtimer.c:1841
 hrtimer_interrupt+0x45b/0xaa0 kernel/time/hrtimer.c:1903
 local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1041 [inline]
 __sysvec_apic_timer_interrupt+0x10b/0x410 arch/x86/kernel/apic/apic.c:1058
 instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1052 [inline]
 sysvec_apic_timer_interrupt+0xa1/0xc0 arch/x86/kernel/apic/apic.c:1052
 </IRQ>
 <TASK>
 asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:697
RIP: 0010:__sanitizer_cov_trace_cmp4+0x4/0x90 kernel/kcov.c:288
Code: 89 74 11 18 48 89 44 11 20 e9 48 ca 8b 09 cc 0f 1f 80 00 00 00 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa <48> 8b 04 24 65 48 8b 14 25 08 e0 6f 92 65 8b 0d a8 a4 af 10 81 e1
RSP: 0018:ffffc900030e7d00 EFLAGS: 00000246
RAX: 0000000000000000 RBX: 0000000000000040 RCX: 0000000000000000
RDX: ffff88807e471e40 RSI: 0000000000000040 RDI: 000000000000000c
RBP: 000000000000000c R08: 0000000000000003 R09: 0000000000000004
R10: dffffc0000000000 R11: fffff5200061cf90 R12: ffff88807ea4d320
R13: dffffc0000000000 R14: ffff88807ea4d3c0 R15: 0000000000000000
 alloc_fd+0x2f3/0x6c0 fs/file.c:595
 do_sys_openat2+0xfc/0x1c0 fs/open.c:1435
 do_sys_open fs/open.c:1452 [inline]
 __do_sys_openat fs/open.c:1468 [inline]
 __se_sys_openat fs/open.c:1463 [inline]
 __x64_sys_openat+0x138/0x170 fs/open.c:1463
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fa6822a7407
Code: 48 89 fa 4c 89 df e8 38 aa 00 00 8b 93 08 03 00 00 59 5e 48 83 f8 fc 74 1a 5b c3 0f 1f 84 00 00 00 00 00 48 8b 44 24 10 0f 05 <5b> c3 0f 1f 80 00 00 00 00 83 e2 39 83 fa 08 75 de e8 23 ff ff ff
RSP: 002b:00007fffd29370d0 EFLAGS: 00000202 ORIG_RAX: 0000000000000101
RAX: ffffffffffffffda RBX: 00007fa682a1a880 RCX: 00007fa6822a7407
RDX: 0000000000080000 RSI: 00007fffd2937250 RDI: ffffffffffffff9c
RBP: 0000000000000008 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000202 R12: 00005600f32347f5
R13: 00005600f32347f5 R14: 0000000000000001 R15: 0000000000000000
 </TASK>
----------------
Code disassembly (best guess):
   0:	89 74 11 18          	mov    %esi,0x18(%rcx,%rdx,1)
   4:	48 89 44 11 20       	mov    %rax,0x20(%rcx,%rdx,1)
   9:	e9 48 ca 8b 09       	jmp    0x98bca56
   e:	cc                   	int3
   f:	0f 1f 80 00 00 00 00 	nopl   0x0(%rax)
  16:	90                   	nop
  17:	90                   	nop
  18:	90                   	nop
  19:	90                   	nop
  1a:	90                   	nop
  1b:	90                   	nop
  1c:	90                   	nop
  1d:	90                   	nop
  1e:	90                   	nop
  1f:	90                   	nop
  20:	90                   	nop
  21:	90                   	nop
  22:	90                   	nop
  23:	90                   	nop
  24:	90                   	nop
  25:	90                   	nop
  26:	f3 0f 1e fa          	endbr64
* 2a:	48 8b 04 24          	mov    (%rsp),%rax <-- trapping instruction
  2e:	65 48 8b 14 25 08 e0 	mov    %gs:0xffffffff926fe008,%rdx
  35:	6f 92
  37:	65 8b 0d a8 a4 af 10 	mov    %gs:0x10afa4a8(%rip),%ecx        # 0x10afa4e6
  3e:	81                   	.byte 0x81
  3f:	e1                   	.byte 0xe1


Tested on:

commit:         72fb0170 Add linux-next specific files for 20251024
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=14b2e3cd980000
kernel config:  https://syzkaller.appspot.com/x/.config?x=bcaf4aad77308158
dashboard link: https://syzkaller.appspot.com/bug?extid=2479e53d0db9b32ae2aa
compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
patch:          https://syzkaller.appspot.com/x/patch.diff?x=13d98be2580000

Re: [syzbot] [kvm?] KASAN: slab-use-after-free Write in kvm_gmem_release

[Hillf Danton]

On Thu, Oct 23, 2025, syzbot wrote:
> syzbot has bisected this issue to:
> 
> commit d1e54dd08f163a9021433020d16a8f8f70ddc41c
> Author: Fuad Tabba <tabba@google.com>
> Date:   Tue Jul 29 22:54:40 2025 +0000
> 
>     KVM: x86: Enable KVM_GUEST_MEMFD for all 64-bit builds
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=12a663cd980000
> start commit:   43e9ad0c55a3 Merge tag 'scsi-fixes' of git://git.kernel.or..
> git tree:       upstream
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=11a663cd980000
> console output: https://syzkaller.appspot.com/x/log.txt?x=16a663cd980000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=67b63a24f3c26fca
> dashboard link: https://syzkaller.appspot.com/bug?extid=2479e53d0db9b32ae2aa
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=173ecd2f980000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14bc2be2580000

Test Sean's fix.

#syz test linux-next master 

--- x/virt/kvm/guest_memfd.c
+++ y/virt/kvm/guest_memfd.c
@@ -708,23 +708,11 @@ err:
 	return r;
 }
 
-void kvm_gmem_unbind(struct kvm_memory_slot *slot)
+static void __kvm_gmem_unbind(struct kvm_memory_slot *slot, struct gmem_file *f)
 {
 	unsigned long start = slot->gmem.pgoff;
 	unsigned long end = start + slot->npages;
-	struct gmem_file *f;
-
-	/*
-	 * Nothing to do if the underlying file was already closed (or is being
-	 * closed right now), kvm_gmem_release() invalidates all bindings.
-	 */
-	CLASS(gmem_get_file, file)(slot);
-	if (!file)
-		return;
-
-	f = file->private_data;
 
-	filemap_invalidate_lock(file->f_mapping);
 	xa_store_range(&f->bindings, start, end - 1, NULL, GFP_KERNEL);
 
 	/*
@@ -732,7 +720,36 @@ void kvm_gmem_unbind(struct kvm_memory_s
 	 * cannot see this memslot.
 	 */
 	WRITE_ONCE(slot->gmem.file, NULL);
-	filemap_invalidate_unlock(file->f_mapping);
+}
+
+void kvm_gmem_unbind(struct kvm_memory_slot *slot)
+{
+       /*
+        * Nothing to do if the underlying file was _already_ closed, as
+        * kvm_gmem_release() invalidates and nullifies all bindings.
+        */
+       if (!slot->gmem.file)
+               return;
+
+       CLASS(gmem_get_file, file)(slot);
+
+       /*
+        * However, if the file is _being_ closed, then the bindings need to be
+        * removed as kvm_gmem_release() might not run until after the memslot
+        * is freed.  Note, modifying the bindings is safe even though the file
+        * is dying as kvm_gmem_release() nullifies slot->gmem.file under
+        * slots_lock, and only puts its reference to KVM after destroying all
+        * bindings.  I.e. reaching this point means kvm_gmem_release() can't
+        * concurrently destroy the bindings or free the gmem_file.
+        */
+       if (!file) {
+               __kvm_gmem_unbind(slot, slot->gmem.file->private_data);
+               return;
+       }
+
+       filemap_invalidate_lock(file->f_mapping);
+       __kvm_gmem_unbind(slot, file->private_data);
+       filemap_invalidate_unlock(file->f_mapping);
 }
 
 /* Returns a locked folio on success.  */
--- x/kernel/printk/printk_ringbuffer.c
+++ y/kernel/printk/printk_ringbuffer.c
@@ -1286,7 +1286,7 @@ static const char *get_data(struct prb_d
 	}
 
 	/* A valid data block will always have at least an ID. */
-	if (WARN_ON_ONCE(*data_size < sizeof(db->id)))
+	if (*data_size < sizeof(db->id))
 		return NULL;
 
 	/* Subtract block ID space from size to reflect data size. */
--

Re: [syzbot] [kvm?] KASAN: slab-use-after-free Write in kvm_gmem_release

[syzbot]

Hello,

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

Reported-by: syzbot+2479e53d0db9b32ae2aa@syzkaller.appspotmail.com
Tested-by: syzbot+2479e53d0db9b32ae2aa@syzkaller.appspotmail.com

Tested on:

commit:         72fb0170 Add linux-next specific files for 20251024
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=15da53e2580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=bcaf4aad77308158
dashboard link: https://syzkaller.appspot.com/bug?extid=2479e53d0db9b32ae2aa
compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
patch:          https://syzkaller.appspot.com/x/patch.diff?x=161c4258580000

Note: testing is done by a robot and is best-effort only.

Re: [syzbot] [kvm?] KASAN: slab-use-after-free Write in kvm_gmem_release

[Sean Christopherson]

On Sat, Oct 25, 2025, Hillf Danton wrote:
> On Thu, Oct 23, 2025, syzbot wrote:
> > syzbot has bisected this issue to:
> > 
> > commit d1e54dd08f163a9021433020d16a8f8f70ddc41c
> > Author: Fuad Tabba <tabba@google.com>
> > Date:   Tue Jul 29 22:54:40 2025 +0000
> > 
> >     KVM: x86: Enable KVM_GUEST_MEMFD for all 64-bit builds
> > 
> > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=12a663cd980000
> > start commit:   43e9ad0c55a3 Merge tag 'scsi-fixes' of git://git.kernel.or..
> > git tree:       upstream
> > final oops:     https://syzkaller.appspot.com/x/report.txt?x=11a663cd980000
> > console output: https://syzkaller.appspot.com/x/log.txt?x=16a663cd980000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=67b63a24f3c26fca
> > dashboard link: https://syzkaller.appspot.com/bug?extid=2479e53d0db9b32ae2aa
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=173ecd2f980000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14bc2be2580000
> 
> Test Sean's fix.
> 
> #syz test linux-next master 

Oh, nice.  Thanks Hillf!