[syzbot] [kvm?] WARNING: suspicious RCU usage in kvm_vcpu_gfn_to_memslot

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 95ec54a420b8 Merge tag 'powerpc-6.14-1' of git://git.kerne..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=15ff45df980000
kernel config: https://syzkaller.appspot.com/x/.config?x=f27a9722e53abeaa
dashboard link: https://syzkaller.appspot.com/bug?extid=cdeaeec70992eca2d920
compiler: Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=133ed618580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=110449f8580000

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/7feb34a89c2a/non_bootable_disk-95ec54a4.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/06bc523d1d96/vmlinux-95ec54a4.xz
kernel image: https://storage.googleapis.com/syzbot-assets/c0b342c3e200/bzImage-95ec54a4.xz

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

=============================
WARNING: suspicious RCU usage
6.13.0-syzkaller-00918-g95ec54a420b8 #0 Not tainted
-----------------------------
./include/linux/kvm_host.h:1038 suspicious rcu_dereference_check() usage!

other info that might help us debug this:


rcu_scheduler_active = 2, debug_locks = 1
no locks held by syz-executor167/5303.

stack backtrace:
CPU: 0 UID: 0 PID: 5303 Comm: syz-executor167 Not tainted 6.13.0-syzkaller-00918-g95ec54a420b8 #0
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120
lockdep_rcu_suspicious+0x226/0x340 kernel/locking/lockdep.c:6845
__kvm_memslots include/linux/kvm_host.h:1036 [inline]
kvm_vcpu_memslots include/linux/kvm_host.h:1050 [inline]
kvm_vcpu_gfn_to_memslot+0x429/0x4c0 virt/kvm/kvm_main.c:2554
kvm_vcpu_write_guest_page virt/kvm/kvm_main.c:3238 [inline]
kvm_vcpu_write_guest+0x7c/0x130 virt/kvm/kvm_main.c:3274
kvm_xen_write_hypercall_page+0x2ff/0x5f0 arch/x86/kvm/xen.c:1299
kvm_set_msr_common+0x150/0x3da0 arch/x86/kvm/x86.c:3751
vmx_set_msr+0x15da/0x2790 arch/x86/kvm/vmx/vmx.c:2487
__kvm_set_msr arch/x86/kvm/x86.c:1877 [inline]
kvm_vcpu_reset+0xbea/0x1740 arch/x86/kvm/x86.c:12456
kvm_arch_vcpu_create+0x8dc/0xa80 arch/x86/kvm/x86.c:12305
kvm_vm_ioctl_create_vcpu+0x3d6/0xa00 virt/kvm/kvm_main.c:4106
kvm_vm_ioctl+0x7e2/0xd30 virt/kvm/kvm_main.c:5019
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:906 [inline]
__se_sys_ioctl+0xf5/0x170 fs/ioctl.c:892
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f9a62a26369
Code: 48 83 c4 28 c3 e8 37 17 00 00 0f 1f 80 00 00 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 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fff83b20f38 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007fff83b21108 RCX: 00007f9a62a26369
RDX: 0000000000000000 RSI: 000000000000ae41 RDI: 0000000000000004
RBP: 00007f9a62a99610 R08: 00007fff83b21108 R09: 00007fff83b21108
R10: 00007fff83b21108 R11: 0000000000000246 R12: 0000000000000001
R13: 00007fff83b210f8 R14: 0000000000000001 R15: 0000000000000001
</TASK>


---
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[Sean Christopherson]

On Thu, Jan 23, 2025, syzbot wrote:
> Call Trace:
> <TASK>
> __dump_stack lib/dump_stack.c:94 [inline]
> dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120
> lockdep_rcu_suspicious+0x226/0x340 kernel/locking/lockdep.c:6845
> __kvm_memslots include/linux/kvm_host.h:1036 [inline]
> kvm_vcpu_memslots include/linux/kvm_host.h:1050 [inline]
> kvm_vcpu_gfn_to_memslot+0x429/0x4c0 virt/kvm/kvm_main.c:2554
> kvm_vcpu_write_guest_page virt/kvm/kvm_main.c:3238 [inline]
> kvm_vcpu_write_guest+0x7c/0x130 virt/kvm/kvm_main.c:3274
> kvm_xen_write_hypercall_page+0x2ff/0x5f0 arch/x86/kvm/xen.c:1299
> kvm_set_msr_common+0x150/0x3da0 arch/x86/kvm/x86.c:3751
> vmx_set_msr+0x15da/0x2790 arch/x86/kvm/vmx/vmx.c:2487
> __kvm_set_msr arch/x86/kvm/x86.c:1877 [inline]

The Xen hypercall page MSR is configured to be MSR_IA32_XSS, which results in KVM's
write of XSS during vCPU creation to do bad things. I'll post a path to restrict
the Xen MSR to the unofficial virtualization-defined range, and cross my fingers
that doing so won't break userspace. There are myriad things that can go wrong if
KVM effectively lets userspace redirect any MSR write.