From: Chao Gao <chao.gao@intel.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, <kvm@vger.kernel.org>,
<linux-kernel@vger.kernel.org>,
Alexander Potapenko <glider@google.com>
Subject: Re: [PATCH 1/2] KVM: x86: Unload "FPU" state on INIT if and only if its currently in-use
Date: Tue, 4 Nov 2025 11:10:32 +0800 [thread overview]
Message-ID: <aQluqPBXclFHxWFD@intel.com> (raw)
In-Reply-To: <20251030185802.3375059-2-seanjc@google.com>
On Thu, Oct 30, 2025 at 11:58:01AM -0700, Sean Christopherson wrote:
>Replace the hack added by commit f958bd2314d1 ("KVM: x86: Fix potential
>put_fpu() w/o load_fpu() on MPX platform") with a more robust approach of
>unloading+reloading guest FPU state based on whether or not the vCPU's FPU
>is currently in-use, i.e. currently loaded. This fixes a bug on hosts
>that support CET but not MPX, where kvm_arch_vcpu_ioctl_get_mpstate()
>neglects to load FPU state (it only checks for MPX support) and leads to
>KVM attempting to put FPU state due to kvm_apic_accept_events() triggering
>INIT emulation. E.g. on a host with CET but not MPX, syzkaller+KASAN
>generates:
>
> Oops: general protection fault, probably for non-canonical address 0xdffffc0000000004: 0000 [#1] SMP KASAN NOPTI
> KASAN: null-ptr-deref in range [0x0000000000000020-0x0000000000000027]
> CPU: 211 UID: 0 PID: 20451 Comm: syz.9.26 Tainted: G S 6.18.0-smp-DEV #7 NONE
> Tainted: [S]=CPU_OUT_OF_SPEC
> Hardware name: Google Izumi/izumi, BIOS 0.20250729.1-0 07/29/2025
> RIP: 0010:fpu_swap_kvm_fpstate+0x3ce/0x610 ../arch/x86/kernel/fpu/core.c:377
> RSP: 0018:ff1100410c167cc0 EFLAGS: 00010202
> RAX: 0000000000000004 RBX: 0000000000000020 RCX: 00000000000001aa
> RDX: 00000000000001ab RSI: ffffffff817bb960 RDI: 0000000022600000
> RBP: dffffc0000000000 R08: ff110040d23c8007 R09: 1fe220081a479000
> R10: dffffc0000000000 R11: ffe21c081a479001 R12: ff110040d23c8d98
> R13: 00000000fffdc578 R14: 0000000000000000 R15: ff110040d23c8d90
> FS: 00007f86dd1876c0(0000) GS:ff11007fc969b000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f86dd186fa8 CR3: 00000040d1dfa003 CR4: 0000000000f73ef0
> PKRU: 80000000
> Call Trace:
> <TASK>
> kvm_vcpu_reset+0x80d/0x12c0 ../arch/x86/kvm/x86.c:11818
> kvm_apic_accept_events+0x1cb/0x500 ../arch/x86/kvm/lapic.c:3489
> kvm_arch_vcpu_ioctl_get_mpstate+0xd0/0x4e0 ../arch/x86/kvm/x86.c:12145
> kvm_vcpu_ioctl+0x5e2/0xed0 ../virt/kvm/kvm_main.c:4539
> __se_sys_ioctl+0x11d/0x1b0 ../fs/ioctl.c:51
> do_syscall_x64 ../arch/x86/entry/syscall_64.c:63 [inline]
> do_syscall_64+0x6e/0x940 ../arch/x86/entry/syscall_64.c:94
> entry_SYSCALL_64_after_hwframe+0x76/0x7e
> RIP: 0033:0x7f86de71d9c9
> </TASK>
>
>with a very simple reproducer:
>
> r0 = openat$kvm(0xffffffffffffff9c, &(0x7f0000000000), 0x80b00, 0x0)
> r1 = ioctl$KVM_CREATE_VM(r0, 0xae01, 0x0)
> ioctl$KVM_CREATE_IRQCHIP(r1, 0xae60)
> r2 = ioctl$KVM_CREATE_VCPU(r1, 0xae41, 0x0)
> ioctl$KVM_SET_IRQCHIP(r1, 0x8208ae63, ...)
> ioctl$KVM_GET_MP_STATE(r2, 0x8004ae98, &(0x7f00000000c0))
>
>Alternatively, the MPX hack in GET_MP_STATE could be extended to cover CET,
>but from a "don't break existing functionality" perspective, that isn't any
>less risky than peeking at the state of in_use, and it's far less robust
>for a long term solution (as evidenced by this bug).
>
>Reported-by: Alexander Potapenko <glider@google.com>
>Fixes: 69cc3e886582 ("KVM: x86: Add XSS support for CET_KERNEL and CET_USER")
>Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
next prev parent reply other threads:[~2025-11-04 3:10 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-10-30 18:58 [PATCH 0/2] KVM: x86: Fix an FPU+CET splat Sean Christopherson
2025-10-30 18:58 ` [PATCH 1/2] KVM: x86: Unload "FPU" state on INIT if and only if its currently in-use Sean Christopherson
2025-11-04 3:10 ` Chao Gao [this message]
2025-10-30 18:58 ` [PATCH 2/2] KVM: x86: Harden KVM against imbalanced load/put of guest FPU state Sean Christopherson
2025-11-04 6:07 ` Chao Gao
2025-10-31 6:28 ` [PATCH 0/2] KVM: x86: Fix an FPU+CET splat Yao Yuan
2025-11-04 17:45 ` Sean Christopherson
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