linux-kernel - Re: [PATCH 1/9] KVM: x86: Immediately reset the MMU context when the SMM flag is cleared

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Message-ID: <eb0b94d4-9edf-6ef6-25f9-27a3e407f60b@redhat.com>
Date:   Thu, 10 Jun 2021 15:19:07 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        syzbot+fb0b6a7e8713aeb0319c@...kaller.appspotmail.com
Subject: Re: [PATCH 1/9] KVM: x86: Immediately reset the MMU context when the
 SMM flag is cleared

On 09/06/21 20:56, Sean Christopherson wrote:
> Immediately reset the MMU context when the vCPU's SMM flag is cleared so
> that the SMM flag in the MMU role is always synchronized with the vCPU's
> flag.  If RSM fails (which isn't correctly emulated), KVM will bail
> without calling post_leave_smm() and leave the MMU in a bad state.
> 
> The bad MMU role can lead to a NULL pointer dereference when grabbing a
> shadow page's rmap for a page fault as the initial lookups for the gfn
> will happen with the vCPU's SMM flag (=0), whereas the rmap lookup will
> use the shadow page's SMM flag, which comes from the MMU (=1).  SMM has
> an entirely different set of memslots, and so the initial lookup can find
> a memslot (SMM=0) and then explode on the rmap memslot lookup (SMM=1).
> 
>    general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN
>    KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
>    CPU: 1 PID: 8410 Comm: syz-executor382 Not tainted 5.13.0-rc5-syzkaller #0
>    Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
>    RIP: 0010:__gfn_to_rmap arch/x86/kvm/mmu/mmu.c:935 [inline]
>    RIP: 0010:gfn_to_rmap+0x2b0/0x4d0 arch/x86/kvm/mmu/mmu.c:947
>    Code: <42> 80 3c 20 00 74 08 4c 89 ff e8 f1 79 a9 00 4c 89 fb 4d 8b 37 44
>    RSP: 0018:ffffc90000ffef98 EFLAGS: 00010246
>    RAX: 0000000000000000 RBX: ffff888015b9f414 RCX: ffff888019669c40
>    RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000001
>    RBP: 0000000000000001 R08: ffffffff811d9cdb R09: ffffed10065a6002
>    R10: ffffed10065a6002 R11: 0000000000000000 R12: dffffc0000000000
>    R13: 0000000000000003 R14: 0000000000000001 R15: 0000000000000000
>    FS:  000000000124b300(0000) GS:ffff8880b9b00000(0000) knlGS:0000000000000000
>    CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>    CR2: 0000000000000000 CR3: 0000000028e31000 CR4: 00000000001526e0
>    DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>    DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>    Call Trace:
>     rmap_add arch/x86/kvm/mmu/mmu.c:965 [inline]
>     mmu_set_spte+0x862/0xe60 arch/x86/kvm/mmu/mmu.c:2604
>     __direct_map arch/x86/kvm/mmu/mmu.c:2862 [inline]
>     direct_page_fault+0x1f74/0x2b70 arch/x86/kvm/mmu/mmu.c:3769
>     kvm_mmu_do_page_fault arch/x86/kvm/mmu.h:124 [inline]
>     kvm_mmu_page_fault+0x199/0x1440 arch/x86/kvm/mmu/mmu.c:5065
>     vmx_handle_exit+0x26/0x160 arch/x86/kvm/vmx/vmx.c:6122
>     vcpu_enter_guest+0x3bdd/0x9630 arch/x86/kvm/x86.c:9428
>     vcpu_run+0x416/0xc20 arch/x86/kvm/x86.c:9494
>     kvm_arch_vcpu_ioctl_run+0x4e8/0xa40 arch/x86/kvm/x86.c:9722
>     kvm_vcpu_ioctl+0x70f/0xbb0 arch/x86/kvm/../../../virt/kvm/kvm_main.c:3460
>     vfs_ioctl fs/ioctl.c:51 [inline]
>     __do_sys_ioctl fs/ioctl.c:1069 [inline]
>     __se_sys_ioctl+0xfb/0x170 fs/ioctl.c:1055
>     do_syscall_64+0x3f/0xb0 arch/x86/entry/common.c:47
>     entry_SYSCALL_64_after_hwframe+0x44/0xae
>    RIP: 0033:0x440ce9
> 
> Reported-by: syzbot+fb0b6a7e8713aeb0319c@...kaller.appspotmail.com
> Fixes: 9ec19493fb86 ("KVM: x86: clear SMM flags before loading state while leaving SMM")
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/x86.c | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 9dd23bdfc6cc..54d212fe9b15 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7106,7 +7106,10 @@ static unsigned emulator_get_hflags(struct x86_emulate_ctxt *ctxt)
>   
>   static void emulator_set_hflags(struct x86_emulate_ctxt *ctxt, unsigned emul_flags)
>   {
> -	emul_to_vcpu(ctxt)->arch.hflags = emul_flags;
> +	struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
> +
> +	vcpu->arch.hflags = emul_flags;
> +	kvm_mmu_reset_context(vcpu);
>   }
>   
>   static int emulator_pre_leave_smm(struct x86_emulate_ctxt *ctxt,
> 

Queued for kvm/master, thanks.

Paolo