Re: [PATCH v3] KVM: X86: Fix userspace set invalid CR4

[Sean Christopherson]

On Wed, Sep 18, 2019 at 05:50:10PM +0800, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> 
> Reported by syzkaller:
> 
> 	WARNING: CPU: 0 PID: 6544 at /home/kernel/data/kvm/arch/x86/kvm//vmx/vmx.c:4689 handle_desc+0x37/0x40 [kvm_intel]
> 	CPU: 0 PID: 6544 Comm: a.out Tainted: G           OE     5.3.0-rc4+ #4
> 	RIP: 0010:handle_desc+0x37/0x40 [kvm_intel]
> 	Call Trace:
> 	 vmx_handle_exit+0xbe/0x6b0 [kvm_intel]
> 	 vcpu_enter_guest+0x4dc/0x18d0 [kvm]
> 	 kvm_arch_vcpu_ioctl_run+0x407/0x660 [kvm]
> 	 kvm_vcpu_ioctl+0x3ad/0x690 [kvm]
> 	 do_vfs_ioctl+0xa2/0x690
> 	 ksys_ioctl+0x6d/0x80
> 	 __x64_sys_ioctl+0x1a/0x20
> 	 do_syscall_64+0x74/0x720
> 	 entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
> When CR4.UMIP is set, guest should have UMIP cpuid flag. Current
> kvm set_sregs function doesn't have such check when userspace inputs
> sregs values. SECONDARY_EXEC_DESC is enabled on writes to CR4.UMIP
> in vmx_set_cr4 though guest doesn't have UMIP cpuid flag. The testcast
> triggers handle_desc warning when executing ltr instruction since
> guest architectural CR4 doesn't set UMIP. This patch fixes it by
> adding valid CR4 and CPUID combination checking in __set_sregs.
> 
> syzkaller source: https://syzkaller.appspot.com/x/repro.c?x=138efb99600000
> 
> Reported-by: syzbot+0f1819555fbdce992df9@xxxxxxxxxxxxxxxxxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> ---

Per Paolo, the additional checks should be ok:

  https://lkml.kernel.org/r/d0c35f21-b262-2c4e-9109-4ab803487705@xxxxxxxxxx

I'm pretty sure userspace can still induce a WARN storm by setting the
cr4_fixed0/1 MSRs for a nested guest, but this is a good change regardless.

Reviewed-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>