On Tue, Sep 17, 2019 at 04:16:25PM +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. Checking CPUID will fix this specific scenario, but it doesn't resolve the underlying issue of __set_sregs() ignoring the return of kvm_x86_ops' set_cr4(), e.g. I think vmx_set_cr4() can still fail if userspace sets a custom MSR_IA32_VMX_CR4_FIXED0 when nested VMX is on. > 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> > --- > arch/x86/kvm/x86.c | 39 ++++++++++++++++++++++++--------------- > 1 file changed, 24 insertions(+), 15 deletions(-) > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index f7cfd8e..cafb4d4 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -884,34 +884,42 @@ int kvm_set_xcr(struct kvm_vcpu *vcpu, u32 index, u64 xcr) > } > EXPORT_SYMBOL_GPL(kvm_set_xcr); > > -int kvm_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4) > +static int kvm_valid_cr4(struct kvm_vcpu *vcpu, unsigned long cr4) > { > - unsigned long old_cr4 = kvm_read_cr4(vcpu); > - unsigned long pdptr_bits = X86_CR4_PGE | X86_CR4_PSE | X86_CR4_PAE | > - X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_PKE; > - > - if (cr4 & CR4_RESERVED_BITS) > - return 1; > - > if (!guest_cpuid_has(vcpu, X86_FEATURE_XSAVE) && (cr4 & X86_CR4_OSXSAVE)) > - return 1; > + return -EINVAL; > > if (!guest_cpuid_has(vcpu, X86_FEATURE_SMEP) && (cr4 & X86_CR4_SMEP)) > - return 1; > + return -EINVAL; > > if (!guest_cpuid_has(vcpu, X86_FEATURE_SMAP) && (cr4 & X86_CR4_SMAP)) > - return 1; > + return -EINVAL; > > if (!guest_cpuid_has(vcpu, X86_FEATURE_FSGSBASE) && (cr4 & X86_CR4_FSGSBASE)) > - return 1; > + return -EINVAL; > > if (!guest_cpuid_has(vcpu, X86_FEATURE_PKU) && (cr4 & X86_CR4_PKE)) > - return 1; > + return -EINVAL; > > if (!guest_cpuid_has(vcpu, X86_FEATURE_LA57) && (cr4 & X86_CR4_LA57)) > - return 1; > + return -EINVAL; > > if (!guest_cpuid_has(vcpu, X86_FEATURE_UMIP) && (cr4 & X86_CR4_UMIP)) > + return -EINVAL; > + > + return 0; > +} > + > +int kvm_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4) > +{ > + unsigned long old_cr4 = kvm_read_cr4(vcpu); > + unsigned long pdptr_bits = X86_CR4_PGE | X86_CR4_PSE | X86_CR4_PAE | > + X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_PKE; > + > + if (cr4 & CR4_RESERVED_BITS) > + return 1; Checking CPUID bits but allowing unconditionally reserved bits to be set feels wrong. Paolo, can you provide an "official" ruling on how KVM_SET_SREGS should interact with reserved bits? It's not at all clear from the git history if skipping the checks was intentional or an oversight. The CR4_RESERVED_BITS check has been in kvm_set_cr4() since the beginning of time (commit 6aa8b732ca01, "[PATCH] kvm: userspace interface"). The first CPUID check came later, in commit 2acf923e38fb ("KVM: VMX: Enable XSAVE/XRSTOR for guest"), but its changelog is decidedly unhelpful. > + > + if (kvm_valid_cr4(vcpu, cr4)) > return 1; > > if (is_long_mode(vcpu)) { > @@ -8675,7 +8683,8 @@ static int __set_sregs(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs) > struct desc_ptr dt; > int ret = -EINVAL; > > - if (kvm_valid_sregs(vcpu, sregs)) > + if (kvm_valid_sregs(vcpu, sregs) || No need for a line break. Even better, call kvm_valid_cr4() from kvm_valid_sregs(), e.g. the X86_FEATURE_XSAVE check in kvm_valid_sregs() is now redundant and can be dropped, and "return kvm_valid_cr4(...)" from kvm_valid_sregs() can likely be optimized into a tail call. > + kvm_valid_cr4(vcpu, sregs->cr4)) > goto out; > > apic_base_msr.data = sregs->apic_base; > -- > 2.7.4 >
