| Message ID | 20190123223925.7558-2-sean.j.christopherson@intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: nVMX: Fix address calculations for VMX instrs | expand |
On Wed, Jan 23, 2019 at 2:39 PM Sean Christopherson <sean.j.christopherson@intel.com> wrote: > > The VMCS.EXIT_QUALIFCATION field reports the displacements of memory > operands for various instructions, including VMX instructions, as a > naturally sized unsigned value, but masks the value by the addr size, > e.g. given a ModRM encoded as -0x28(%ebp), the -0x28 displacement is > reported as 0xffffffd8 for a 32-bit address size. Despite some weird > wording regarding sign extension, the SDM explicitly states that bits > beyond the instructions address size are undefined: > > In all cases, bits of this field beyond the instruction’s address > size are undefined. > > Failure to sign extend the displacement results in KVM incorrectly > treating a negative displacement as a large positive displacement when > the address size of the VMX instruction is smaller than KVM's native > size, e.g. a 32-bit address size on a 64-bit KVM. > > The very original decoding, added by commit 064aea774768 ("KVM: nVMX: > Decoding memory operands of VMX instructions"), sort of modeled sign > extension by truncating the final virtual/linear address for a 32-bit > address size. I.e. it messed up the effective address but made it work > by adjusting the final address. > > When segmentation checks were added, the truncation logic was kept > as-is and no sign extension logic was introduced. In other words, it > kept calculating the wrong effective address while mostly generating > the correct virtual/linear address. As the effective address is what's > used in the segment limit checks, this results in KVM incorreclty > injecting #GP/#SS faults due to non-existent segment violations when > a nested VMM uses negative displacements with an address size smaller > than KVM's native address size. > > Using the -0x28(%ebp) example, an EBP value of 0x1000 will result in > KVM using 0x100000fd8 as the effective address when checking for a > segment limit violation. This causes a 100% failure rate when running > a 32-bit KVM build as L1 on top of a 64-bit KVM L0. > > Fixes: f9eb4af67c9d ("KVM: nVMX: VMX instructions: add checks for #GP/#SS exceptions") > Cc: stable@vger.kernel.org > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com> Reviewed-by: Jim Mattson <jmattson@google.com>
diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c index 2616bd2c7f2c..40b6e331dce9 100644 --- a/arch/x86/kvm/vmx/nested.c +++ b/arch/x86/kvm/vmx/nested.c @@ -4030,6 +4030,10 @@ int get_vmx_mem_address(struct kvm_vcpu *vcpu, unsigned long exit_qualification, /* Addr = segment_base + offset */ /* offset = base + [index * scale] + displacement */ off = exit_qualification; /* holds the displacement */ + if (addr_size == 1) + off = (gva_t)sign_extend64(off, 31); + else if (addr_size == 0) + off = (gva_t)sign_extend64(off, 15); if (base_is_valid) off += kvm_register_read(vcpu, base_reg); if (index_is_valid)
The VMCS.EXIT_QUALIFCATION field reports the displacements of memory operands for various instructions, including VMX instructions, as a naturally sized unsigned value, but masks the value by the addr size, e.g. given a ModRM encoded as -0x28(%ebp), the -0x28 displacement is reported as 0xffffffd8 for a 32-bit address size. Despite some weird wording regarding sign extension, the SDM explicitly states that bits beyond the instructions address size are undefined: In all cases, bits of this field beyond the instruction’s address size are undefined. Failure to sign extend the displacement results in KVM incorrectly treating a negative displacement as a large positive displacement when the address size of the VMX instruction is smaller than KVM's native size, e.g. a 32-bit address size on a 64-bit KVM. The very original decoding, added by commit 064aea774768 ("KVM: nVMX: Decoding memory operands of VMX instructions"), sort of modeled sign extension by truncating the final virtual/linear address for a 32-bit address size. I.e. it messed up the effective address but made it work by adjusting the final address. When segmentation checks were added, the truncation logic was kept as-is and no sign extension logic was introduced. In other words, it kept calculating the wrong effective address while mostly generating the correct virtual/linear address. As the effective address is what's used in the segment limit checks, this results in KVM incorreclty injecting #GP/#SS faults due to non-existent segment violations when a nested VMM uses negative displacements with an address size smaller than KVM's native address size. Using the -0x28(%ebp) example, an EBP value of 0x1000 will result in KVM using 0x100000fd8 as the effective address when checking for a segment limit violation. This causes a 100% failure rate when running a 32-bit KVM build as L1 on top of a 64-bit KVM L0. Fixes: f9eb4af67c9d ("KVM: nVMX: VMX instructions: add checks for #GP/#SS exceptions") Cc: stable@vger.kernel.org Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com> --- arch/x86/kvm/vmx/nested.c | 4 ++++ 1 file changed, 4 insertions(+)