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@xxxxxxxxxxxxxxx
Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
---
arch/x86/kvm/vmx/nested.c | 4 ++++
1 file changed, 4 insertions(+)
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)
--
2.20.1
