[PATCH] x86/vmx: Revert "x86/VMX: sanitize rIP before re-entering guest"

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

At the time of XSA-170, the x86 instruction emulator really was broken, and
would allow arbitrary non-canonical values to be loaded into %rip.  This was
fixed after the embargo by c/s 81d3a0b26c1 "x86emul: limit-check branch
targets".

However, in a demonstration that off-by-one errors really are one of the
hardest programming issues we face, everyone involved with XSA-170, myself
included, mistook the statement in the SDM which says:

  If the processor supports N < 64 linear-address bits, bits 63:N must be 
identical

to mean "must be canonical".  A real canonical check is bits 63:N-1.

VMEntries really do tolerate a not-quite-canonical %rip, specifically to cater
to the boundary condition at 0x0000800000000000.

Now that the emulator has been fixed, revert the XSA-170 change to fix
architectural behaviour at the boundary case.  The XTF test case for XSA-170
exercises this corner case, and still passes.

Fixes: ffbbfda377 ("x86/VMX: sanitize rIP before re-entering guest")
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Jun Nakajima <jun.nakajima@xxxxxxxxx>
CC: Kevin Tian <kevin.tian@xxxxxxxxx>
---
 xen/arch/x86/hvm/vmx/vmx.c | 34 +---------------------------------
 1 file changed, 1 insertion(+), 33 deletions(-)

diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index 86b8916a5d..28d09c1ca0 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -3650,7 +3650,7 @@ static int vmx_handle_apic_write(void)
 void vmx_vmexit_handler(struct cpu_user_regs *regs)
 {
     unsigned long exit_qualification, exit_reason, idtv_info, intr_info = 0;
-    unsigned int vector = 0, mode;
+    unsigned int vector = 0;
     struct vcpu *v = current;
     struct domain *currd = v->domain;
 
@@ -4280,38 +4280,6 @@ void vmx_vmexit_handler(struct cpu_user_regs *regs)
 out:
     if ( nestedhvm_vcpu_in_guestmode(v) )
         nvmx_idtv_handling();
-
-    /*
-     * VM entry will fail (causing the guest to get crashed) if rIP (and
-     * rFLAGS, but we don't have an issue there) doesn't meet certain
-     * criteria. As we must not allow less than fully privileged mode to have
-     * such an effect on the domain, we correct rIP in that case (accepting
-     * this not being architecturally correct behavior, as the injected #GP
-     * fault will then not see the correct [invalid] return address).
-     * And since we know the guest will crash, we crash it right away if it
-     * already is in most privileged mode.
-     */
-    mode = vmx_guest_x86_mode(v);
-    if ( mode == 8 ? !is_canonical_address(regs->rip)
-                   : regs->rip != regs->eip )
-    {
-        gprintk(XENLOG_WARNING, "Bad rIP %lx for mode %u\n", regs->rip, mode);
-
-        if ( vmx_get_cpl() )
-        {
-            __vmread(VM_ENTRY_INTR_INFO, &intr_info);
-            if ( !(intr_info & INTR_INFO_VALID_MASK) )
-                hvm_inject_hw_exception(TRAP_gp_fault, 0);
-            /* Need to fix rIP nevertheless. */
-            if ( mode == 8 )
-                regs->rip = (long)(regs->rip << (64 - VADDR_BITS)) >>
-                            (64 - VADDR_BITS);
-            else
-                regs->rip = regs->eip;
-        }
-        else
-            domain_crash(v->domain);
-    }
 }
 
 static void lbr_tsx_fixup(void)
-- 
2.11.0