[xen stable-4.17] x86/vmx: Revert "x86/VMX: sanitize rIP before re-entering guest"

[patchbot@xxxxxxx]

commit 8be85d8c0df2445c012fac42117396b483db5db0
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Tue Sep 5 08:53:31 2023 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Sep 5 08:53:31 2023 +0200

    x86/vmx: Revert "x86/VMX: sanitize rIP before re-entering guest"
    
    At the time of XSA-170, the x86 instruction emulator was genuinely broken.  
It
    would load arbitrary values into %rip and putting a check here probably was
    the best stopgap security fix.  It should have been reverted following c/s
    81d3a0b26c1 "x86emul: limit-check branch targets" which corrected the 
emulator
    behaviour.
    
    However, everyone involved in XSA-170, myself included, failed to read the 
SDM
    correctly.  On the subject of %rip consistency checks, the SDM stated:
    
      If the processor supports N < 64 linear-address bits, bits 63:N must be
      identical
    
    A non-canonical %rip (and SSP more recently) is an explicitly legal state in
    x86, and the VMEntry consistency checks are intentionally off-by-one from a
    regular canonical check.
    
    The consequence of this bug is that Xen will currently take a legal x86 
state
    which would successfully VMEnter, and corrupt it into having 
non-architectural
    behaviour.
    
    Furthermore, in the time this bugfix has been pending in public, I
    successfully persuaded Intel to clarify the SDM, adding the following
    clarification:
    
      The guest RIP value is not required to be canonical; the value of bit N-1
      may differ from that of bit N.
    
    Fixes: ffbbfda377 ("x86/VMX: sanitize rIP before re-entering guest")
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    master commit: 10c83bb0f5d158d101d983883741b76f927e54a3
    master date: 2023-08-23 18:44:59 +0100
---
 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 f256dc2635..072288a5ef 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -3975,7 +3975,7 @@ static void undo_nmis_unblocked_by_iret(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;
 
@@ -4650,38 +4650,6 @@ 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)
--
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