File 64e6459b-revert-VMX-sanitize-rIP-before-reentering.patch of Package xen.31421
# Commit 10c83bb0f5d158d101d983883741b76f927e54a3
# Date 2023-08-23 18:44:59 +0100
# Author Andrew Cooper <andrew.cooper3@citrix.com>
# Committer Andrew Cooper <andrew.cooper3@citrix.com>
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@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -3939,7 +3939,7 @@ 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;
@@ -4566,38 +4566,6 @@ void vmx_vmexit_handler(struct cpu_user_
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)