[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [xen stable-4.17] x86/vmx: Revert "x86/VMX: sanitize rIP before re-entering guest"
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) -- generated by git-patchbot for /home/xen/git/xen.git#stable-4.17
|
![]() |
Lists.xenproject.org is hosted with RackSpace, monitoring our |