From: Jan Beulich <jbeulich@suse.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Xen-devel <xen-devel@lists.xenproject.org>,
"Roger Pau Monné" <roger.pau@citrix.com>, "Wei Liu" <wl@xen.org>,
"Jun Nakajima" <jun.nakajima@intel.com>,
"Kevin Tian" <kevin.tian@intel.com>
Subject: Re: [PATCH] x86/vmx: Revert "x86/VMX: sanitize rIP before re-entering guest"
Date: Tue, 13 Oct 2020 17:58:50 +0200 [thread overview]
Message-ID: <fb4a7a1d-95ad-0b59-7cb9-4a94c3600960@suse.com> (raw)
In-Reply-To: <20201009150948.31063-1-andrew.cooper3@citrix.com>
On 09.10.2020 17:09, Andrew Cooper wrote:
> 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@citrix.com>
But why revert the change rather than fix ...
> @@ -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)
... the wrong use of is_canonical_address() here? By reverting
you open up avenues for XSAs in case we get things wrong elsewhere,
including ...
> - : regs->rip != regs->eip )
... for 32-bit guests.
Jan
next prev parent reply other threads:[~2020-10-13 15:59 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-09 15:09 [PATCH] x86/vmx: Revert "x86/VMX: sanitize rIP before re-entering guest" Andrew Cooper
2020-10-13 15:58 ` Jan Beulich [this message]
2020-10-14 13:57 ` Andrew Cooper
2020-10-15 8:01 ` Jan Beulich
2020-10-16 15:38 ` Andrew Cooper
2020-10-19 9:09 ` Jan Beulich
2020-10-19 16:12 ` Andrew Cooper
2020-10-20 8:09 ` Jan Beulich
2020-10-23 6:14 ` Tian, Kevin
2023-04-05 21:52 Andrew Cooper
2023-04-06 7:10 ` Jan Beulich
2023-08-23 11:15 ` Roger Pau Monné
2023-08-23 11:56 ` Andrew Cooper
2023-08-23 13:31 ` Roger Pau Monné
2023-08-23 14:09 ` Andrew Cooper
2023-08-24 4:26 ` Tian, Kevin
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