From: Jan Beulich <jbeulich@suse.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Xen-devel <xen-devel@lists.xenproject.org>,
"Wei Liu" <wl@xen.org>, "Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [PATCH] x86/traps: Rework #PF[Rsvd] bit handling
Date: Tue, 19 May 2020 10:14:07 +0200 [thread overview]
Message-ID: <bf0d9e00-cb42-34b1-26ee-93628eea094c@suse.com> (raw)
In-Reply-To: <20200518153820.18170-1-andrew.cooper3@citrix.com>
On 18.05.2020 17:38, Andrew Cooper wrote:
> The reserved_bit_page_fault() paths effectively turn reserved bit faults into
> a warning, but in the light of L1TF, the real impact is far more serious.
>
> Xen does not have any reserved bits set in its pagetables, nor do we permit PV
> guests to write any. An HVM shadow guest may have reserved bits via the MMIO
> fastpath, but those faults are handled in the VMExit #PF intercept, rather
> than Xen's #PF handler.
>
> There is no need to disable interrupts (in spurious_page_fault()) for
> __page_fault_type() to look at the rsvd bit, nor should extable fixup be
> tolerated.
I'm afraid I don't understand the connection of the first half of this
to the patch - you don't alter spurious_page_fault() in this regard (at
all, actually).
As to extable fixup, I'm not sure: If a reserved bit ends up slipping
into the non-Xen parts of the page tables, and if guest accessors then
become able to trip a corresponding #PF, the bug will need an XSA with
the proposed change, while - afaict - it won't if the exception gets
recovered from. (There may then still be log spam issue, I admit.)
> @@ -1439,6 +1418,18 @@ void do_page_fault(struct cpu_user_regs *regs)
> if ( unlikely(fixup_page_fault(addr, regs) != 0) )
> return;
>
> + /*
> + * Xen have reserved bits in its pagetables, nor do we permit PV guests to
> + * write any. Such entries would be vulnerable to the L1TF sidechannel.
> + *
> + * The only logic which intentionally sets reserved bits is the shadow
> + * MMIO fastpath (SH_L1E_MMIO_*), which is careful not to be
> + * L1TF-vulnerable, and handled via the VMExit #PF intercept path, rather
> + * than here.
> + */
> + if ( error_code & PFEC_reserved_bit )
> + goto fatal;
Judging from the description, wouldn't this then better go even further
up, ahead of the fixup_page_fault() invocation? In fact the function
has two PFEC_reserved_bit checks to _avoid_ taking action, which look
like they could then be dropped.
Jan
next prev parent reply other threads:[~2020-05-19 8:14 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-18 15:38 [PATCH] x86/traps: Rework #PF[Rsvd] bit handling Andrew Cooper
2020-05-18 15:40 ` Andrew Cooper
2020-05-19 8:14 ` Jan Beulich [this message]
2020-05-19 14:29 ` Andrew Cooper
2020-05-19 14:55 ` Jan Beulich
2020-05-19 15:59 ` Andrew Cooper
2020-05-19 8:34 ` Jan Beulich
2020-05-19 14:11 ` Andrew Cooper
2020-05-19 14:48 ` Jan Beulich
2020-05-19 15:33 ` Andrew Cooper
2020-05-19 16:09 ` Jan Beulich
2020-05-19 18:00 ` Andrew Cooper
2020-05-20 7:48 ` Jan Beulich
2020-05-20 15:48 ` Andrew Cooper
2020-05-20 7:10 ` Tim Deegan
2020-05-21 15:43 ` [PATCH v2] " Andrew Cooper
2020-05-22 13:51 ` Jan Beulich
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