[PATCH L1TF v10 5/8] common/memory: block speculative out-of-bound accesses

From: Norbert Manthey <nmanthey@amazon.de>
To: xen-devel@lists.xenproject.org
Cc: Juergen Gross <jgross@suse.com>, Tim Deegan <tim@xen.org>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Wei Liu <wei.liu2@citrix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>,
	Dario Faggioli <dfaggioli@suse.com>,
	Martin Pohlack <mpohlack@amazon.de>,
	Pawel Wieczorkiewicz <wipawel@amazon.de>,
	Julien Grall <julien.grall@arm.com>,
	David Woodhouse <dwmw@amazon.co.uk>,
	Jan Beulich <jbeulich@suse.com>,
	Martin Mazein <amazein@amazon.de>,
	Bjoern Doebel <doebel@amazon.de>,
	Norbert Manthey <nmanthey@amazon.de>
Subject: [PATCH L1TF v10 5/8] common/memory: block speculative out-of-bound accesses
Date: Thu, 14 Mar 2019 13:50:08 +0100	[thread overview]
Message-ID: <1552567811-5301-6-git-send-email-nmanthey@amazon.de> (raw)
In-Reply-To: <1552567811-5301-1-git-send-email-nmanthey@amazon.de>

The get_page_from_gfn method returns a pointer to a page that belongs
to a gfn. Before returning the pointer, the gfn is checked for being
valid. Under speculation, these checks can be bypassed, so that
the function get_page is still executed partially. Consequently, the
function page_get_owner_and_reference might be executed partially as
well. In this function, the computed pointer is accessed, resulting in
a speculative out-of-bound address load. As the gfn can be controlled by
a guest, this access is problematic.

To mitigate the root cause, an lfence instruction is added via the
evaluate_nospec macro. To make the protection generic, we do not
introduce the lfence instruction for this single check, but add it to
the mfn_valid function. This way, other potentially problematic accesses
are protected as well.

This is part of the speculative hardening effort.

Signed-off-by: Norbert Manthey <nmanthey@amazon.de>
Acked-by: Jan Beulich <jbeulich@suse.com>

---
 xen/common/pdx.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/xen/common/pdx.c b/xen/common/pdx.c
--- a/xen/common/pdx.c
+++ b/xen/common/pdx.c
@@ -18,6 +18,7 @@
 #include <xen/init.h>
 #include <xen/mm.h>
 #include <xen/bitops.h>
+#include <xen/nospec.h>
 
 /* Parameters for PFN/MADDR compression. */
 unsigned long __read_mostly max_pdx;
@@ -33,8 +34,9 @@ unsigned long __read_mostly pdx_group_valid[BITS_TO_LONGS(
 
 bool __mfn_valid(unsigned long mfn)
 {
-    return likely(mfn < max_page) &&
-           likely(!(mfn & pfn_hole_mask)) &&
+    if ( unlikely(evaluate_nospec(mfn >= max_page)) )
+        return false;
+    return likely(!(mfn & pfn_hole_mask)) &&
            likely(test_bit(pfn_to_pdx(mfn) / PDX_GROUP_COUNT,
                            pdx_group_valid));
 }
-- 
2.7.4




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