[PATCH 14/15] x86, fpu: check to ensure increasing-offset xstate offsets

[Dave Hansen]

From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

The xstate CPUID leaves enumerate where each state component is
inside the XSAVE buffer, along with the size of the entire
buffer.  Our new XSAVE sanity-checking code extrapolates an
expected _total_ buffer size by looking at the last component
that it encounters.

That method requires that the highest-numbered component also
be the one with the highest offset.  This is a pretty safe
assumption, but let's add some code to ensure it stays true.

To make this check work correctly, we also need to ensure we
only consider the offsets from enabled features because the
offset register (ebx) will return 0 on unsupported features.

This also means that we will preserve the -1's that we
initialized xstate_offsets/sizes[] with.  That will help
find bugs.

Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
---

 b/arch/x86/kernel/fpu/xstate.c |   35 ++++++++++++++++++++++++-----------
 1 file changed, 24 insertions(+), 11 deletions(-)

diff -puN arch/x86/kernel/fpu/xstate.c~x86-fpu-increasing-offset-assumption arch/x86/kernel/fpu/xstate.c
--- a/arch/x86/kernel/fpu/xstate.c~x86-fpu-increasing-offset-assumption	2015-08-31 15:17:37.174009326 -0700
+++ b/arch/x86/kernel/fpu/xstate.c	2015-08-31 15:17:37.177009463 -0700
@@ -185,18 +185,41 @@ void fpu__init_cpu_xstate(void)
 }
 
 /*
+ * Note that in the future we will likely need a pair of
+ * functions here: one for user xstates and the other for
+ * system xstates.  For now, they are the same.
+ */
+static int xfeature_enabled(enum xfeature xfeature)
+{
+	return !!(xfeatures_mask & (1UL << xfeature));
+}
+
+/*
  * Record the offsets and sizes of various xstates contained
  * in the XSAVE state memory layout.
  */
 static void __init setup_xstate_features(void)
 {
 	u32 eax, ebx, ecx, edx, i;
+	/* start at the beginnning of the "extended state" */
+	unsigned int last_good_offset = offsetof(struct xregs_state,
+						 extended_state_area);
 
 	for (i = FIRST_EXTENDED_XFEATURE; i < XFEATURE_MAX; i++) {
-		cpuid_count(XSTATE_CPUID, i, &eax, &ebx, &ecx, &edx);
+		if (!xfeature_enabled(i))
+			continue;
 
+		cpuid_count(XSTATE_CPUID, i, &eax, &ebx, &ecx, &edx);
 		xstate_offsets[i] = ebx;
 		xstate_sizes[i] = eax;
+		/*
+		 * In our xstate size checks, we assume that the
+		 * highest-numbered xstate feature has the
+		 * highest offset in the buffer.  Ensure it does.
+		 */
+		WARN_ONCE(last_good_offset > xstate_offsets[i],
+			"x86/fpu: misordered xstate at %d\n", last_good_offset);
+		last_good_offset = xstate_offsets[i];
 
 		printk(KERN_INFO "x86/fpu: xstate_offset[%d]: %4d, xstate_sizes[%d]: %4d\n", i, ebx, i, eax);
 	}
@@ -226,16 +249,6 @@ static void __init print_xstate_features
 }
 
 /*
- * Note that in the future we will likely need a pair of
- * functions here: one for user xstates and the other for
- * system xstates.  For now, they are the same.
- */
-static int xfeature_enabled(enum xfeature xfeature)
-{
-	return !!(xfeatures_mask & (1UL << xfeature));
-}
-
-/*
  * This function sets up offsets and sizes of all extended states in
  * xsave area. This supports both standard format and compacted format
  * of the xsave aread.
_
--
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