linux-kernel - [PATCH 01/19] x86, mpx, xsave: fix up bad get_xsave

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Message-Id: <20150508185948.01B91DEB@viggo.jf.intel.com>
Date:	Fri, 08 May 2015 11:59:48 -0700
From:	Dave Hansen <dave@...1.net>
To:	linux-kernel@...r.kernel.org
Cc:	x86@...nel.org, tglx@...utronix.de, Dave Hansen <dave@...1.net>,
	dave.hansen@...ux.intel.com
Subject: [PATCH 01/19] x86, mpx, xsave: fix up bad get_xsave_addr() assumptions


From: Dave Hansen <dave.hansen@...ux.intel.com>

get_xsave_addr() assumes that if an xsave bit is present in the
hardware (pcntxt_mask) that it is present in a given xsave
buffer.  Due to an bug in the xsave code on all of the systems
that have MPX (and thus all the users of this code), that has
been a true assumption.

But, the bug is getting fixed, so our assumption is not going
to hold any more.

It's quite possible (and normal) for an enabled state to be
present on 'pcntxt_mask', but *not* in 'xstate_bv'.  We need
to consult 'xstate_bv'.

Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
---

 b/arch/x86/kernel/xsave.c |   41 ++++++++++++++++++++++++++++++++++-------
 1 file changed, 34 insertions(+), 7 deletions(-)

diff -puN arch/x86/kernel/xsave.c~consullt-xstate_bv arch/x86/kernel/xsave.c
--- a/arch/x86/kernel/xsave.c~consullt-xstate_bv	2015-05-08 11:46:10.595563814 -0700
+++ b/arch/x86/kernel/xsave.c	2015-05-08 11:46:10.598563949 -0700
@@ -706,19 +706,46 @@ void __init_refok eager_fpu_init(void)
  * This is the API that is called to get xstate address in either
  * standard format or compacted format of xsave area.
  *
+ * Note that if there is no data for the field in the xsave buffer
+ * this will return NULL.
+ *
  * Inputs:
- *	xsave: base address of the xsave area;
- *	xstate: state which is defined in xsave.h (e.g. XSTATE_FP, XSTATE_SSE,
- *	etc.)
+ *	xstate: the thread's storage area for all FPU data
+ *	xstate_field: state which is defined in xsave.h (e.g. XSTATE_FP,
+ *	XSTATE_SSE, etc...)
  * Output:
  *	address of the state in the xsave area.
  */
-void *get_xsave_addr(struct xsave_struct *xsave, int xstate)
+void *get_xsave_addr(struct xsave_struct *xsave, int xstate_field)
 {
-	int feature = fls64(xstate) - 1;
-	if (!test_bit(feature, (unsigned long *)&pcntxt_mask))
+	int feature_nr = fls64(xstate_field) - 1;
+	/*
+	 * Do we even *have* xsave state?
+	 */
+	if (!boot_cpu_has(X86_FEATURE_XSAVE))
+		return NULL;
+
+	xsave = &current->thread.fpu.state->xsave;
+	/*
+	 * We should not ever be requesting fields that we
+	 * have not enabled.  Remember that pcntxt_mask is
+	 * what we write to the XCR0 register.
+	 */
+	WARN_ONCE(!(pcntxt_mask & xstate_field), "get of unsupported state");
+	/*
+	 * This assumes the last 'xsave*' instruction to
+	 * have requested that 'xstate_field' be saved.
+	 * If it did not, we might be seeing and old value
+	 * of the field in the buffer.
+	 *
+	 * This can happen because the last 'xsave' did not
+	 * request that this feature be saved (unlikely)
+	 * or because the "init optimization" caused it
+	 * to not be saved.
+	 */
+	if (!(xsave->xsave_hdr.xstate_bv & xstate_field))
 		return NULL;
 
-	return (void *)xsave + xstate_comp_offsets[feature];
+	return (void *)xsave + xstate_comp_offsets[feature_nr];
 }
 EXPORT_SYMBOL_GPL(get_xsave_addr);
_
--
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