linux-kernel - Re: [PATCH 1/3] x86/fpu/xstate: Fix last_good_offset in setup_xstate

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Date:   Tue, 11 Feb 2020 19:56:35 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/3] x86/fpu/xstate: Fix last_good_offset in
 setup_xstate_features()

On Thu, Jan 09, 2020 at 01:14:50PM -0800, Yu-cheng Yu wrote:
> The function setup_xstate_features() uses CPUID to find each xfeature's
> standard-format offset and size.  Since XSAVES always uses the compacted
> format, supervisor xstates are *NEVER* in the standard-format and their
> offsets are left as -1's.  However, they are still being tracked as
> last_good_offset.
> 
> Fix it by tracking only user xstate offsets.
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
> Reviewed-by: Dave Hansen <dave.hansen@...ux.intel.com>
> ---
>  arch/x86/kernel/fpu/xstate.c | 24 ++++++++++++++----------
>  1 file changed, 14 insertions(+), 10 deletions(-)

How about cleaning it up some more, while at it?

---
>From c12e13dcd814023a903399ec5ac2e7082d664b8b Mon Sep 17 00:00:00 2001
From: Yu-cheng Yu <yu-cheng.yu@...el.com>
Date: Thu, 9 Jan 2020 13:14:50 -0800
Subject: [PATCH] x86/fpu/xstate: Fix last_good_offset in
 setup_xstate_features()

The function setup_xstate_features() uses CPUID to find each xfeature's
standard-format offset and size.  Since XSAVES always uses the compacted
format, supervisor xstates are *NEVER* in the standard-format and their
offsets are left as -1's.  However, they are still being tracked as
last_good_offset.

Fix it by tracking only user xstate offsets.

 [ bp: Use xfeature_is_supervisor() and save an indentation level. Drop
   now unused xfeature_is_user(). ]

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
Signed-off-by: Borislav Petkov <bp@...e.de>
Reviewed-by: Dave Hansen <dave.hansen@...ux.intel.com>
Link: https://lkml.kernel.org/r/20200109211452.27369-2-yu-cheng.yu@intel.com
---
 arch/x86/kernel/fpu/xstate.c | 27 +++++++++++++--------------
 1 file changed, 13 insertions(+), 14 deletions(-)

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index a1806598aaa4..fe67cabfb4a1 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -120,11 +120,6 @@ static bool xfeature_is_supervisor(int xfeature_nr)
 	return ecx & 1;
 }
 
-static bool xfeature_is_user(int xfeature_nr)
-{
-	return !xfeature_is_supervisor(xfeature_nr);
-}
-
 /*
  * When executing XSAVEOPT (or other optimized XSAVE instructions), if
  * a processor implementation detects that an FPU state component is still
@@ -265,21 +260,25 @@ static void __init setup_xstate_features(void)
 
 		cpuid_count(XSTATE_CPUID, i, &eax, &ebx, &ecx, &edx);
 
+		xstate_sizes[i] = eax;
+
 		/*
-		 * If an xfeature is supervisor state, the offset
-		 * in EBX is invalid. We leave it to -1.
+		 * If an xfeature is supervisor state, the offset in EBX is
+		 * invalid, leave it to -1.
 		 */
-		if (xfeature_is_user(i))
-			xstate_offsets[i] = ebx;
+		if (xfeature_is_supervisor(i))
+			continue;
+
+		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.
+		 * 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);
+			  "x86/fpu: misordered xstate at %d\n", last_good_offset);
+
 		last_good_offset = xstate_offsets[i];
 	}
 }
-- 
2.21.0

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette