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[PATCH] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS if forced to zero

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When running as Xen pv guest X86_BUG_SYSRET_SS_ATTRS must not be set
on AMD cpus. Xen will disable this via setup_clear_cpu_cap(), so test
cpu_caps_cleared to not have disabled this bit.

This bug/feature bit is kind of special as it will be used very early
when switching threads. Setting the bit and clearing it a little bit
later leaves a critical window where things can go wrong. This time
window has enlarged a little bit by using setup_clear_cpu_cap() instead
of the hypervisor's set_cpu_features callback. It seems this larger
window now makes it rather easy to hit the problem.

The proper solution is to never set the bit in case of Xen.

Signed-off-by: Juergen Gross <jgross@suse.com>
---
 arch/x86/kernel/cpu/amd.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index c36140d788fe..f659b6f534b7 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -800,7 +800,9 @@ static void init_amd(struct cpuinfo_x86 *c)
 			set_cpu_cap(c, X86_FEATURE_3DNOWPREFETCH);
 
 	/* AMD CPUs don't reset SS attributes on SYSRET */
-	set_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS);
+	if (!test_bit(X86_BUG_SYSRET_SS_ATTRS,
+		      (unsigned long *)cpu_caps_cleared))
+		set_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS);
 }
 
 #ifdef CONFIG_X86_32
-- 
2.12.0

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[PATCH] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS if forced to zero Juergen Gross <jgross@suse.com> - 2017-04-25 20:10 +0200
  Re: [PATCH] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS if forced to  zero Borislav Petkov <bp@alien8.de> - 2017-04-25 20:30 +0200
    Re: [PATCH] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS if forced to  zero Juergen Gross <jgross@suse.com> - 2017-04-25 20:40 +0200
      Re: [PATCH] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS if forced to  zero Borislav Petkov <bp@alien8.de> - 2017-04-25 21:20 +0200
        Re: [Xen-devel] [PATCH] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS if  forced to zero Andrew Cooper <andrew.cooper3@citrix.com> - 2017-04-25 22:20 +0200
          Re: [Xen-devel] [PATCH] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS  if forced to zero Borislav Petkov <bp@alien8.de> - 2017-04-25 22:30 +0200
        Re: [PATCH] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS if forced to  zero Juergen Gross <jgross@suse.com> - 2017-04-26 06:50 +0200
          Re: [PATCH] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS if forced to  zero Borislav Petkov <bp@alien8.de> - 2017-04-26 08:40 +0200
            Re: [PATCH] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS if forced to  zero Juergen Gross <jgross@suse.com> - 2017-04-26 20:30 +0200
              Re: [PATCH] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS if forced to  zero Borislav Petkov <bp@alien8.de> - 2017-04-27 00:10 +0200
                Re: [PATCH] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS if forced to  zero Juergen Gross <jgross@suse.com> - 2017-04-27 06:50 +0200

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