lkml.org 
[lkml]   [2017]   [Apr]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS if forced to zero
Date
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
\
 
 \ /
  Last update: 2017-04-25 20:00    [W:1.776 / U:0.416 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site