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x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS when running under Xen

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commit def9331a12 upstream

When running as Xen pv guest X86_BUG_SYSRET_SS_ATTRS must not be set
on AMD cpus.

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>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
David Woodhouse
2018-05-20 20:51:10 +01:00
committed by Greg Kroah-Hartman
parent b2d748b3a8
commit 944e0fc51a
2 changed files with 4 additions and 5 deletions
Download Patch File Download Diff File Expand all files Collapse all files

5
arch/x86/kernel/cpu/amd.c
View File

@ -824,8 +824,9 @@ static void init_amd(struct cpuinfo_x86 *c)
if (cpu_has(c, X86_FEATURE_3DNOW) || cpu_has(c, X86_FEATURE_LM))
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);
/* AMD CPUs don't reset SS attributes on SYSRET, Xen does. */
if (!cpu_has(c, X86_FEATURE_XENPV))
set_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS);
}
#ifdef CONFIG_X86_32

4
arch/x86/xen/enlighten.c
View File

@ -1977,10 +1977,8 @@ EXPORT_SYMBOL_GPL(xen_hvm_need_lapic);
static void xen_set_cpu_features(struct cpuinfo_x86 *c)
{
if (xen_pv_domain()) {
clear_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS);
if (xen_pv_domain())
set_cpu_cap(c, X86_FEATURE_XENPV);
}
}
static void xen_pin_vcpu(int cpu)