4.9-stable review patch. If anyone has any objections, please let me know.
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From: Juergen Gross jgross@suse.com
Upstream commit: def9331a12977770cc6132d79f8e6565871e8e38 ("x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS when running under Xen")
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: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/kernel/cpu/amd.c | 5 +++-- arch/x86/xen/enlighten.c | 4 +--- 2 files changed, 4 insertions(+), 5 deletions(-)
--- a/arch/x86/kernel/cpu/amd.c +++ b/arch/x86/kernel/cpu/amd.c @@ -857,8 +857,9 @@ static void init_amd(struct cpuinfo_x86 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 --- a/arch/x86/xen/enlighten.c +++ b/arch/x86/xen/enlighten.c @@ -474,10 +474,8 @@ static void __init xen_init_cpuid_mask(v
static void __init xen_init_capabilities(void) { - if (xen_pv_domain()) { - setup_clear_cpu_cap(X86_BUG_SYSRET_SS_ATTRS); + if (xen_pv_domain()) setup_force_cpu_cap(X86_FEATURE_XENPV); - } }
static void xen_set_debugreg(int reg, unsigned long val)