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

BugLink: https://bugs.launchpad.net/bugs/1791080

commit def9331a 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>
Signed-off-by: Srivatsa S. Bhat <srivatsa@csail.mit.edu>
Reviewed-by: Matt Helsley (VMware) <matt.helsley@gmail.com>
Reviewed-by: Alexey Makhalov <amakhalov@vmware.com>
Reviewed-by: Bo Gan <ganb@vmware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

arch/x86/kernel/cpu/amd.c

@@ -831,8 +831,9 @@ 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);
 
 	/*
 	 * On AMD family 0x10, 0x12 and 0x16 processors that do not support the

arch/x86/xen/enlighten.c

@@ -463,10 +463,8 @@ static void __init xen_init_cpuid_mask(void)
 
 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)