[Xen-devel] Patch "x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS when running under Xen" has been added to the 4.9-stable tree

[<gregkh@xxxxxxxxxxxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

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

to the 4.9-stable tree which can be found at:
    
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     x86-amd-don-t-set-x86_bug_sysret_ss_attrs-when-running-under-xen.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.


From foo@baz Sat Jun  2 15:10:07 CEST 2018
From: Juergen Gross <jgross@xxxxxxxx>
Date: Wed, 30 May 2018 13:09:58 +0200
Subject: x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS when running under Xen
To: stable@xxxxxxxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx
Cc: dwmw@xxxxxxxxxxxx, boris.ostrovsky@xxxxxxxxxx, Juergen Gross 
<jgross@xxxxxxxx>
Message-ID: <20180530110958.19413-4-jgross@xxxxxxxx>

From: Juergen Gross <jgross@xxxxxxxx>

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@xxxxxxxx>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Acked-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 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
@@ -473,10 +473,8 @@ 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)


Patches currently in stable-queue which might be from jgross@xxxxxxxx are

queue-4.9/x86-amd-don-t-set-x86_bug_sysret_ss_attrs-when-running-under-xen.patch
queue-4.9/x86-amd-revert-commit-944e0fc51a89c9827b9.patch
queue-4.9/xen-set-cpu-capabilities-from-xen_start_kernel.patch

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