FAILED: patch "[PATCH] x86/cpu: Fix migration safety with X86_BUG_NULL_SEL" failed to apply to 4.4-stable tree

The patch below does not apply to the 4.4-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 415de44076640483648d6c0f6d645a9ee61328ad Mon Sep 17 00:00:00 2001
From: Jane Malalane <[email protected]>
Date: Thu, 21 Oct 2021 11:47:44 +0100
Subject: [PATCH] x86/cpu: Fix migration safety with X86_BUG_NULL_SEL

Currently, Linux probes for X86_BUG_NULL_SEL unconditionally which makes it unsafe to migrate in a virtualised environment as the properties across the migration pool might differ.

To be specific, the case which goes wrong is:

1. Zen1 (or earlier) and Zen2 (or later) in a migration pool
2. Linux boots on Zen2, probes and finds the absence of X86_BUG_NULL_SEL
3. Linux is then migrated to Zen1

Linux is now running on a X86_BUG_NULL_SEL-impacted CPU while believing that the bug is fixed.

The only way to address the problem is to fully trust the "no longer affected" CPUID bit when virtualised, because in the above case it would be clear deliberately to indicate the fact "you might migrate to somewhere which has this behaviour".

Zen3 adds the NullSelectorClearsBase CPUID bit to indicate that loading a NULL segment selector zeroes the base and limit fields, as well as just attributes. Zen2 also has this behaviour but doesn't have the NSCB bit.

[ bp: Minor touchups. ]

index 2131af9f2fa2..4edb6f0f628c 100644
@@ -989,6 +989,8 @@ static void init_amd(struct cpuinfo_x86 *c)
 	if (cpu_has(c, X86_FEATURE_IRPERF) &&
 	    !cpu_has_amd_erratum(c, amd_erratum_1054))
 		msr_set_bit(MSR_K7_HWCR, MSR_K7_HWCR_IRPERF_EN_BIT);
+
+	check_null_seg_clears_base(c);
 }
 
 #ifdef CONFIG_X86_32

index 95521302630d..ee6f23f7587d 100644
@@ -75,6 +75,7 @@ extern int detect_extended_topology_early(struct cpuinfo_x86 *c);
 extern int detect_extended_topology(struct cpuinfo_x86 *c);
 extern int detect_ht_early(struct cpuinfo_x86 *c);
 extern void detect_ht(struct cpuinfo_x86 *c);
+extern void check_null_seg_clears_base(struct cpuinfo_x86 *c);
 
 unsigned int aperfmperf_get_khz(int cpu);
 

index 6d50136f7ab9..3fcdda4c1e11 100644
@@ -335,6 +335,8 @@ static void init_hygon(struct cpuinfo_x86 *c)
 	/* Hygon 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);
+
+	check_null_seg_clears_base(c);
 }
 
 static void cpu_detect_tlb_hygon(struct cpuinfo_x86 *c)