x86/speculation: Fix SPEC_CTRL write on SMT state change

jpoimboe · suryasaimadhu · commit 56aa4d221f1e · 2022-06-27T10:34:00.000+02:00
If the SMT state changes, SSBD might get accidentally disabled.  Fix
that.

Signed-off-by: Josh Poimboeuf &lt;jpoimboe@kernel.org&gt;
Signed-off-by: Peter Zijlstra (Intel) &lt;peterz@infradead.org&gt;
Signed-off-by: Borislav Petkov &lt;bp@suse.de&gt;
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
@@ -1458,7 +1458,8 @@ static void __init spectre_v2_select_mitigation(void)
 
 static void update_stibp_msr(void * __unused)
 {
-	write_spec_ctrl_current(x86_spec_ctrl_base, true);
+	u64 val = spec_ctrl_current() | (x86_spec_ctrl_base & SPEC_CTRL_STIBP);
+	write_spec_ctrl_current(val, true);
 }
 
 /* Update x86_spec_ctrl_base in case SMT state changed. */

Original file line number	Diff line number	Diff line change
`@@ -1458,7 +1458,8 @@ static void __init spectre_v2_select_mitigation(void)`
`1458`	`1458`
`1459`	`1459`	`static void update_stibp_msr(void * __unused)`
`1460`	`1460`	`{`
`1461`		`- write_spec_ctrl_current(x86_spec_ctrl_base, true);`
	`1461`	`+ u64 val = spec_ctrl_current() \| (x86_spec_ctrl_base & SPEC_CTRL_STIBP);`
	`1462`	`+ write_spec_ctrl_current(val, true);`
`1462`	`1463`	`}`
`1463`	`1464`
`1464`	`1465`	`/* Update x86_spec_ctrl_base in case SMT state changed. */`