x86/bugs: Add X86_BUG_SPECTRE_V2_USER

All CPU vulnerabilities with command line options map to a single X86_BUG bit
except for Spectre V2 where both the spectre_v2 and spectre_v2_user command
line options are related to the same bug.

The spectre_v2 command line options mostly relate to user->kernel and
guest->host mitigations, while the spectre_v2_user command line options relate
to user->user or guest->guest protections.

Define a new X86_BUG bit for spectre_v2_user so each *_select_mitigation()
function in bugs.c is related to a unique X86_BUG bit.

No functional changes.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20250108202515.385902-2-david.kaplan@amd.com

arch/x86/include/asm/cpufeatures.h

@@ -537,4 +537,5 @@
 #define X86_BUG_RFDS			X86_BUG(1*32 + 2) /* "rfds" CPU is vulnerable to Register File Data Sampling */
 #define X86_BUG_BHI			X86_BUG(1*32 + 3) /* "bhi" CPU is affected by Branch History Injection */
 #define X86_BUG_IBPB_NO_RET	   	X86_BUG(1*32 + 4) /* "ibpb_no_ret" IBPB omits return target predictions */
+#define X86_BUG_SPECTRE_V2_USER		X86_BUG(1*32 + 5) /* "spectre_v2_user" CPU is affected by Spectre variant 2 attack between user processes */
 #endif /* _ASM_X86_CPUFEATURES_H */

arch/x86/kernel/cpu/common.c

@@ -1331,8 +1331,10 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
 
 	setup_force_cpu_bug(X86_BUG_SPECTRE_V1);
 
-	if (!cpu_matches(cpu_vuln_whitelist, NO_SPECTRE_V2))
+	if (!cpu_matches(cpu_vuln_whitelist, NO_SPECTRE_V2)) {
 		setup_force_cpu_bug(X86_BUG_SPECTRE_V2);
+		setup_force_cpu_bug(X86_BUG_SPECTRE_V2_USER);
+	}
 
 	if (!cpu_matches(cpu_vuln_whitelist, NO_SSB) &&
 	    !(x86_arch_cap_msr & ARCH_CAP_SSB_NO) &&