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* [PATCH v3 1/2] x86/speculation: Allow enabling STIBP with legacy IBRS
@ 2023-02-27  6:05 KP Singh
  2023-02-27  6:05 ` [PATCH v3 2/2] Documentation/hw-vuln: Document the interaction between IBRS and STIBP KP Singh
  2023-02-27  6:29 ` [PATCH v3 1/2] x86/speculation: Allow enabling STIBP with legacy IBRS Greg KH
  0 siblings, 2 replies; 5+ messages in thread
From: KP Singh @ 2023-02-27  6:05 UTC (permalink / raw)
  To: linux-kernel
  Cc: pjt, evn, jpoimboe, tglx, x86, hpa, peterz, pawan.kumar.gupta,
	kim.phillips, alexandre.chartre, daniel.sneddon, corbet, bp,
	linyujun809, kpsingh, jmattson, mingo, seanjc, andrew.cooper3,
	José Oliveira, Rodrigo Branco, stable

When plain IBRS is enabled (not enhanced IBRS), the logic in
spectre_v2_user_select_mitigation() determines that STIBP is not needed.

The IBRS bit implicitly protects against cross-thread branch target
injection. However, with legacy IBRS, the IBRS bit is cleared on
returning to userspace for performance reasons which leaves userspace
threads vulnerable to cross-thread branch target injection against which
STIBP protects.

Exclude IBRS from the spectre_v2_in_ibrs_mode() check to allow for
enabling STIBP (through seccomp/prctl() by default or always-on, if
selected by spectre_v2_user kernel cmdline parameter).

Fixes: 7c693f54c873 ("x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS")
Reported-by: José Oliveira <joseloliveira11@gmail.com>
Reported-by: Rodrigo Branco <rodrigo@kernelhacking.com>
Cc: stable@vger.kernel.org
Signed-off-by: KP Singh <kpsingh@kernel.org>
---
 arch/x86/kernel/cpu/bugs.c | 26 +++++++++++++++++++-------
 1 file changed, 19 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index cf81848b72f4..44e22cda7fb3 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1133,14 +1133,18 @@ spectre_v2_parse_user_cmdline(void)
 	return SPECTRE_V2_USER_CMD_AUTO;
 }
 
-static inline bool spectre_v2_in_ibrs_mode(enum spectre_v2_mitigation mode)
+static inline bool spectre_v2_in_eibrs_mode(enum spectre_v2_mitigation mode)
 {
-	return mode == SPECTRE_V2_IBRS ||
-	       mode == SPECTRE_V2_EIBRS ||
+	return mode == SPECTRE_V2_EIBRS ||
 	       mode == SPECTRE_V2_EIBRS_RETPOLINE ||
 	       mode == SPECTRE_V2_EIBRS_LFENCE;
 }
 
+static inline bool spectre_v2_in_ibrs_mode(enum spectre_v2_mitigation mode)
+{
+	return spectre_v2_in_eibrs_mode(mode) || mode == SPECTRE_V2_IBRS;
+}
+
 static void __init
 spectre_v2_user_select_mitigation(void)
 {
@@ -1203,12 +1207,20 @@ spectre_v2_user_select_mitigation(void)
 	}
 
 	/*
-	 * If no STIBP, IBRS or enhanced IBRS is enabled, or SMT impossible,
-	 * STIBP is not required.
+	 * If no STIBP, enhanced IBRS is enabled, or SMT impossible, STIBP
+	 * is not required.
+	 *
+	 * Enhanced IBRS also protects against cross-thread branch target
+	 * injection in user-mode as the IBRS bit remains always set which
+	 * implicitly enables cross-thread protections.  However, in legacy IBRS
+	 * mode, the IBRS bit is set only on kernel entry and cleared on return
+	 * to userspace. This disables the implicit
+	 * cross-thread protection, so allow for STIBP to be selected in that
+	 * case.
 	 */
 	if (!boot_cpu_has(X86_FEATURE_STIBP) ||
 	    !smt_possible ||
-	    spectre_v2_in_ibrs_mode(spectre_v2_enabled))
+	    spectre_v2_in_eibrs_mode(spectre_v2_enabled))
 		return;
 
 	/*
@@ -2340,7 +2352,7 @@ static ssize_t mmio_stale_data_show_state(char *buf)
 
 static char *stibp_state(void)
 {
-	if (spectre_v2_in_ibrs_mode(spectre_v2_enabled))
+	if (spectre_v2_in_eibrs_mode(spectre_v2_enabled))
 		return "";
 
 	switch (spectre_v2_user_stibp) {
-- 
2.39.2.637.g21b0678d19-goog


^ permalink raw reply related	[flat|nested] 5+ messages in thread

end of thread, other threads:[~2023-02-27 19:59 UTC | newest]

Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2023-02-27  6:05 [PATCH v3 1/2] x86/speculation: Allow enabling STIBP with legacy IBRS KP Singh
2023-02-27  6:05 ` [PATCH v3 2/2] Documentation/hw-vuln: Document the interaction between IBRS and STIBP KP Singh
2023-02-27  6:30   ` Greg KH
2023-02-27 19:58   ` [tip: x86/urgent] " tip-bot2 for KP Singh
2023-02-27  6:29 ` [PATCH v3 1/2] x86/speculation: Allow enabling STIBP with legacy IBRS Greg KH

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