Re: [PATCH v2 1/2] x86/speculation: Allow enabling STIBP with legacy IBRS

[Borislav Petkov <bp@alien8.de>]

On Tue, Feb 21, 2023 at 07:49:07PM +0100, KP Singh wrote:
> Setting the IBRS bit implicitly enables STIBP to protect against
> cross-thread branch target injection. With enhanced IBRS, the bit it set
> once and is not cleared again. However, on CPUs with just legacy IBRS,
> IBRS bit set on user -> kernel and cleared on kernel -> user (a.k.a
> KERNEL_IBRS). Clearing this bit also disables the implicitly enabled
> STIBP, thus requiring some form of cross-thread protection in userspace.
> 
> Enable STIBP, either opt-in via prctl or seccomp, or always on depending
> on the choice of mitigation selected via spectre_v2_user.
> 
> Reported-by: José Oliveira <joseloliveira11@gmail.com>
> Reported-by: Rodrigo Branco <rodrigo@kernelhacking.com>
> Reviewed-by: Alexandra Sandulescu <aesa@google.com>
> Fixes: 7c693f54c873 ("x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS")
> Cc: stable@vger.kernel.org
> Signed-off-by: KP Singh <kpsingh@kernel.org>
> ---
>  arch/x86/kernel/cpu/bugs.c | 33 ++++++++++++++++++++++-----------
>  1 file changed, 22 insertions(+), 11 deletions(-)

Below is what I'd like to see. Holler if something's wrong.

It is totally untested ofc.

---
From: KP Singh <kpsingh@kernel.org>
Date: Tue, 21 Feb 2023 19:49:07 +0100
Subject: [PATCH] x86/speculation: Allow enabling STIBP with legacy IBRS
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

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

However, on return to userspace, the IBRS bit is cleared for performance
reasons. That leaves userspace threads vulnerable to cross-thread
predictions influence against which STIBP protects.

Exclude IBRS from the spectre_v2_in_ibrs_mode() check to allow for
enabling STIBP through seccomp/prctl().

  [ bp: Rewrite commit message and massage. ]

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>
Signed-off-by: KP Singh <kpsingh@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20230221184908.2349578-1-kpsingh@kernel.org
---
 arch/x86/kernel/cpu/bugs.c | 25 ++++++++++++++++++-------
 1 file changed, 18 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index cf81848b72f4..9a969ab0e62a 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 +1207,19 @@ 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 protects also against user-mode attacks 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 protections 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 +2351,7 @@ 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) {
-- 
2.35.1

-- 
Regards/Gruss,
    Boris.

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