linux-kernel - [tip: x86/urgent] x86/speculation: Fix prctl() when spectre_v2

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Message-ID: <160633225579.3364.12052138578908275403.tip-bot2@tip-bot2>
Date:   Wed, 25 Nov 2020 19:24:15 -0000
From:   "tip-bot2 for Anand K Mistry" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Anand K Mistry <amistry@...gle.com>, Borislav Petkov <bp@...e.de>,
        <stable@...r.kernel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/urgent] x86/speculation: Fix prctl() when
 spectre_v2_user={seccomp,prctl},ibpb

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     33fc379df76b4991e5ae312f07bcd6820811971e
Gitweb:        https://git.kernel.org/tip/33fc379df76b4991e5ae312f07bcd6820811971e
Author:        Anand K Mistry <amistry@...gle.com>
AuthorDate:    Tue, 10 Nov 2020 12:33:53 +11:00
Committer:     Borislav Petkov <bp@...e.de>
CommitterDate: Wed, 25 Nov 2020 20:17:09 +01:00

x86/speculation: Fix prctl() when spectre_v2_user={seccomp,prctl},ibpb

When spectre_v2_user={seccomp,prctl},ibpb is specified on the command
line, IBPB is force-enabled and STIPB is conditionally-enabled (or not
available).

However, since

  21998a351512 ("x86/speculation: Avoid force-disabling IBPB based on STIBP and enhanced IBRS.")

the spectre_v2_user_ibpb variable is set to SPECTRE_V2_USER_{PRCTL,SECCOMP}
instead of SPECTRE_V2_USER_STRICT, which is the actual behaviour.
Because the issuing of IBPB relies on the switch_mm_*_ibpb static
branches, the mitigations behave as expected.

Since

  1978b3a53a74 ("x86/speculation: Allow IBPB to be conditionally enabled on CPUs with always-on STIBP")

this discrepency caused the misreporting of IB speculation via prctl().

On CPUs with STIBP always-on and spectre_v2_user=seccomp,ibpb,
prctl(PR_GET_SPECULATION_CTRL) would return PR_SPEC_PRCTL |
PR_SPEC_ENABLE instead of PR_SPEC_DISABLE since both IBPB and STIPB are
always on. It also allowed prctl(PR_SET_SPECULATION_CTRL) to set the IB
speculation mode, even though the flag is ignored.

Similarly, for CPUs without SMT, prctl(PR_GET_SPECULATION_CTRL) should
also return PR_SPEC_DISABLE since IBPB is always on and STIBP is not
available.

 [ bp: Massage commit message. ]

Fixes: 21998a351512 ("x86/speculation: Avoid force-disabling IBPB based on STIBP and enhanced IBRS.")
Fixes: 1978b3a53a74 ("x86/speculation: Allow IBPB to be conditionally enabled on CPUs with always-on STIBP")
Signed-off-by: Anand K Mistry <amistry@...gle.com>
Signed-off-by: Borislav Petkov <bp@...e.de>
Cc: <stable@...r.kernel.org>
Link: https://lkml.kernel.org/r/20201110123349.1.Id0cbf996d2151f4c143c90f9028651a5b49a5908@changeid
---
 arch/x86/kernel/cpu/bugs.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 581fb72..d41b70f 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -739,11 +739,13 @@ spectre_v2_user_select_mitigation(enum spectre_v2_mitigation_cmd v2_cmd)
 	if (boot_cpu_has(X86_FEATURE_IBPB)) {
 		setup_force_cpu_cap(X86_FEATURE_USE_IBPB);
 
+		spectre_v2_user_ibpb = mode;
 		switch (cmd) {
 		case SPECTRE_V2_USER_CMD_FORCE:
 		case SPECTRE_V2_USER_CMD_PRCTL_IBPB:
 		case SPECTRE_V2_USER_CMD_SECCOMP_IBPB:
 			static_branch_enable(&switch_mm_always_ibpb);
+			spectre_v2_user_ibpb = SPECTRE_V2_USER_STRICT;
 			break;
 		case SPECTRE_V2_USER_CMD_PRCTL:
 		case SPECTRE_V2_USER_CMD_AUTO:
@@ -757,8 +759,6 @@ spectre_v2_user_select_mitigation(enum spectre_v2_mitigation_cmd v2_cmd)
 		pr_info("mitigation: Enabling %s Indirect Branch Prediction Barrier\n",
 			static_key_enabled(&switch_mm_always_ibpb) ?
 			"always-on" : "conditional");
-
-		spectre_v2_user_ibpb = mode;
 	}
 
 	/*