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@google.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: <stable@vger.kernel.org>
Link: https://lkml.kernel.org/r/20201110123349.1.Id0cbf996d2151f4c143c90f9028651a5b49a5908@changeid

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 581fb72..d41b70f 100644 (file)
--- 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;
        }
 
        /*