[PATCH 3.16 57/86] x86/speculation: Enable prctl mode for spectre_v2_user

Ben Hutchings
2019-05-16T16:03:54Z

From: Thomas Gleixner <tglx@linutronix.de>

commit 7cc765a67d8e04ef7d772425ca5a2a1e2b894c15 upstream.

Now that all prerequisites are in place:

- Add the prctl command line option

- Default the 'auto' mode to 'prctl'

- When SMT state changes, update the static key which controls the
conditional STIBP evaluation on context switch.

- At init update the static key which controls the conditional IBPB
evaluation on context switch.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Casey Schaufler <casey.schaufler@intel.com>
Cc: Asit Mallick <asit.k.mallick@intel.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Jon Masters <jcm@redhat.com>
Cc: Waiman Long <longman9394@gmail.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Dave Stewart <david.c.stewart@intel.com>
Cc: Kees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/20181125185005.958421388@linutronix.de
[bwh: Backported to 3.16: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
Docuemntation/kernel-parameters.txt | 7 +++-
arch/x86/kernel/cpu/bugs.c | 41 +++++++++++++++----
2 files changed, 38 insertions(+), 10 deletions(-)

--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3218,9 +3218,14 @@ bytes respectively. Such letter suffixes
off - Unconditionally disable mitigations. Is
enforced by spectre_v2=off

+ prctl - Indirect branch speculation is enabled,
+ but mitigation can be enabled via prctl
+ per thread. The mitigation control state
+ is inherited on fork.
+
auto - Kernel selects the mitigation depending on
the available CPU features and vulnerability.
- Default is off.
+ Default is prctl.

Not specifying this option is equivalent to
spectre_v2_user=auto.
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -307,11 +307,13 @@ enum spectre_v2_user_cmd {
SPECTRE_V2_USER_CMD_NONE,
SPECTRE_V2_USER_CMD_AUTO,
SPECTRE_V2_USER_CMD_FORCE,
+ SPECTRE_V2_USER_CMD_PRCTL,
};

static const char * const spectre_v2_user_strings[] = {
[SPECTRE_V2_USER_NONE] = "User space: Vulnerable",
[SPECTRE_V2_USER_STRICT] = "User space: Mitigation: STIBP protection",
+ [SPECTRE_V2_USER_PRCTL] = "User space: Mitigation: STIBP via prctl",
};

static const struct {
@@ -322,6 +324,7 @@ static const struct {
{ "auto", SPECTRE_V2_USER_CMD_AUTO, false },
{ "off", SPECTRE_V2_USER_CMD_NONE, false },
{ "on", SPECTRE_V2_USER_CMD_FORCE, true },
+ { "prctl", SPECTRE_V2_USER_CMD_PRCTL, false },
};

static void __init spec_v2_user_print_cond(const char *reason, bool secure)
@@ -375,12 +378,15 @@ spectre_v2_user_select_mitigation(enum s
smt_possible = false;

switch (spectre_v2_parse_user_cmdline(v2_cmd)) {
- case SPECTRE_V2_USER_CMD_AUTO:
case SPECTRE_V2_USER_CMD_NONE:
goto set_mode;
case SPECTRE_V2_USER_CMD_FORCE:
mode = SPECTRE_V2_USER_STRICT;
break;
+ case SPECTRE_V2_USER_CMD_AUTO:
+ case SPECTRE_V2_USER_CMD_PRCTL:
+ mode = SPECTRE_V2_USER_PRCTL;
+ break;
}

/* Initialize Indirect Branch Prediction Barrier */
@@ -391,6 +397,9 @@ spectre_v2_user_select_mitigation(enum s
case SPECTRE_V2_USER_STRICT:
static_branch_enable(&switch_mm_always_ibpb);
break;
+ case SPECTRE_V2_USER_PRCTL:
+ static_branch_enable(&switch_mm_cond_ibpb);
+ break;
default:
break;
}
@@ -403,6 +412,12 @@ spectre_v2_user_select_mitigation(enum s
if (spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)
return;

+ /*
+ * If SMT is not possible or STIBP is not available clear the STIPB
+ * mode.
+ */
+ if (!smt_possible || !boot_cpu_has(X86_FEATURE_STIBP))
+ mode = SPECTRE_V2_USER_NONE;
set_mode:
spectre_v2_user = mode;
/* Only print the STIBP mode when SMT possible */
@@ -610,6 +625,15 @@ static void update_stibp_strict(void)
on_each_cpu(update_stibp_msr, NULL, 1);
}

+/* Update the static key controlling the evaluation of TIF_SPEC_IB */
+static void update_indir_branch_cond(void)
+{
+ if (sched_smt_active())
+ static_branch_enable(&switch_to_cond_stibp);
+ else
+ static_branch_disable(&switch_to_cond_stibp);
+}
+
void arch_smt_update(void)
{
/* Enhanced IBRS implies STIBP. No update required. */
@@ -625,6 +649,7 @@ void arch_smt_update(void)
update_stibp_strict();
break;
case SPECTRE_V2_USER_PRCTL:
+ update_indir_branch_cond();
break;
}

@@ -1012,7 +1037,8 @@ static char *stibp_state(void)
case SPECTRE_V2_USER_STRICT:
return ", STIBP: forced";
case SPECTRE_V2_USER_PRCTL:
- return "";
+ if (static_key_enabled(&switch_to_cond_stibp))
+ return ", STIBP: conditional";
}
return "";
}
@@ -1020,14 +1046,11 @@ static char *stibp_state(void)
static char *ibpb_state(void)
{
if (boot_cpu_has(X86_FEATURE_IBPB)) {
- switch (spectre_v2_user) {
- case SPECTRE_V2_USER_NONE:
- return ", IBPB: disabled";
- case SPECTRE_V2_USER_STRICT:
+ if (static_key_enabled(&switch_mm_always_ibpb))
return ", IBPB: always-on";
- case SPECTRE_V2_USER_PRCTL:
- return "";
- }
+ if (static_key_enabled(&switch_mm_cond_ibpb))
+ return ", IBPB: conditional";
+ return ", IBPB: disabled";
}
return "";
}