x86/speculation: Add prctl for Speculative Store Bypass mitigation

Add prctl based control for Speculative Store Bypass mitigation and make it
the default mitigation for Intel and AMD.

Andi Kleen provided the following rationale (slightly redacted):

There are multiple levels of impact of Speculative Store Bypass:

1) JITed sandbox.
It cannot invoke system calls, but can do PRIME+PROBE and may have call
interfaces to other code

2) Native code process.
No protection inside the process at this level.

3) Kernel.

4) Between processes.

The prctl tries to protect against case (1) doing attacks.

If the untrusted code can do random system calls then control is already
lost in a much worse way. So there needs to be system call protection in
some way (using a JIT not allowing them or seccomp). Or rather if the
process can subvert its environment somehow to do the prctl it can already
execute arbitrary code, which is much worse than SSB.

To put it differently, the point of the prctl is to not allow JITed code
to read data it shouldn't read from its JITed sandbox. If it already has
escaped its sandbox then it can already read everything it wants in its
address space, and do much worse.

The ability to control Speculative Store Bypass allows to enable the
protection selectively without affecting overall system performance.

Based on an initial patch from Tim Chen. Completely rewritten.

Signed-off-by: Thomas Gleixner <[e-mail]>
Reviewed-by: Konrad Rzeszutek Wilk <[e-mail]>

Documentation/admin-guide/kernel-parameters.txt

···
 			off    - Unconditionally enable Speculative Store Bypass
 			auto   - Kernel detects whether the CPU model contains an
 				 implementation of Speculative Store Bypass and
-				 picks the most appropriate mitigation
+				 picks the most appropriate mitigation.
+			prctl  - Control Speculative Store Bypass per thread
+				 via prctl. Speculative Store Bypass is enabled
+				 for a process by default. The state of the control
+				 is inherited on fork.
 
 			Not specifying this option is equivalent to
 			spec_store_bypass_disable=auto.

arch/x86/include/asm/nospec-branch.h

···
 enum ssb_mitigation {
 	SPEC_STORE_BYPASS_NONE,
 	SPEC_STORE_BYPASS_DISABLE,
+	SPEC_STORE_BYPASS_PRCTL,
 };
 
 extern char __indirect_thunk_start[];

arch/x86/kernel/cpu/bugs.c

···
 #include <linux/utsname.h>
 #include <linux/cpu.h>
 #include <linux/module.h>
+#include <linux/nospec.h>
+#include <linux/prctl.h>
 
 #include <asm/spec-ctrl.h>
 #include <asm/cmdline.h>
···
 	SPEC_STORE_BYPASS_CMD_NONE,
 	SPEC_STORE_BYPASS_CMD_AUTO,
 	SPEC_STORE_BYPASS_CMD_ON,
+	SPEC_STORE_BYPASS_CMD_PRCTL,
 };
 
 static const char *ssb_strings[] = {
 	[SPEC_STORE_BYPASS_NONE]	= "Vulnerable",
-	[SPEC_STORE_BYPASS_DISABLE]	= "Mitigation: Speculative Store Bypass disabled"
+	[SPEC_STORE_BYPASS_DISABLE]	= "Mitigation: Speculative Store Bypass disabled",
+	[SPEC_STORE_BYPASS_PRCTL]	= "Mitigation: Speculative Store Bypass disabled via prctl"
 };
 
 static const struct {
 	const char *option;
 	enum ssb_mitigation_cmd cmd;
 } ssb_mitigation_options[] = {
-	{ "auto",	SPEC_STORE_BYPASS_CMD_AUTO }, /* Platform decides */
-	{ "on",		SPEC_STORE_BYPASS_CMD_ON },   /* Disable Speculative Store Bypass */
-	{ "off",	SPEC_STORE_BYPASS_CMD_NONE }, /* Don't touch Speculative Store Bypass */
+	{ "auto",	SPEC_STORE_BYPASS_CMD_AUTO },  /* Platform decides */
+	{ "on",		SPEC_STORE_BYPASS_CMD_ON },    /* Disable Speculative Store Bypass */
+	{ "off",	SPEC_STORE_BYPASS_CMD_NONE },  /* Don't touch Speculative Store Bypass */
+	{ "prctl",	SPEC_STORE_BYPASS_CMD_PRCTL }, /* Disable Speculative Store Bypass via prctl */
 };
 
 static enum ssb_mitigation_cmd __init ssb_parse_cmdline(void)
···
 
 	switch (cmd) {
 	case SPEC_STORE_BYPASS_CMD_AUTO:
-		/*
-		 * AMD platforms by default don't need SSB mitigation.
-		 */
-		if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD)
-			break;
+		/* Choose prctl as the default mode */
+		mode = SPEC_STORE_BYPASS_PRCTL;
+		break;
 	case SPEC_STORE_BYPASS_CMD_ON:
 		mode = SPEC_STORE_BYPASS_DISABLE;
+		break;
+	case SPEC_STORE_BYPASS_CMD_PRCTL:
+		mode = SPEC_STORE_BYPASS_PRCTL;
 		break;
 	case SPEC_STORE_BYPASS_CMD_NONE:
 		break;
···
 	 *  - X86_FEATURE_RDS - CPU is able to turn off speculative store bypass
 	 *  - X86_FEATURE_SPEC_STORE_BYPASS_DISABLE - engage the mitigation
 	 */
-	if (mode != SPEC_STORE_BYPASS_NONE) {
+	if (mode == SPEC_STORE_BYPASS_DISABLE) {
 		setup_force_cpu_cap(X86_FEATURE_SPEC_STORE_BYPASS_DISABLE);
 		/*
 		 * Intel uses the SPEC CTRL MSR Bit(2) for this, while AMD uses
···
 }
 
 #undef pr_fmt
+
+static int ssb_prctl_set(unsigned long ctrl)
+{
+	bool rds = !!test_tsk_thread_flag(current, TIF_RDS);
+
+	if (ssb_mode != SPEC_STORE_BYPASS_PRCTL)
+		return -ENXIO;
+
+	if (ctrl == PR_SPEC_ENABLE)
+		clear_tsk_thread_flag(current, TIF_RDS);
+	else
+		set_tsk_thread_flag(current, TIF_RDS);
+
+	if (rds != !!test_tsk_thread_flag(current, TIF_RDS))
+		speculative_store_bypass_update();
+
+	return 0;
+}
+
+static int ssb_prctl_get(void)
+{
+	switch (ssb_mode) {
+	case SPEC_STORE_BYPASS_DISABLE:
+		return PR_SPEC_DISABLE;
+	case SPEC_STORE_BYPASS_PRCTL:
+		if (test_tsk_thread_flag(current, TIF_RDS))
+			return PR_SPEC_PRCTL | PR_SPEC_DISABLE;
+		return PR_SPEC_PRCTL | PR_SPEC_ENABLE;
+	default:
+		if (boot_cpu_has_bug(X86_BUG_SPEC_STORE_BYPASS))
+			return PR_SPEC_ENABLE;
+		return PR_SPEC_NOT_AFFECTED;
+	}
+}
+
+int arch_prctl_spec_ctrl_set(unsigned long which, unsigned long ctrl)
+{
+	if (ctrl != PR_SPEC_ENABLE && ctrl != PR_SPEC_DISABLE)
+		return -ERANGE;
+
+	switch (which) {
+	case PR_SPEC_STORE_BYPASS:
+		return ssb_prctl_set(ctrl);
+	default:
+		return -ENODEV;
+	}
+}
+
+int arch_prctl_spec_ctrl_get(unsigned long which)
+{
+	switch (which) {
+	case PR_SPEC_STORE_BYPASS:
+		return ssb_prctl_get();
+	default:
+		return -ENODEV;
+	}
+}
 
 void x86_spec_ctrl_setup_ap(void)
 {