x86/speculation: Make "seccomp" the default mode for Speculative Store Bypass

Unless explicitly opted out of, anything running under seccomp will have
SSB mitigations enabled. Choosing the "prctl" mode will disable this.

[ tglx: Adjusted it to the new arch_seccomp_spec_mitigate() mechanism ]

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index a8d2ae1e335bd3dd8878d3df3271b8ce529ca597..f2040d46f0956bcc8c4502404d8f6d46892f0351 100644 (file)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4049,19 +4049,27 @@
                        This parameter controls whether the Speculative Store
                        Bypass optimization is used.
 
-                       on     - Unconditionally disable Speculative Store Bypass
-                       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.
-                       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.
+                       on      - Unconditionally disable Speculative Store Bypass
+                       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. If the
+                                 CPU is not vulnerable, "off" is selected. If the
+                                 CPU is vulnerable the default mitigation is
+                                 architecture and Kconfig dependent. See below.
+                       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.
+                       seccomp - Same as "prctl" above, but all seccomp threads
+                                 will disable SSB unless they explicitly opt out.
 
                        Not specifying this option is equivalent to
                        spec_store_bypass_disable=auto.
 
+                       Default mitigations:
+                       X86:    If CONFIG_SECCOMP=y "seccomp", otherwise "prctl"
+
        spia_io_base=   [HW,MTD]
        spia_fio_base=
        spia_pedr=

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 71ad01422655409898cf83e15e3bc00e98bd0999..328ea3cb769f85097f9fc7489bf38984aa9993b4 100644 (file)
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -233,6 +233,7 @@ enum ssb_mitigation {
        SPEC_STORE_BYPASS_NONE,
        SPEC_STORE_BYPASS_DISABLE,
        SPEC_STORE_BYPASS_PRCTL,
+       SPEC_STORE_BYPASS_SECCOMP,
 };
 
 extern char __indirect_thunk_start[];

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 5dab4c3d26e7600c81ead42fb73f31809ea040d4..563d8e54c8633e200badfd934252cb4e54498aa1 100644 (file)
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -416,22 +416,25 @@ enum ssb_mitigation_cmd {
        SPEC_STORE_BYPASS_CMD_AUTO,
        SPEC_STORE_BYPASS_CMD_ON,
        SPEC_STORE_BYPASS_CMD_PRCTL,
+       SPEC_STORE_BYPASS_CMD_SECCOMP,
 };
 
 static const char *ssb_strings[] = {
        [SPEC_STORE_BYPASS_NONE]        = "Vulnerable",
        [SPEC_STORE_BYPASS_DISABLE]     = "Mitigation: Speculative Store Bypass disabled",
-       [SPEC_STORE_BYPASS_PRCTL]       = "Mitigation: Speculative Store Bypass disabled via prctl"
+       [SPEC_STORE_BYPASS_PRCTL]       = "Mitigation: Speculative Store Bypass disabled via prctl",
+       [SPEC_STORE_BYPASS_SECCOMP]     = "Mitigation: Speculative Store Bypass disabled via prctl and seccomp",
 };
 
 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 */
-       { "prctl",      SPEC_STORE_BYPASS_CMD_PRCTL }, /* Disable Speculative Store Bypass via prctl */
+       { "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 */
+       { "seccomp",    SPEC_STORE_BYPASS_CMD_SECCOMP }, /* Disable Speculative Store Bypass via prctl and seccomp */
 };
 
 static enum ssb_mitigation_cmd __init ssb_parse_cmdline(void)
@@ -481,8 +484,15 @@ static enum ssb_mitigation_cmd __init __ssb_select_mitigation(void)
 
        switch (cmd) {
        case SPEC_STORE_BYPASS_CMD_AUTO:
-               /* Choose prctl as the default mode */
-               mode = SPEC_STORE_BYPASS_PRCTL;
+       case SPEC_STORE_BYPASS_CMD_SECCOMP:
+               /*
+                * Choose prctl+seccomp as the default mode if seccomp is
+                * enabled.
+                */
+               if (IS_ENABLED(CONFIG_SECCOMP))
+                       mode = SPEC_STORE_BYPASS_SECCOMP;
+               else
+                       mode = SPEC_STORE_BYPASS_PRCTL;
                break;
        case SPEC_STORE_BYPASS_CMD_ON:
                mode = SPEC_STORE_BYPASS_DISABLE;
@@ -530,12 +540,14 @@ static void ssb_select_mitigation()
 }
 
 #undef pr_fmt
+#define pr_fmt(fmt)     "Speculation prctl: " fmt
 
 static int ssb_prctl_set(struct task_struct *task, unsigned long ctrl)
 {
        bool update;
 
-       if (ssb_mode != SPEC_STORE_BYPASS_PRCTL)
+       if (ssb_mode != SPEC_STORE_BYPASS_PRCTL &&
+           ssb_mode != SPEC_STORE_BYPASS_SECCOMP)
                return -ENXIO;
 
        switch (ctrl) {
@@ -583,7 +595,8 @@ int arch_prctl_spec_ctrl_set(struct task_struct *task, unsigned long which,
 #ifdef CONFIG_SECCOMP
 void arch_seccomp_spec_mitigate(struct task_struct *task)
 {
-       ssb_prctl_set(task, PR_SPEC_FORCE_DISABLE);
+       if (ssb_mode == SPEC_STORE_BYPASS_SECCOMP)
+               ssb_prctl_set(task, PR_SPEC_FORCE_DISABLE);
 }
 #endif
 
@@ -592,6 +605,7 @@ static int ssb_prctl_get(struct task_struct *task)
        switch (ssb_mode) {
        case SPEC_STORE_BYPASS_DISABLE:
                return PR_SPEC_DISABLE;
+       case SPEC_STORE_BYPASS_SECCOMP:
        case SPEC_STORE_BYPASS_PRCTL:
                if (task_spec_ssb_force_disable(task))
                        return PR_SPEC_PRCTL | PR_SPEC_FORCE_DISABLE;