x86/bugs: Add AMD retbleed= boot parameter

commit 7fbf47c7ce50b38a64576b150e7011ae73d54669 upstream.

Add the "retbleed=<value>" boot parameter to select a mitigation for
RETBleed. Possible values are "off", "auto" and "unret"
(JMP2RET mitigation). The default value is "auto".

Currently, "retbleed=auto" will select the unret mitigation on
AMD and Hygon and no mitigation on Intel (JMP2RET is not effective on
Intel).

  [peterz: rebase; add hygon]
  [jpoimboe: cleanups]

Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 9f4f3e2..961843a 100644 (file)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4968,6 +4968,21 @@
 
        retain_initrd   [RAM] Keep initrd memory after extraction
 
+       retbleed=       [X86] Control mitigation of RETBleed (Arbitrary
+                       Speculative Code Execution with Return Instructions)
+                       vulnerability.
+
+                       off         - unconditionally disable
+                       auto        - automatically select a migitation
+                       unret       - force enable untrained return thunks,
+                                     only effective on AMD Zen {1,2}
+                                     based systems.
+
+                       Selecting 'auto' will choose a mitigation method at run
+                       time according to the CPU.
+
+                       Not specifying this option is equivalent to retbleed=auto.
+
        rfkill.default_state=
                0       "airplane mode".  All wifi, bluetooth, wimax, gps, fm,
                        etc. communication is blocked by default.

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 1d0f16b..babe46b 100644 (file)
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -471,6 +471,9 @@ config RETPOLINE
 config CC_HAS_SLS
        def_bool $(cc-option,-mharden-sls=all)
 
+config CC_HAS_RETURN_THUNK
+       def_bool $(cc-option,-mfunction-return=thunk-extern)
+
 config SLS
        bool "Mitigate Straight-Line-Speculation"
        depends on CC_HAS_SLS && X86_64

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index c62ea37..3e01a88 100644 (file)
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -37,6 +37,7 @@
 #include "cpu.h"
 
 static void __init spectre_v1_select_mitigation(void);
+static void __init retbleed_select_mitigation(void);
 static void __init spectre_v2_select_mitigation(void);
 static void __init ssb_select_mitigation(void);
 static void __init l1tf_select_mitigation(void);
@@ -120,6 +121,12 @@ void __init check_bugs(void)
 
        /* Select the proper CPU mitigations before patching alternatives: */
        spectre_v1_select_mitigation();
+       retbleed_select_mitigation();
+       /*
+        * spectre_v2_select_mitigation() relies on the state set by
+        * retbleed_select_mitigation(); specifically the STIBP selection is
+        * forced for UNRET.
+        */
        spectre_v2_select_mitigation();
        ssb_select_mitigation();
        l1tf_select_mitigation();
@@ -746,6 +753,100 @@ static int __init nospectre_v1_cmdline(char *str)
 early_param("nospectre_v1", nospectre_v1_cmdline);
 
 #undef pr_fmt
+#define pr_fmt(fmt)     "RETBleed: " fmt
+
+enum retbleed_mitigation {
+       RETBLEED_MITIGATION_NONE,
+       RETBLEED_MITIGATION_UNRET,
+};
+
+enum retbleed_mitigation_cmd {
+       RETBLEED_CMD_OFF,
+       RETBLEED_CMD_AUTO,
+       RETBLEED_CMD_UNRET,
+};
+
+const char * const retbleed_strings[] = {
+       [RETBLEED_MITIGATION_NONE]      = "Vulnerable",
+       [RETBLEED_MITIGATION_UNRET]     = "Mitigation: untrained return thunk",
+};
+
+static enum retbleed_mitigation retbleed_mitigation __ro_after_init =
+       RETBLEED_MITIGATION_NONE;
+static enum retbleed_mitigation_cmd retbleed_cmd __ro_after_init =
+       RETBLEED_CMD_AUTO;
+
+static int __init retbleed_parse_cmdline(char *str)
+{
+       if (!str)
+               return -EINVAL;
+
+       if (!strcmp(str, "off"))
+               retbleed_cmd = RETBLEED_CMD_OFF;
+       else if (!strcmp(str, "auto"))
+               retbleed_cmd = RETBLEED_CMD_AUTO;
+       else if (!strcmp(str, "unret"))
+               retbleed_cmd = RETBLEED_CMD_UNRET;
+       else
+               pr_err("Unknown retbleed option (%s). Defaulting to 'auto'\n", str);
+
+       return 0;
+}
+early_param("retbleed", retbleed_parse_cmdline);
+
+#define RETBLEED_UNTRAIN_MSG "WARNING: BTB untrained return thunk mitigation is only effective on AMD/Hygon!\n"
+#define RETBLEED_COMPILER_MSG "WARNING: kernel not compiled with RETPOLINE or -mfunction-return capable compiler!\n"
+
+static void __init retbleed_select_mitigation(void)
+{
+       if (!boot_cpu_has_bug(X86_BUG_RETBLEED) || cpu_mitigations_off())
+               return;
+
+       switch (retbleed_cmd) {
+       case RETBLEED_CMD_OFF:
+               return;
+
+       case RETBLEED_CMD_UNRET:
+               retbleed_mitigation = RETBLEED_MITIGATION_UNRET;
+               break;
+
+       case RETBLEED_CMD_AUTO:
+       default:
+               if (!boot_cpu_has_bug(X86_BUG_RETBLEED))
+                       break;
+
+               if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD ||
+                   boot_cpu_data.x86_vendor == X86_VENDOR_HYGON)
+                       retbleed_mitigation = RETBLEED_MITIGATION_UNRET;
+               break;
+       }
+
+       switch (retbleed_mitigation) {
+       case RETBLEED_MITIGATION_UNRET:
+
+               if (!IS_ENABLED(CONFIG_RETPOLINE) ||
+                   !IS_ENABLED(CONFIG_CC_HAS_RETURN_THUNK)) {
+                       pr_err(RETBLEED_COMPILER_MSG);
+                       retbleed_mitigation = RETBLEED_MITIGATION_NONE;
+                       break;
+               }
+
+               setup_force_cpu_cap(X86_FEATURE_RETHUNK);
+               setup_force_cpu_cap(X86_FEATURE_UNRET);
+
+               if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD &&
+                   boot_cpu_data.x86_vendor != X86_VENDOR_HYGON)
+                       pr_err(RETBLEED_UNTRAIN_MSG);
+               break;
+
+       default:
+               break;
+       }
+
+       pr_info("%s\n", retbleed_strings[retbleed_mitigation]);
+}
+
+#undef pr_fmt
 #define pr_fmt(fmt)     "Spectre V2 : " fmt
 
 static enum spectre_v2_mitigation spectre_v2_enabled __ro_after_init =
@@ -1989,7 +2090,12 @@ static ssize_t srbds_show_state(char *buf)
 
 static ssize_t retbleed_show_state(char *buf)
 {
-       return sprintf(buf, "Vulnerable\n");
+       if (retbleed_mitigation == RETBLEED_MITIGATION_UNRET &&
+           (boot_cpu_data.x86_vendor != X86_VENDOR_AMD &&
+            boot_cpu_data.x86_vendor != X86_VENDOR_HYGON))
+               return sprintf(buf, "Vulnerable: untrained return thunk on non-Zen uarch\n");
+
+       return sprintf(buf, "%s\n", retbleed_strings[retbleed_mitigation]);
 }
 
 static ssize_t cpu_show_common(struct device *dev, struct device_attribute *attr,