x86/bugs: Add AMD retbleed= boot parameter

commit 09d1ae07168b9534d7d9ff6b0b25ace87d2d71d9 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>
[cascardo: this effectively remove the UNRET mitigation as an option, so it
 has to be complemented by a later pick of the same commit later. This is
 done in order to pick retbleed_select_mitigation]
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 db9d53b879f89d8925f90c41776a963cb0aa1140..e0fb1e5e5113207cd1d8997b943706d22f41c927 100644 (file)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4298,6 +4298,18 @@
 
        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
+
+                       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/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 769071688c110522903dbf2d6b1fe2cf27ddd19e..24380bcc3d3dc1c860899717870de7a4e2ce630d 100644 (file)
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -36,6 +36,7 @@
 #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);
@@ -111,6 +112,12 @@ 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();
@@ -705,6 +712,71 @@ 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,
+};
+
+enum retbleed_mitigation_cmd {
+       RETBLEED_CMD_OFF,
+       RETBLEED_CMD_AUTO,
+};
+
+const char * const retbleed_strings[] = {
+       [RETBLEED_MITIGATION_NONE]      = "Vulnerable",
+};
+
+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
+               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_AUTO:
+       default:
+               if (!boot_cpu_has_bug(X86_BUG_RETBLEED))
+                       break;
+
+               break;
+       }
+
+       switch (retbleed_mitigation) {
+       default:
+               break;
+       }
+
+       pr_info("%s\n", retbleed_strings[retbleed_mitigation]);
+}
+
 #undef pr_fmt
 #define pr_fmt(fmt)     "Spectre V2 : " fmt
 
@@ -1913,7 +1985,7 @@ static ssize_t srbds_show_state(char *buf)
 
 static ssize_t retbleed_show_state(char *buf)
 {
-       return sprintf(buf, "Vulnerable\n");
+       return sprintf(buf, "%s\n", retbleed_strings[retbleed_mitigation]);
 }
 
 static ssize_t cpu_show_common(struct device *dev, struct device_attribute *attr,