[PATCH 5.18 25/70] x86/bugs: Add AMD retbleed= boot parameter

Greg Kroah-Hartman
2022-07-22T09:10:29Z

From: Alexandre Chartre <alexandre.chartre@oracle.com>

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>
---
Documentation/admin-guide/kernel-parameters.txt | 15 +++
arch/x86/Kconfig | 3
arch/x86/kernel/cpu/bugs.c | 108 +++++++++++++++++++++++-
3 files changed, 125 insertions(+), 1 deletion(-)

--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5124,6 +5124,21 @@

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.
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -469,6 +469,9 @@ 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
--- 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(c
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 *bu

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,