Commit e8ec1b6e authored by Kim Phillips's avatar Kim Phillips Committed by Borislav Petkov
Browse files

x86/bugs: Enable STIBP for JMP2RET 



For untrained return thunks to be fully effective, STIBP must be enabled
or SMT disabled.

Co-developed-by: default avatarJosh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: default avatarJosh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: default avatarKim Phillips <kim.phillips@amd.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarBorislav Petkov <bp@suse.de>
parent 7fbf47c7

Documentation/admin-guide/kernel-parameters.txt

+11 −5
Original line number Diff line number Diff line
@@ -5201,11 +5201,17 @@ 
			Speculative Code Execution with Return Instructions)

			vulnerability.



			off         - unconditionally disable

			off          - no mitigation

			auto         - automatically select a migitation

			auto,nosmt   - automatically select a mitigation,

				       disabling SMT if necessary for

				       the full mitigation (only on Zen1

				       and older without STIBP).

			unret        - force enable untrained return thunks,

				      only effective on AMD Zen {1,2}

				       only effective on AMD f15h-f17h

				       based systems.

			unret,nosmt  - like unret, will disable SMT when STIBP

			               is not available.



			Selecting 'auto' will choose a mitigation method at run

			time according to the CPU.

arch/x86/kernel/cpu/bugs.c

+46 −12
Original line number Diff line number Diff line
@@ -783,19 +783,34 @@ static enum retbleed_mitigation retbleed_mitigation __ro_after_init = 
static enum retbleed_mitigation_cmd retbleed_cmd __ro_after_init =

	RETBLEED_CMD_AUTO;



static int __ro_after_init retbleed_nosmt = false;



static int __init retbleed_parse_cmdline(char *str)

{

	if (!str)

		return -EINVAL;



	if (!strcmp(str, "off"))

	while (str) {

		char *next = strchr(str, ',');

		if (next) {

			*next = 0;

			next++;

		}



		if (!strcmp(str, "off")) {

			retbleed_cmd = RETBLEED_CMD_OFF;

	else if (!strcmp(str, "auto"))

		} else if (!strcmp(str, "auto")) {

			retbleed_cmd = RETBLEED_CMD_AUTO;

	else if (!strcmp(str, "unret"))

		} else if (!strcmp(str, "unret")) {

			retbleed_cmd = RETBLEED_CMD_UNRET;

	else

		pr_err("Unknown retbleed option (%s). Defaulting to 'auto'\n", str);

		} else if (!strcmp(str, "nosmt")) {

			retbleed_nosmt = true;

		} else {

			pr_err("Ignoring unknown retbleed option (%s).", str);

		}



		str = next;

	}



	return 0;

}
@@ -841,6 +856,10 @@ static void __init retbleed_select_mitigation(void) 
		setup_force_cpu_cap(X86_FEATURE_RETHUNK);

		setup_force_cpu_cap(X86_FEATURE_UNRET);



		if (!boot_cpu_has(X86_FEATURE_STIBP) &&

		    (retbleed_nosmt || cpu_mitigations_auto_nosmt()))

			cpu_smt_disable(false);



		if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD &&

		    boot_cpu_data.x86_vendor != X86_VENDOR_HYGON)

			pr_err(RETBLEED_UNTRAIN_MSG);
@@ -1087,6 +1106,13 @@ spectre_v2_user_select_mitigation(enum spectre_v2_mitigation_cmd v2_cmd) 
	    boot_cpu_has(X86_FEATURE_AMD_STIBP_ALWAYS_ON))

		mode = SPECTRE_V2_USER_STRICT_PREFERRED;



	if (retbleed_mitigation == RETBLEED_MITIGATION_UNRET) {

		if (mode != SPECTRE_V2_USER_STRICT &&

		    mode != SPECTRE_V2_USER_STRICT_PREFERRED)

			pr_info("Selecting STIBP always-on mode to complement retbleed mitigation'\n");

		mode = SPECTRE_V2_USER_STRICT_PREFERRED;

	}



	spectre_v2_user_stibp = mode;



set_mode:
@@ -2097,11 +2123,19 @@ static ssize_t srbds_show_state(char *buf) 


static ssize_t retbleed_show_state(char *buf)

{

	if (retbleed_mitigation == RETBLEED_MITIGATION_UNRET &&

	    (boot_cpu_data.x86_vendor != X86_VENDOR_AMD &&

	     boot_cpu_data.x86_vendor != X86_VENDOR_HYGON))

	if (retbleed_mitigation == RETBLEED_MITIGATION_UNRET) {

	    if (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; SMT %s\n",

			   retbleed_strings[retbleed_mitigation],

			   !sched_smt_active() ? "disabled" :

			   spectre_v2_user_stibp == SPECTRE_V2_USER_STRICT ||

			   spectre_v2_user_stibp == SPECTRE_V2_USER_STRICT_PREFERRED ?

			   "enabled with STIBP protection" : "vulnerable");

	}



	return sprintf(buf, "%s\n", retbleed_strings[retbleed_mitigation]);

}