From: Kim Phillips kim.phillips@amd.com
commit e8ec1b6e08a2102d8755ccb06fa26d540f26a2fa upstream.
For untrained return thunks to be fully effective, STIBP must be enabled or SMT disabled.
Co-developed-by: Josh Poimboeuf jpoimboe@redhat.com Signed-off-by: Josh Poimboeuf jpoimboe@redhat.com Signed-off-by: Kim Phillips kim.phillips@amd.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.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 | 16 ++++-- arch/x86/kernel/cpu/bugs.c | 58 +++++++++++++++++++----- 2 files changed, 57 insertions(+), 17 deletions(-)
--- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -5128,11 +5128,17 @@ 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. + 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 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. --- a/arch/x86/kernel/cpu/bugs.c +++ b/arch/x86/kernel/cpu/bugs.c @@ -776,19 +776,34 @@ static enum retbleed_mitigation retbleed 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")) - 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); + 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")) { + retbleed_cmd = RETBLEED_CMD_AUTO; + } else if (!strcmp(str, "unret")) { + retbleed_cmd = RETBLEED_CMD_UNRET; + } else if (!strcmp(str, "nosmt")) { + retbleed_nosmt = true; + } else { + pr_err("Ignoring unknown retbleed option (%s).", str); + } + + str = next; + }
return 0; } @@ -834,6 +849,10 @@ static void __init retbleed_select_mitig 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); @@ -1080,6 +1099,13 @@ spectre_v2_user_select_mitigation(enum s 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: @@ -2090,10 +2116,18 @@ static ssize_t srbds_show_state(char *bu
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)) - return sprintf(buf, "Vulnerable: untrained return thunk on non-Zen uarch\n"); + 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]); }