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Fix the "IBPB mitigated RETBleed" mode of operation on AMD CPUs

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(not turned on by default), which also need STIBP enabled (if
 available) to be '100% safe' on even the shortest speculation
 windows.
 
 Signed-off-by: Ingo Molnar <mingo@kernel.org>
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Merge tag 'x86-urgent-2022-08-13' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 fix from Ingo Molnar:
 "Fix the 'IBPB mitigated RETBleed' mode of operation on AMD CPUs (not
  turned on by default), which also need STIBP enabled (if available) to
  be '100% safe' on even the shortest speculation windows"

* tag 'x86-urgent-2022-08-13' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/bugs: Enable STIBP for IBPB mitigated RETBleed
This commit is contained in:
Linus Torvalds 2022-08-13 14:24:12 -07:00
commit c5f1e32e32
2 changed files with 27 additions and 12 deletions
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29
Documentation/admin-guide/kernel-parameters.txt
View File

@ -5274,20 +5274,33 @@
Speculative Code Execution with Return Instructions)
vulnerability.
AMD-based UNRET and IBPB mitigations alone do not stop
sibling threads from influencing the predictions of other
sibling threads. For that reason, STIBP is used on pro-
cessors that support it, and mitigate SMT on processors
that don't.
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).
ibpb - mitigate short speculation windows on
basic block boundaries too. Safe, highest
perf impact.
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.
ibpb - On AMD, mitigate short speculation
windows on basic block boundaries too.
Safe, highest perf impact. It also
enables STIBP if present. Not suitable
on Intel.
ibpb,nosmt - Like "ibpb" above but will disable SMT
when STIBP is not available. This is
the alternative for systems which do not
have STIBP.
unret - Force enable untrained return thunks,
only effective on AMD f15h-f17h based
systems.
unret,nosmt - Like unret, but will disable SMT when STIBP
is not available. This is the alternative for
systems which do not have STIBP.
Selecting 'auto' will choose a mitigation method at run
time according to the CPU.

10
arch/x86/kernel/cpu/bugs.c
View File

@ -152,7 +152,7 @@ void __init check_bugs(void)
/*
* spectre_v2_user_select_mitigation() relies on the state set by
* retbleed_select_mitigation(); specifically the STIBP selection is
* forced for UNRET.
* forced for UNRET or IBPB.
*/
spectre_v2_user_select_mitigation();
ssb_select_mitigation();
@ -1179,7 +1179,8 @@ spectre_v2_user_select_mitigation(void)
boot_cpu_has(X86_FEATURE_AMD_STIBP_ALWAYS_ON))
mode = SPECTRE_V2_USER_STRICT_PREFERRED;
if (retbleed_mitigation == RETBLEED_MITIGATION_UNRET) {
if (retbleed_mitigation == RETBLEED_MITIGATION_UNRET ||
retbleed_mitigation == RETBLEED_MITIGATION_IBPB) {
if (mode != SPECTRE_V2_USER_STRICT &&
mode != SPECTRE_V2_USER_STRICT_PREFERRED)
pr_info("Selecting STIBP always-on mode to complement retbleed mitigation\n");
@ -2360,10 +2361,11 @@ static ssize_t srbds_show_state(char *buf)
static ssize_t retbleed_show_state(char *buf)
{
if (retbleed_mitigation == RETBLEED_MITIGATION_UNRET) {
if (retbleed_mitigation == RETBLEED_MITIGATION_UNRET ||
retbleed_mitigation == RETBLEED_MITIGATION_IBPB) {
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, "Vulnerable: untrained return thunk / IBPB on non-AMD based uarch\n");
return sprintf(buf, "%s; SMT %s\n",
retbleed_strings[retbleed_mitigation],