From: Waiman Long longman@redhat.com
commit 64870ed1b12e235cfca3f6c6da75b542c973ff78 upstream.
For MDS vulnerable processors with TSX support, enabling either MDS or TAA mitigations will enable the use of VERW to flush internal processor buffers at the right code path. IOW, they are either both mitigated or both not. However, if the command line options are inconsistent, the vulnerabilites sysfs files may not report the mitigation status correctly.
For example, with only the "mds=off" option:
vulnerabilities/mds:Vulnerable; SMT vulnerable vulnerabilities/tsx_async_abort:Mitigation: Clear CPU buffers; SMT vulnerable
The mds vulnerabilities file has wrong status in this case. Similarly, the taa vulnerability file will be wrong with mds mitigation on, but taa off.
Change taa_select_mitigation() to sync up the two mitigation status and have them turned off if both "mds=off" and "tsx_async_abort=off" are present.
Update documentation to emphasize the fact that both "mds=off" and "tsx_async_abort=off" have to be specified together for processors that are affected by both TAA and MDS to be effective.
[ bp: Massage and add kernel-parameters.txt change too. ]
Fixes: 1b42f017415b ("x86/speculation/taa: Add mitigation for TSX Async Abort") Signed-off-by: Waiman Long longman@redhat.com Signed-off-by: Borislav Petkov bp@suse.de Cc: Greg Kroah-Hartman gregkh@linuxfoundation.org Cc: "H. Peter Anvin" hpa@zytor.com Cc: Ingo Molnar mingo@redhat.com Cc: Jiri Kosina jkosina@suse.cz Cc: Jonathan Corbet corbet@lwn.net Cc: Josh Poimboeuf jpoimboe@redhat.com Cc: linux-doc@vger.kernel.org Cc: Mark Gross mgross@linux.intel.com Cc: stable@vger.kernel.org Cc: Pawan Gupta pawan.kumar.gupta@linux.intel.com Cc: Peter Zijlstra peterz@infradead.org Cc: Thomas Gleixner tglx@linutronix.de Cc: Tim Chen tim.c.chen@linux.intel.com Cc: Tony Luck tony.luck@intel.com Cc: Tyler Hicks tyhicks@canonical.com Cc: x86-ml x86@kernel.org Link: https://lkml.kernel.org/r/20191115161445.30809-2-longman@redhat.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- Documentation/admin-guide/hw-vuln/mds.rst | 7 +++++-- Documentation/admin-guide/hw-vuln/tsx_async_abort.rst | 5 ++++- Documentation/admin-guide/kernel-parameters.txt | 11 +++++++++++ arch/x86/kernel/cpu/bugs.c | 17 +++++++++++++++-- 4 files changed, 35 insertions(+), 5 deletions(-)
--- a/Documentation/admin-guide/hw-vuln/mds.rst +++ b/Documentation/admin-guide/hw-vuln/mds.rst @@ -265,8 +265,11 @@ time with the option "mds=". The valid a
============ =============================================================
-Not specifying this option is equivalent to "mds=full". - +Not specifying this option is equivalent to "mds=full". For processors +that are affected by both TAA (TSX Asynchronous Abort) and MDS, +specifying just "mds=off" without an accompanying "tsx_async_abort=off" +will have no effect as the same mitigation is used for both +vulnerabilities.
Mitigation selection guide -------------------------- --- a/Documentation/admin-guide/hw-vuln/tsx_async_abort.rst +++ b/Documentation/admin-guide/hw-vuln/tsx_async_abort.rst @@ -174,7 +174,10 @@ the option "tsx_async_abort=". The valid CPU is not vulnerable to cross-thread TAA attacks. ============ =============================================================
-Not specifying this option is equivalent to "tsx_async_abort=full". +Not specifying this option is equivalent to "tsx_async_abort=full". For +processors that are affected by both TAA and MDS, specifying just +"tsx_async_abort=off" without an accompanying "mds=off" will have no +effect as the same mitigation is used for both vulnerabilities.
The kernel command line also allows to control the TSX feature using the parameter "tsx=" on CPUs which support TSX control. MSR_IA32_TSX_CTRL is used --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -2359,6 +2359,12 @@ SMT on vulnerable CPUs off - Unconditionally disable MDS mitigation
+ On TAA-affected machines, mds=off can be prevented by + an active TAA mitigation as both vulnerabilities are + mitigated with the same mechanism so in order to disable + this mitigation, you need to specify tsx_async_abort=off + too. + Not specifying this option is equivalent to mds=full.
@@ -4773,6 +4779,11 @@ vulnerable to cross-thread TAA attacks. off - Unconditionally disable TAA mitigation
+ On MDS-affected machines, tsx_async_abort=off can be + prevented by an active MDS mitigation as both vulnerabilities + are mitigated with the same mechanism so in order to disable + this mitigation, you need to specify mds=off too. + Not specifying this option is equivalent to tsx_async_abort=full. On CPUs which are MDS affected and deploy MDS mitigation, TAA mitigation is not --- a/arch/x86/kernel/cpu/bugs.c +++ b/arch/x86/kernel/cpu/bugs.c @@ -304,8 +304,12 @@ static void __init taa_select_mitigation return; }
- /* TAA mitigation is turned off on the cmdline (tsx_async_abort=off) */ - if (taa_mitigation == TAA_MITIGATION_OFF) + /* + * TAA mitigation via VERW is turned off if both + * tsx_async_abort=off and mds=off are specified. + */ + if (taa_mitigation == TAA_MITIGATION_OFF && + mds_mitigation == MDS_MITIGATION_OFF) goto out;
if (boot_cpu_has(X86_FEATURE_MD_CLEAR)) @@ -339,6 +343,15 @@ static void __init taa_select_mitigation if (taa_nosmt || cpu_mitigations_auto_nosmt()) cpu_smt_disable(false);
+ /* + * Update MDS mitigation, if necessary, as the mds_user_clear is + * now enabled for TAA mitigation. + */ + if (mds_mitigation == MDS_MITIGATION_OFF && + boot_cpu_has_bug(X86_BUG_MDS)) { + mds_mitigation = MDS_MITIGATION_FULL; + mds_select_mitigation(); + } out: pr_info("%s\n", taa_strings[taa_mitigation]); }