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x86/speculation: Fix incorrect MDS/TAA mitigation status 

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: 1b42f01741 ("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
alistair/sunxi64-5.5-dsi
Waiman Long 2019-11-15 11:14:44 -05:00 committed by Borislav Petkov
parent eb094f0696
commit 64870ed1b1
4 changed files with 35 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
Documentation/admin-guide/hw-vuln/mds.rst
View File

@ -265,8 +265,11 @@ time with the option "mds=". The valid arguments for this option are:
============ =============================================================
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
--------------------------

5
Documentation/admin-guide/hw-vuln/tsx_async_abort.rst
View File

@ -174,7 +174,10 @@ the option "tsx_async_abort=". The valid arguments for this option are:
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

11
Documentation/admin-guide/kernel-parameters.txt
View File

@ -2473,6 +2473,12 @@
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.
@ -4931,6 +4937,11 @@
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

17
arch/x86/kernel/cpu/bugs.c
View File

@ -304,8 +304,12 @@ static void __init taa_select_mitigation(void)
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(void)
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]);
}