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Revision 706a1ea65e6faaf853427a0e931f59d604dd45e3 authored by Linus Torvalds on 23 August 2018, 21:55:01 UTC, committed by Linus Torvalds on 23 August 2018, 21:55:01 UTC 
Merge branch 'tlb-fixes'
 
Merge fixes for missing TLB shootdowns.

This fixes a couple of cases that involved us possibly freeing page
table structures before the required TLB shootdown had been done.

There are a few cleanup patches to make the code easier to follow, and
to avoid some of the more problematic cases entirely when not necessary.

To make this easier for backports, it undoes the recent lazy TLB
patches, because the cleanups and fixes are more important, and Rik is
ok with re-doing them later when things have calmed down.

The missing TLB flush was only delayed, and the wrong ordering only
happened under memory pressure (and in theory under a couple of other
fairly theoretical situations), so this may have been all very unlikely
to have hit people in practice.

But getting the TLB shootdown wrong is _so_ hard to debug and see that I
consider this a crticial fix.

Many thanks to Jann Horn for having debugged this.

* tlb-fixes:
  x86/mm: Only use tlb_remove_table() for paravirt
  mm: mmu_notifier fix for tlb_end_vma
  mm/tlb, x86/mm: Support invalidating TLB caches for RCU_TABLE_FREE
  mm/tlb: Remove tlb_remove_table() non-concurrent condition
  mm: move tlb_table_flush to tlb_flush_mmu_free
  x86/mm/tlb: Revert the recent lazy TLB patches
2 parent s d40acad + 48a8b97
Raw File
SAK.txt 
=========================================
Linux Secure Attention Key (SAK) handling
=========================================

:Date: 18 March 2001
:Author: Andrew Morton

An operating system's Secure Attention Key is a security tool which is
provided as protection against trojan password capturing programs.  It
is an undefeatable way of killing all programs which could be
masquerading as login applications.  Users need to be taught to enter
this key sequence before they log in to the system.

From the PC keyboard, Linux has two similar but different ways of
providing SAK.  One is the ALT-SYSRQ-K sequence.  You shouldn't use
this sequence.  It is only available if the kernel was compiled with
sysrq support.

The proper way of generating a SAK is to define the key sequence using
``loadkeys``.  This will work whether or not sysrq support is compiled
into the kernel.

SAK works correctly when the keyboard is in raw mode.  This means that
once defined, SAK will kill a running X server.  If the system is in
run level 5, the X server will restart.  This is what you want to
happen.

What key sequence should you use? Well, CTRL-ALT-DEL is used to reboot
the machine.  CTRL-ALT-BACKSPACE is magical to the X server.  We'll
choose CTRL-ALT-PAUSE.

In your rc.sysinit (or rc.local) file, add the command::

	echo "control alt keycode 101 = SAK" | /bin/loadkeys

And that's it!  Only the superuser may reprogram the SAK key.


.. note::

  1. Linux SAK is said to be not a "true SAK" as is required by
     systems which implement C2 level security.  This author does not
     know why.


  2. On the PC keyboard, SAK kills all applications which have
     /dev/console opened.

     Unfortunately this includes a number of things which you don't
     actually want killed.  This is because these applications are
     incorrectly holding /dev/console open.  Be sure to complain to your
     Linux distributor about this!

     You can identify processes which will be killed by SAK with the
     command::

	# ls -l /proc/[0-9]*/fd/* | grep console
	l-wx------    1 root     root           64 Mar 18 00:46 /proc/579/fd/0 -> /dev/console

     Then::

	# ps aux|grep 579
	root       579  0.0  0.1  1088  436 ?        S    00:43   0:00 gpm -t ps/2

     So ``gpm`` will be killed by SAK.  This is a bug in gpm.  It should
     be closing standard input.  You can work around this by finding the
     initscript which launches gpm and changing it thusly:

     Old::

	daemon gpm

     New::

	daemon gpm < /dev/null

     Vixie cron also seems to have this problem, and needs the same treatment.

     Also, one prominent Linux distribution has the following three
     lines in its rc.sysinit and rc scripts::

	exec 3<&0
	exec 4>&1
	exec 5>&2

     These commands cause **all** daemons which are launched by the
     initscripts to have file descriptors 3, 4 and 5 attached to
     /dev/console.  So SAK kills them all.  A workaround is to simply
     delete these lines, but this may cause system management
     applications to malfunction - test everything well.
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