ARM: 7403/1: tls: remove covert channel via TPIDRURW
authorWill Deacon <will.deacon@arm.com>
Fri, 27 Apr 2012 11:45:07 +0000 (12:45 +0100)
committerVarun Wadekar <vwadekar@nvidia.com>
Mon, 14 May 2012 12:53:22 +0000 (17:53 +0530)
commitca7ef8d648e1a280b198f667224965952d2f88a7
tree5c6f8221650aca0807d2b4c3b126f94c5965aebc
parent22b5dc2eb23c3d0ae74b68884474a876b59139b0
ARM: 7403/1: tls: remove covert channel via TPIDRURW

TPIDRURW is a user read/write register forming part of the group of
thread registers in more recent versions of the ARM architecture (~v6+).

Currently, the kernel does not touch this register, which allows tasks
to communicate covertly by reading and writing to the register without
context-switching affecting its contents.

This patch clears TPIDRURW when TPIDRURO is updated via the set_tls
macro, which is called directly from __switch_to. Since the current
behaviour makes the register useless to userspace as far as thread
pointers are concerned, simply clearing the register (rather than saving
and restoring it) will not cause any problems to userspace.

Cc: stable@vger.kernel.org
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
arch/arm/include/asm/tls.h