[ 30/47] ARM: 7403/1: tls: remove covert channel via TPIDRURW

Greg KH
2012-05-04T20:49:31Z

From: Will Deacon <will.deacon@arm.com>

commit 6a1c53124aa161eb624ce7b1e40ade728186d34c upstream.

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.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/arm/include/asm/tls.h | 4 ++++
1 file changed, 4 insertions(+)

--- a/arch/arm/include/asm/tls.h
+++ b/arch/arm/include/asm/tls.h
@@ -7,6 +7,8 @@

.macro set_tls_v6k, tp, tmp1, tmp2
mcr p15, 0, \tp, c13, c0, 3 @ set TLS register
+ mov \tmp1, #0
+ mcr p15, 0, \tmp1, c13, c0, 2 @ clear user r/w TLS register
.endm

.macro set_tls_v6, tp, tmp1, tmp2
@@ -15,6 +17,8 @@
mov \tmp2, #0xffff0fff
tst \tmp1, #HWCAP_TLS @ hardware TLS available?
mcrne p15, 0, \tp, c13, c0, 3 @ yes, set TLS register
+ movne \tmp1, #0
+ mcrne p15, 0, \tmp1, c13, c0, 2 @ clear user r/w TLS register
streq \tp, [\tmp2, #-15] @ set TLS value at 0xffff0ff0
.endm