[PATCH] ARM: tls: remove covert channel via TPIDRURW

[Michał Mirosław]

2012/1/16 Will Deacon <will.deacon at arm.com>:
> 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.

So why not fix it instead of leaving it useless?

Best Regards,
Michał Mirosław