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Date: Fri, 9 Nov 2007 12:22:08 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: David Miller <davem@davemloft.net>
Cc: torvalds@linux-foundation.org, linux-kernel@vger.kernel.org,
       sparclinux@vger.kernel.org, linux-arch@vger.kernel.org, bernd@bzed.de,
       joy@entuzijast.net, fabbione@ubuntu.com, arnd@arndb.de,
       stable@kernel.org
Subject: Re: Fix for sparc64 cpu hangs.
Message-Id: <20071109122208.5b6794ea.akpm@linux-foundation.org>
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On Tue, 06 Nov 2007 21:13:56 -0800 (PST)
David Miller <davem@davemloft.net> wrote:

> From: David Miller <davem@davemloft.net>
> Date: Tue, 06 Nov 2007 20:34:33 -0800 (PST)
> 
> > [FUTEX]: Fix address computation in compat code.
> 
> Sorry, I just noticed there is a second handle_futex_death()
> call in compat_exit_robust_list() which has the same
> address computation bug.
> 
> Here is an updated patch:
> 
> [FUTEX]: Fix address computation in compat code.
> 
> compat_exit_robust_list() computes a pointer to the
> futex entry in userspace as follows:
> 
> 	(void __user *)entry + futex_offset
> 
> 'entry' is a 'struct robust_list __user *', and
> 'futex_offset' is a 'compat_long_t' (typically a 's32').
> 
> Things explode if the 32-bit sign bit is set in futex_offset.
> 
> Type promotion sign extends futex_offset to a 64-bit value before
> adding it to 'entry'.
> 
> This triggered a problem on sparc64 running 32-bit applications which
> would lock up a cpu looping forever in the fault handling for the
> userspace load in handle_futex_death().
> 
> Compat userspace runs with address masking (wherein the cpu zeros out
> the top 32-bits of every effective address given to a memory operation
> instruction) so the sparc64 fault handler accounts for this by
> zero'ing out the top 32-bits of the fault address too.
> 
> Since the kernel properly uses the compat_uptr interfaces, kernel side
> accesses to compat userspace work too since they will only use
> addresses with the top 32-bit clear.
> 
> Because of this compat futex layer bug we get into the following loop
> when executing the get_user() load near the top of handle_futex_death():
> 
> 1) load from address '0xfffffffff7f16bd8', FAULT
> 2) fault handler clears upper 32-bits, processes fault
>    for address '0xf7f16bd8' which succeeds
> 3) goto #1
> 
> I want to thank Bernd Zeimetz, Josip Rodin, and Fabio Massimo Di Nitto
> for their tireless efforts helping me track down this bug.
> 

I tagged this as needed-in-2.6.23.x.  Please let me know if that is not
appropriate.