Re: [PATCH] MIPS: Use __copy_{to,from}_user() for emulated FP loads/stores

From: Paul Burton <paulburton@kernel.org>
To: David Laight <David.Laight@aculab.com>
Cc: "linux-mips@vger.kernel.org" <linux-mips@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"stable@vger.kernel.org" <stable@vger.kernel.org>
Subject: Re: [PATCH] MIPS: Use __copy_{to,from}_user() for emulated FP loads/stores
Date: Wed, 4 Dec 2019 07:40:48 -0800	[thread overview]
Message-ID: <20191204154048.eotzglp4rdlx4yzl@lantea.localdomain> (raw)
In-Reply-To: <f5e09155580d417e9dcd07b1c20786ed@AcuMS.aculab.com>

Hi David,

On Wed, Dec 04, 2019 at 11:14:08AM +0000, David Laight wrote:
> From: Paul Burton
> > Sent: 03 December 2019 20:50
> > Our FPU emulator currently uses __get_user() & __put_user() to perform
> > emulated loads & stores. This is problematic because __get_user() &
> > __put_user() are only suitable for naturally aligned memory accesses,
> > and the address we're accessing is entirely under the control of
> > userland.
> > 
> > This allows userland to cause a kernel panic by simply performing an
> > unaligned floating point load or store - the kernel will handle the
> > address error exception by attempting to emulate the instruction, and in
> > the process it may generate another address error exception itself.
> > This time the exception is taken with EPC pointing at the kernels FPU
> > emulation code, and we hit a die_if_kernel() in
> > emulate_load_store_insn().
> 
> Won't this be true of almost all code that uses get_user() and put_user()
> (with or without the leading __).

Only if the address being accessed is under the control of userland to
the extent that it can create an unaligned address. You're right that
may be more widespread though; it needs checking...

We used to have separate get_user_unaligned() & put_user_unaligned()
which would suggest that it's expected that get_user() & put_user()
require their accesses be aligned, but they were removed by commit
3170d8d226c2 ("kill {__,}{get,put}_user_unaligned()") in v4.13.

But perhaps we should just take the second AdEL exception & recover via
the fixups table. We definitely don't right now... Needs further
investigation...

> > Fix this up by using __copy_from_user() instead of __get_user() and
> > __copy_to_user() instead of __put_user(). These replacements will handle
> > arbitrary alignment without problems.
> 
> They'll also kill performance.....

Sure they're heavier, but if you're hitting the FPU emulator you're
already slow - trapping to the kernel for instruction emulation is
hardly a hot path. If you care about performance at all then this is
already a code path to avoid at all costs.

Thanks,
    Paul