linux-kernel - Re: [PATCH v6 12/20] arm64:ilp32: add sys_ilp32.c and a separate table (in entry.S) to use it

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Message-ID: <20160106171047.GP6301@e104818-lin.cambridge.arm.com>
Date:	Wed, 6 Jan 2016 17:10:47 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Andrew Pinski <pinskia@...il.com>,
	"Kapoor, Prasun" <Prasun.Kapoor@...iumnetworks.com>,
	broonie@...nel.org, Nathan Lynch <Nathan_Lynch@...tor.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexander Graf <agraf@...e.de>,
	Alexey Klimov <klimov.linux@...il.com>,
	Yury Norov <ynorov@...iumnetworks.com>,
	Jan Dakinevich <jan.dakinevich@...il.com>,
	Andrew Pinski <apinski@...ium.com>,
	David Daney <ddaney.cavm@...il.com>,
	Andreas Schwab <schwab@...e.de>,
	"Zhangjian (Bamvor)" <bamvor.zhangjian@...wei.com>,
	Philipp Tomsich <philipp.tomsich@...obroma-systems.com>,
	"Joseph S. Myers" <joseph@...esourcery.com>,
	christoph.muellner@...obroma-systems.com
Subject: Re: [PATCH v6 12/20] arm64:ilp32: add sys_ilp32.c and a separate
 table (in entry.S) to use it

On Tue, Jan 05, 2016 at 10:12:20PM +0100, Arnd Bergmann wrote:
> On Tuesday 05 January 2016 18:26:57 Yury Norov wrote:
> > > So the calling conventions avoid the problem of being able to set
> > > the upper bits from malicious user space when the kernel assumes they
> > > are zeroed out (we had security bugs in this area, before we introduced
> > > SYSCALL_DEFINEx()), but it means that we need wrappers around each
> > > syscall that takes an argument that is different length between user
> > > and kernel space (as Catalin guessed). arch/s390 has the same problem and
> > > works around it with code in arch/s390/kernel/compat_wrapper.c, while
> > > other architectures (at least powerpc, x86 and tile IIRC, don't know much
> > > about mips, parisc and sparc) don't have the problem because of their
> > > calling conventions.
> > > 
> > > This also means that we cannot work around it in glibc at all, because
> > > we have to be able to handle malicious user space, so it has to be
> > > done in the kernel using something similar to what s390 does.
> > 
> > So it seems like we (should) have 2 compat modes - with and without access
> > to upper half of register. I'm thinking now on how put it in generic
> > unistd.h less painfull way.
> 
> I think we can do that by slightly modifying the existing __SYSCALL/__SC_3264/
> __SC_COMP/__SC_COMP_3264 macros: The first two need extra wrappers for
> arm64-ilp32 and s390, the other two don't.
> 
> We can use some clever string concatenation to add a ##_wrapper to the name
> of the handler where needed and then just have a file that implements
> the wrappers, copied from s390.
> 
> Unfortunately, we can't just zero out all the upper halves and be done with
> it: even if we went back to passing 64-bit arguments as separate 32-bit
> registers, we'd still need to deal with sign-extending negative 32-bit
> numbers.

How many syscalls would we need sign-extension for? Most are probably
already handled by specific compat_sys_* functions, otherwise A32 compat
wouldn't work properly.

Anyway, I think we can get away with not modifying the generic __SYSCALL
definition and only use something like
arch/s390/kernel/compat_wrapper.c. In sys_ilp32.c, we would make
__SYSCALL expand the function name with some ilp32_ prefix.

For existing compat_* syscalls, we only need to handle the pointer types
(something like the s390's __TYPE_IS_PTR). I think other types are
already handled by defining the prototype with compat_ulong_t etc.

For native syscalls like sys_read, apart from pointers we also need to
handle size_t. The wrapper would need to be defined using compat types:

ILP32_SYSCALL_DEFINE3(read, unsigned int, fd, char __user *, buf, compat_size_t, count)

and let the compiler handle the conversion to size_t automatically when
calling sys_read from the wrapper.

> > Beside of that, I think I almost finished with all current comments. As
> > this issue is not related to ILP32 directly, I think, it's better to show
> > it now, as there is pretty massive rework. What do you think?
> 
> Good idea, yes.

Note that we still need to sort the 0/sign extension out before we
"declare" the ILP32 ABI stable.

-- 
Catalin
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