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

Catalin Marinas
2016-01-06T17:10:58Z

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