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

Yury Norov
2016-01-05T15:27:26Z

On Thu, Dec 17, 2015 at 09:50:52PM +0100, Arnd Bergmann wrote:
> On Thursday 17 December 2015 12:14:20 Andrew Pinski wrote:
> > On Thu, Dec 17, 2015 at 12:10 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> > > On Thursday 17 December 2015 18:27:53 Catalin Marinas wrote:
> > >> On Wed, Dec 16, 2015 at 12:42:38AM +0300, Yury Norov wrote:
> > >
> > >> > +#define compat_sys_lookup_dcookie sys_lookup_dcookie
> > >> > +#define compat_sys_pread64 sys_pread64
> > >> > +#define compat_sys_pwrite64 sys_pwrite64
> > >> > +#define compat_sys_readahead sys_readahead
> > >> > +#define compat_sys_shmat sys_shmat
> > >>
> > >> I wonder whether we need wrappers (actually, not only for these but
> > >> sys_read etc.). These functions take either a pointer or a size_t
> > >> argument which are 32-bit with ILP32 but treated as 64-bit by an LP64
> > >> kernel. Can we guarantee that user space zeros the top 32-bit of the
> > >> arguments passed here?
> > >
> > > I'm pretty sure that is safe. I haven't read the calling conventions
> > > specification for arm64 ilp32, but usually all function arguments are
> > > passed as 64-bit registers with proper sign-extend or zero-extend.
> >
> > Well (just like LP64 on AARCH64), when passing a 32bit value to a
> > function, the upper 32bits are undefined. I ran into this when I was
> > debugging the GCC go library on ILP32 (though reproduced with pure C
> > code) and the assembly functions inside glibc where pointers are
> > passed with the upper 32bits as undefined.
> > So we have an issue if called with syscall function or using pure
> > assembly to create the syscall functions (which glibc does).
>
> Ok, I see :-(
>
> 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.
>
> Arnd

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.

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?