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

Catalin Marinas
2016-01-07T17:23:39Z

On Thu, Jan 07, 2016 at 03:13:37PM +0100, Arnd Bergmann wrote:
> On Wednesday 06 January 2016 17:10:47 Catalin Marinas wrote:
> > 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.
>
> Good point. I suppose any system call that expects a negative argument
> may run into this on all architectures and require a COMPAT_SYSCALL handler,
> but only s390 cares about doing the extension for the entire set of syscalls.
>
> This may be to work around a peculiarity of s390, which has now two
> but three possible 32-to-64 extension modes: signed int, unsigned int
> and pointer. The third one sets the top 33 bits to zero, clearing the
> top bit of the 31-bit pointer value in the process. Nothing else needs
> this, so if we just clear the upper bits on all system calls and go
> back to passing 64-bit arguments as pairs, we are fine and have a much
> simpler solution.

It would be indeed simpler from a kernel perspective. I'm not sure about
the performance impact (a wrapper which does "mov wn, wn" for the first
6 registers, on top of existing wrappers). OTOH, with explicit wrappers
we have the overhead of an additional function call, so we may be better
off with the former.

> > 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.
>
> Correct. I don't think we need an ILP32_SYSCALL_DEFINEx set of macros
> though, the existing COMPAT_SYSCALL_DEFINEx ones should get this right
> already.

The existing COMPAT_SYSCALL_DEFINEx macros generate the wrapper and
definition for the compat_sys_* functions. What I meant by an
ILP32_SYSCALL_DEFINEx is a macro which only generates a wrapper that
calls into the native syscall (after sanitizing the arguments).

--
Catalin