Re: [PATCH v4 32/35] s390/uaccess: Add KMSAN support to put_user() and get_user()

Ilya Leoshkevich
2024-06-18T09:40:55Z

On Tue, 2024-06-18 at 11:24 +0200, Alexander Potapenko wrote:
> On Thu, Jun 13, 2024 at 5:39 PM Ilya Leoshkevich <iii@linux.ibm.com>
> wrote:
> >
> > put_user() uses inline assembly with precise constraints, so Clang
> > is
> > in principle capable of instrumenting it automatically.
> > Unfortunately,
> > one of the constraints contains a dereferenced user pointer, and
> > Clang
> > does not currently distinguish user and kernel pointers. Therefore
> > KMSAN attempts to access shadow for user pointers, which is not a
> > right
> > thing to do.
> >
> > An obvious fix to add __no_sanitize_memory to __put_user_fn() does
> > not
> > work, since it's __always_inline. And __always_inline cannot be
> > removed
> > due to the __put_user_bad() trick.
> >
> > A different obvious fix of using the "a" instead of the "+Q"
> > constraint
> > degrades the code quality, which is very important here, since it's
> > a
> > hot path.
> >
> > Instead, repurpose the __put_user_asm() macro to define
> > __put_user_{char,short,int,long}_noinstr() functions and mark them
> > with
> > __no_sanitize_memory. For the non-KMSAN builds make them
> > __always_inline in order to keep the generated code quality. Also
> > define __put_user_{char,short,int,long}() functions, which call the
> > aforementioned ones and which *are* instrumented, because they call
> > KMSAN hooks, which may be implemented as macros.
>
> I am not really familiar with s390 assembly, but I think you still
> need to call kmsan_copy_to_user() and kmsan_copy_from_user() to
> properly initialize the copied data and report infoleaks.
> Would it be possible to insert calls to linux/instrumented.h hooks
> into uaccess functions?

Aren't the existing instrument_get_user() / instrument_put_user() calls
sufficient?