Re: [PATCH v13 04/20] mm, arm64: untag user pointers passed to memory syscalls

[Andrey Konovalov]

On Fri, Mar 29, 2019 at 11:30 AM Catalin Marinas
<catalin.marinas@xxxxxxx> wrote:
>
> (I trimmed down the cc list a bit since it's always bouncing)
>
> On Thu, Mar 28, 2019 at 02:19:34PM -0400, Steven Rostedt wrote:
> > On Thu, 28 Mar 2019 19:10:07 +0100
> > Andrey Konovalov <andreyknvl@xxxxxxxxxx> wrote:
> >
> > > > > Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> > > > > ---
> > > > >  ipc/shm.c      | 2 ++
> > > > >  mm/madvise.c   | 2 ++
> > > > >  mm/mempolicy.c | 5 +++++
> > > > >  mm/migrate.c   | 1 +
> > > > >  mm/mincore.c   | 2 ++
> > > > >  mm/mlock.c     | 5 +++++
> > > > >  mm/mmap.c      | 7 +++++++
> > > > >  mm/mprotect.c  | 1 +
> > > > >  mm/mremap.c    | 2 ++
> > > > >  mm/msync.c     | 2 ++
> > > > >  10 files changed, 29 insertions(+)
> > > >
> > > > I wonder whether it's better to keep these as wrappers in the arm64
> > > > code.
> > >
> > > I don't think I understand what you propose, could you elaborate?
> >
> > I believe Catalin is saying that instead of placing things like:
> >
> > @@ -1593,6 +1593,7 @@ SYSCALL_DEFINE3(shmat, int, shmid, char __user *, shmaddr, int, shmflg)
> >       unsigned long ret;
> >       long err;
> >
> > +     shmaddr = untagged_addr(shmaddr);
> >
> > To instead have the shmaddr set to the untagged_addr() before calling
> > the system call, and passing the untagged addr to the system call, as
> > that goes through the arm64 architecture specific code first.
>
> Indeed. For example, we already have a SYSCALL_DEFINE6(mmap, ...) in
> arch/arm64/kernel/sys.c, just add the untagging there. We could do
> something similar for the other syscalls. I don't mind doing this in the
> generic code but if it's only needed for arm64, I'd rather keep the
> generic changes to a minimum.

Do I understand correctly, that I'll need to add ksys_ wrappers for
each of the memory syscalls, and then redefine them in
arch/arm64/kernel/sys.c with arm64_ prefix, like it is done for the
personality syscall right now? This will require generic changes as
well.

>
> (I had a hack overriding __SC_CAST to do this automatically for pointer
> arguments but this wouldn't work on mmap() and friends as the argument
> is unsigned long)
>
> --
> Catalin