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

[Catalin Marinas]

(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.

(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