Re: [PATCH] Fix compat regression in process_vm_rw()

[Al Viro]

On Mon, Oct 26, 2020 at 06:03:18PM -0600, Jens Axboe wrote:
> The removal of compat_process_vm_{readv,writev} didn't change
> process_vm_rw(), which always assumes it's not doing a compat syscall.
> Instead of passing in 'false' unconditionally for 'compat', make it
> conditional on in_compat_syscall().
> 
> Fixes: c3973b401ef2 ("mm: remove compat_process_vm_{readv,writev}")
> Reported-by: Kyle Huey <me@xxxxxxxxxxxx>
> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>

ACK with some reservations - I suspect that we want an explicit flag
for process_vm_{read,write}v() that would force the 64bit layout for
the vector refering to the foreign process.  It's not relevant for
regression fix; however, as it is these syscalls are not usable for
32bit process trying to access memory of 64bit one - there's no way
to specify the addresses past 4G.