binfmt_elf: take the mmap lock around find_extend_vma()

create_elf_tables() runs after setup_new_exec(), so other tasks can
already access our new mm and do things like process_madvise() on it.  (At
the time I'm writing this commit, process_madvise() is not in mainline
yet, but has been in akpm's tree for some time.)

While I believe that there are currently no APIs that would actually allow
another process to mess up our VMA tree (process_madvise() is limited to
MADV_COLD and MADV_PAGEOUT, and uring and userfaultfd cannot reach an mm
under which no syscalls have been executed yet), this seems like an
accident waiting to happen.

Let's make sure that we always take the mmap lock around GUP paths as long
as another process might be able to see the mm.

(Yes, this diff looks suspicious because we drop the lock before doing
anything with `vma`, but that's because we actually don't do anything with
it apart from the NULL check.)

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Michel Lespinasse <walken@google.com>
Cc: "Eric W . Biederman" <ebiederm@xmission.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Sakari Ailus <sakari.ailus@linux.intel.com>
Link: https://lkml.kernel.org/r/CAG48ez1-PBCdv3y8pn-Ty-b+FmBSLwDuVKFSt8h7wARLy0dF-Q@mail.gmail.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>