mm: check that mm is still valid in madvise()

IORING_OP_MADVISE can end up basically doing mprotect() on the VM of
another process, which means that it can race with our crazy core dump
handling which accesses the VM state without holding the mmap_sem
(because it incorrectly thinks that it is the final user).

This is clearly a core dumping problem, but we've never fixed it the
right way, and instead have the notion of "check that the mm is still
ok" using mmget_still_valid() after getting the mmap_sem for writing in
any situation where we're not the original VM thread.

See commit 04f5866e ("coredump: fix race condition between
mmget_not_zero()/get_task_mm() and core dumping") for more background on
this whole mmget_still_valid() thing.  You might want to have a barf bag
handy when you do.

We're discussing just fixing this properly in the only remaining core
dumping routines.  But even if we do that, let's make do_madvise() do
the right thing, and then when we fix core dumping, we can remove all
these mmget_still_valid() checks.

Change-Id: I503f628ca80bd759e101d03a96f3600294a0726a
Reported-and-tested-by: Jann Horn <jannh@google.com>
Fixes: c1ca757bd6f4 ("io_uring: add IORING_OP_MADVISE")
Acked-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Git-commit: d208cbe90b0f39abbd019a383514306997ba47e3
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>