On Tue, Aug 03, 2021 at 11:07:35PM +0000, Liam Howlett wrote:

* Luigi Rizzo [off-list ref] [210803 17:49]:

quoted

On Tue, Aug 3, 2021 at 6:08 PM Jason Gunthorpe [off-list ref] wrote:

quoted

On Sat, Jul 31, 2021 at 10:53:41AM -0700, Luigi Rizzo wrote:

quoted

find_vma() and variants need protection when used.
This patch adds mmap_assert_lock() calls in the functions.

To make sure the invariant is satisfied, we also need to add a
mmap_read_loc() around the get_user_pages_remote() call in
get_arg_page(). The lock is not strictly necessary because the mm
has been newly created, but the extra cost is limited because
the same mutex was also acquired shortly before in __bprm_mm_init(),
so it is hot and uncontended.

Signed-off-by: Luigi Rizzo <redacted>
 fs/exec.c | 2 ++
 mm/mmap.c | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/fs/exec.c b/fs/exec.c
index 38f63451b928..ac7603e985b4 100644
+++ b/fs/exec.c

@@ -217,8 +217,10 @@ static struct page *get_arg_page(struct linux_binprm *bprm, unsigned long pos,
       * We are doing an exec().  'current' is the process
       * doing the exec and bprm->mm is the new process's mm.
       */
+     mmap_read_lock(bprm->mm);
      ret = get_user_pages_remote(bprm->mm, pos, 1, gup_flags,
                      &page, NULL, NULL);
+     mmap_read_unlock(bprm->mm);
      if (ret <= 0)
              return NULL;

Wasn't Jann Horn working on something like this too?

https://lore.kernel.org/linux-mm/20201016225713.1971256-1-jannh@google.com/ (local)

IIRC it was very tricky here, are you sure it is OK to obtain this lock
here?

I cannot comment on Jann's patch series but no other thread knows
about this mm at this point in the code so the lock is definitely
safe to acquire (shortly before there was also a write lock acquired
on the same mm, in the same conditions).

If there is no other code that knows about this mm, then does one need
the lock at all?  Is this just to satisfy the new check you added?

If you want to make this change, I would suggest writing it in a way to
ensure the call to expand_downwards() in the same function also holds
the lock.  I believe this is technically required as well?  What do you
think?

This is essentially what Jann was doing. Since the mm is newly created
we can create it write locked and then we can add proper locking tests
to many of the functions called along this path.

Adding useless locks around each troublesome callsite just seems
really confusing to me.

Jason