On Tue 17-01-23 17:53:00, Suren Baghdasaryan wrote:

On Tue, Jan 17, 2023 at 7:42 AM 'Michal Hocko' via kernel-team
[off-list ref] wrote:

quoted

On Mon 09-01-23 12:53:21, Suren Baghdasaryan wrote:

quoted

Assert there are no holders of VMA lock for reading when it is about to be
destroyed.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 include/linux/mm.h | 8 ++++++++
 kernel/fork.c      | 2 ++
 2 files changed, 10 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 594e835bad9c..c464fc8a514c 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h

@@ -680,6 +680,13 @@ static inline void vma_assert_write_locked(struct vm_area_struct *vma)
      VM_BUG_ON_VMA(vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq), vma);
 }

+static inline void vma_assert_no_reader(struct vm_area_struct *vma)
+{
+     VM_BUG_ON_VMA(rwsem_is_locked(&vma->lock) &&
+                   vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq),
+                   vma);

Do we really need to check for vm_lock_seq? rwsem_is_locked should tell
us something is wrong on its own, no? This could be somebody racing with
the vma destruction and using the write lock. Unlikely but I do not see
why to narrow debugging scope.

I wanted to ensure there are no page fault handlers (read-lockers)
when we are destroying the VMA and rwsem_is_locked(&vma->lock) alone
could trigger if someone is concurrently calling vma_write_lock(). But
I don't think we expect someone to be write-locking the VMA while we

That would be UAF, no?

are destroying it, so you are right, I'm overcomplicating things here.
I think I can get rid of vma_assert_no_reader() and add
VM_BUG_ON_VMA(rwsem_is_locked(&vma->lock)) directly in
__vm_area_free(). WDYT?

Yes, that adds some debugging. Not sure it is really necessary buyt it
is VM_BUG_ON so why not.
-- 
Michal Hocko
SUSE Labs

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