On Tue, Jan 17, 2023 at 11:00 AM Jann Horn [off-list ref] wrote:

On Tue, Jan 17, 2023 at 7:55 PM Suren Baghdasaryan [off-list ref] wrote:

quoted

On Tue, Jan 17, 2023 at 10:47 AM Matthew Wilcox [off-list ref] wrote:

quoted

On Tue, Jan 17, 2023 at 10:36:42AM -0800, Suren Baghdasaryan wrote:

quoted

On Tue, Jan 17, 2023 at 10:31 AM Matthew Wilcox [off-list ref] wrote:

quoted

On Tue, Jan 17, 2023 at 10:26:32AM -0800, Suren Baghdasaryan wrote:

quoted

On Tue, Jan 17, 2023 at 10:12 AM Jann Horn [off-list ref] wrote:

quoted

On Mon, Jan 9, 2023 at 9:55 PM Suren Baghdasaryan [off-list ref] wrote:

quoted

rw_semaphore is a sizable structure of 40 bytes and consumes
considerable space for each vm_area_struct. However vma_lock has
two important specifics which can be used to replace rw_semaphore
with a simpler structure:

[...]

quoted

 static inline void vma_read_unlock(struct vm_area_struct *vma)
 {
-       up_read(&vma->vm_lock->lock);
+       if (atomic_dec_and_test(&vma->vm_lock->count))
+               wake_up(&vma->vm_mm->vma_writer_wait);
 }

I haven't properly reviewed this, but this bit looks like a
use-after-free because you're accessing the vma after dropping your
reference on it. You'd have to first look up the vma->vm_mm, then do
the atomic_dec_and_test(), and afterwards do the wake_up() without
touching the vma. Or alternatively wrap the whole thing in an RCU
read-side critical section if the VMA is freed with RCU delay.

vm_lock->count does not control the lifetime of the VMA, it's a
counter of how many readers took the lock or it's negative if the lock
is write-locked.

Yes, but ...

        Task A:
        atomic_dec_and_test(&vma->vm_lock->count)
                        Task B:
                        munmap()
                        write lock
                        free VMA
                        synchronize_rcu()
                        VMA is really freed
        wake_up(&vma->vm_mm->vma_writer_wait);

... vma is freed.

Now, I think this doesn't occur.  I'm pretty sure that every caller of
vma_read_unlock() is holding the RCU read lock.  But maybe we should
have that assertion?

Yep, that's what this patch is doing
https://lore.kernel.org/all/20230109205336.3665937-27-surenb@google.com/ (local)
by calling vma_assert_no_reader() from __vm_area_free().

That's not enough though.  Task A still has a pointer to vma after it
has called atomic_dec_and_test(), even after vma has been freed by
Task B, and before Task A dereferences vma->vm_mm.

Ah, I see your point now. I guess I'll have to store vma->vm_mm in a
local variable and call mmgrab() before atomic_dec_and_test(), then
use it in wake_up() and call mmdrop(). Is that what you are thinking?

You shouldn't need mmgrab()/mmdrop(), because whoever is calling you
for page fault handling must be keeping the mm_struct alive.

Good point. Will update in the next revision to store mm before
dropping the count. Thanks for all the comments folks!

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