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charset="UTF-8" X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 4DAA5C000B X-Rspam-User: X-Stat-Signature: tzpd3e9xfdbw5cggymjx9ajrjp8uyjim X-HE-Tag: 1673980614-813226 X-HE-Meta: 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 b42EpvRH kLe0DCYKysb6fpGkFJfMeDh9csMp2bAJLf7VIsQhzeUNaMtbBWt9Cy5b3Pw== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Jan 17, 2023 at 10:31 AM Matthew Wilcox wrote: > > On Tue, Jan 17, 2023 at 10:26:32AM -0800, Suren Baghdasaryan wrote: > > On Tue, Jan 17, 2023 at 10:12 AM Jann Horn wrote: > > > > > > On Mon, Jan 9, 2023 at 9:55 PM Suren Baghdasaryan wrote: > > > > 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: > > > [...] > > > > 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/ by calling vma_assert_no_reader() from __vm_area_free(). >