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charset="UTF-8" X-Stat-Signature: ppcptje6gzyfitccqeous39cc7jrxbq3 X-Rspam-User: X-Rspamd-Queue-Id: 71E0220013 X-Rspamd-Server: rspam06 X-HE-Tag: 1673982007-693666 X-HE-Meta: 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 GSO7dNf5 A6z2Re62xS05220CY4Wpm3K5/tOPM84nzx7Wz+cp35qp49AqQasxylnlA3IhAaotxbJBc 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 7:55 PM Suren Baghdasaryan wrote: > On Tue, Jan 17, 2023 at 10:47 AM Matthew Wilcox wrote: > > > > On Tue, Jan 17, 2023 at 10:36:42AM -0800, Suren Baghdasaryan wrote: > > > 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(). > > > > 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.