On Tue, Jan 17, 2023 at 10:26:32AM -0800, Suren Baghdasaryan wrote: > On Tue, Jan 17, 2023 at 10:12 AM Jann Horn <ja...@google.com> wrote: > > > > On Mon, Jan 9, 2023 at 9:55 PM Suren Baghdasaryan <sur...@google.com> 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?