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charset="UTF-8" X-Rspamd-Queue-Id: D79D2A001C X-Rspamd-Server: rspam09 X-Rspam-User: X-Stat-Signature: 3iitbbqfafia5oae31xbiwdqm4p19czk X-HE-Tag: 1673982378-91825 X-HE-Meta: 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 L3r2ddAy FVWi14m/aAtoosn/DBzrBztoNgfVkesEcmH1GlMcvcvu7NBIh4/YixQ6AeTmFwdWh2qau 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 11:00 AM Jann Horn wrote: > > 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. Good point. Will update in the next revision to store mm before dropping the count. Thanks for all the comments folks!