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 6L66A84s jWTdqVLIgcH89dYbUi86VGfGMajPQxa9FgVTxrl/ipmDCirPvViRWTqFwDLqpo3hh9j3CbQHjkWK1k5lvXhQ3uCxcug== 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: List-Subscribe: List-Unsubscribe: On Mon, Jun 23, 2025 at 12:22:26PM -0400, Liam R. Howlett wrote: > * Suren Baghdasaryan [250623 11:39]: > > On Mon, Jun 23, 2025 at 8:00 AM Vlastimil Babka wrote: > > > > > > Since the introduction in 9a10064f5625 ("mm: add a field to store names > > > for private anonymous memory") the code to set anon_name on a vma has > > > been using madvise_update_vma() to call replace_vma_anon_name(). Since > > > > s/replace_vma_anon_name()/replace_anon_vma_name() > > > > > the former is called also by a number of other madvise behaviours that > > > do not set a new anon_name, they have been passing the existing > > > anon_name of the vma to make replace_vma_anon_name() a no-op. > > > > > > This is rather wasteful as it needs anon_vma_name_eq() to determine the > > > no-op situations, and checks for when replace_vma_anon_name() is allowed > > > (the vma is anon/shmem) duplicate the checks already done earlier in > > > madvise_vma_behavior(). It has also lead to commit 942341dcc574 ("mm: > > > fix use-after-free when anon vma name is used after vma is freed") > > > adding anon_name refcount get/put operations exactly to the cases that > > > actually do not change anon_name - just so the replace_vma_anon_name() > > > can keep safely determining it has nothing to do. > > > > > > The recent madvise cleanups made this suboptimal handling very obvious, > > > but happily also allow for an easy fix. madvise_update_vma() now has the > > > complete information whether it's been called to set a new anon_name, so > > > stop passing it the existing vma's name and doing the refcount get/put > > > in its only caller madvise_vma_behavior(). > > > > > > In madvise_update_vma() itself, limit calling of replace_anon_vma_name() > > > only to cases where we are setting a new name, otherwise we know it's a > > > no-op. We can rely solely on the __MADV_SET_ANON_VMA_NAME behaviour and > > > can remove the duplicate checks for vma being anon/shmem that were done > > > already in madvise_vma_behavior(). > > > > > > The remaining reason to obtain the vma's existing anon_name is to pass > > > it to vma_modify_flags_name() for the splitting and merging to work > > > properly. In case of merging, the vma might be freed along with the > > > anon_name, but madvise_update_vma() will not access it afterwards > > > > This is quite subtle. Can we add a comment in the code that anon_name > > might be freed as a result of vma merge after vma_modify_flags_name() > > gets called and anon_name should not be accessed afterwards? > > Surely that's not the common pattern since the anon vma name is ref > counted? > > And it's probably the case for more than just the anon name? This is all quite tricky. I think the key thing is that madvise_set_anon_name() is invoked by prctl_set_vma() (yuck) which allocates a brand new anon_vma_name. When we merge, we don't actually set that anon_vma_name to anything, but we might put (and thereby maybe free) an anon_vma_name that is identical in terms of the string as part of the merge. But that'll be the vma->anon_vma_name that gets killed, not anon_vma_name in madvise_update_vma(), as that hasn't been set yet :) The problem was when doing so and referencing vma->anon_vma_name, and then afterwards invoking replace_anon_vma_name(). The VMA being changed might have got deleted as part of a merge, and so this could then be a dangling pointer. But the irony is, in that case, there's really no need to call replace_anon_vma_name() the one thing that actually uses anon_vma_name after the merge... because trivially anon_vma_name_eq() effectively comparing vma->anon_vma_name to itself will always be true and thus the operation will always be a no-op. Except it'll be a no-op referencing a dangling pointer... The previous cleanups made this whole thing clearer (see, this all sounds very claer right? :P) meaning the get/put are just totally unnecessary. TL;DR - vma->anon_vma_name is _not being used after the merge_ here in any case. > > ... > > Thanks, > Liam