On Tue 22-02-22 07:43:40, Suren Baghdasaryan wrote: > On Tue, Feb 22, 2022 at 12:06 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > On Mon 21-02-22 21:40:25, Suren Baghdasaryan wrote: > > > When adjacent vmas are being merged it can result in the vma that was > > > originally passed to madvise_update_vma being destroyed. In the current > > > implementation, the name parameter passed to madvise_update_vma points > > > directly to vma->anon_name->name and it is used after the call to > > > vma_merge. In the cases when vma_merge merges the original vma and > > > destroys it, this will result in use-after-free bug as shown below: > > > > > > madvise_vma_behavior << passes vma->anon_name->name as name param > > > madvise_update_vma(name) > > > vma_merge > > > __vma_adjust > > > vm_area_free <-- frees the vma > > > replace_vma_anon_name(name) <-- UAF > > > > This seems to be stale because bare const char pointer is not passed in > > the call chain. In fact I am not even sure there is any actual UAF here > > after the rework. > > Could you be more specific in describing the scenario? > > Yes, sorry, I need to update the part of the description talking about > passing vma->anon_name->name directly. > I think UAF is still there, it's just harder to reproduce (admittedly > I could not reproduce it with the previous reproducer). The scenario > would be when a vma with vma->anon_name->kref == 1 is being merged > with another one and freed in the process: > > madvise_vma_behavior > anon_name = vma_anon_name(vma) <-- does not increase refcount > madvise_update_vma(anon_name) > *prev = vma_merge <-- returns another vma > __vma_adjust > vm_area_free(vma) > free_vma_anon_name > anon_vma_name_put > vma_anon_name_free <-- frees the vma->anon_name > vma = *prev <-- original vma was freed How come this is not a UAF in the first place? > replace_vma_anon_name(vma, >>anon_name<<) <-- UAF > > Does this make sense or did I miss something? Sorry for being dense but I still do not see it. If *prev has been freed then we already have a different UAF. Admittedly, I am not really fluent at vma_merge code path so I am not really sure your chain above is really possible. I will try to double check later. -- Michal Hocko SUSE Labs
