On Wed 23-02-22 06:10:54, Suren Baghdasaryan wrote: > On Wed, Feb 23, 2022 at 12:55 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > 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? > > Sorry, I got you confused. The original vma that was passed as a > parameter to vma_merge(vma) was freed and vma_merge() returns the area > it was merged with: > > *prev = vma_merge(vma_a) <-- vma_a is merged with adjacent vma_b, > vma_a is freed and vma_b is returned > vma = *prev <-- "vma" now points to vma_b OK, I see the problem now. Essentially if we have two adjacent vmas which have the same name but wrapped by two anon_vma_name instances and we merge them together then the first one can be freed along with its anon_vma_name which is still used by replace_anon_vma_name. Thanks for the patience. -- Michal Hocko SUSE Labs
