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
replace_vma_anon_name(vma, >>anon_name<<) <-- UAF
Does this make sense or did I miss something?
Thanks,
Suren.
>
> > Fix this by raising the name refcount and stabilizing it.
> >
> > Fixes: 9a10064f5625 ("mm: add a field to store names for private anonymous memory")
> > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > Reported-by: syzbot+aa7b3d4b35f9dc46a366@xxxxxxxxxxxxxxxxxxxxxxxxx
> > ---
> > changes in v3:
> > - Reapplied the fix after code refactoring, per Michal Hocko
> >
> > mm/madvise.c | 4 ++++
> > 1 file changed, 4 insertions(+)
> >
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index a395884aeecb..00e8105430e9 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -140,6 +140,8 @@ static int replace_vma_anon_name(struct vm_area_struct *vma,
> > /*
> > * Update the vm_flags on region of a vma, splitting it or merging it as
> > * necessary. Must be called with mmap_sem held for writing;
> > + * Caller should ensure anon_name stability by raising its refcount even when
> > + * anon_name belongs to a valid vma because this function might free that vma.
> > */
> > static int madvise_update_vma(struct vm_area_struct *vma,
> > struct vm_area_struct **prev, unsigned long start,
> > @@ -1021,8 +1023,10 @@ static int madvise_vma_behavior(struct vm_area_struct *vma,
> > }
> >
> > anon_name = vma_anon_name(vma);
> > + anon_vma_name_get(anon_name);
> > error = madvise_update_vma(vma, prev, start, end, new_flags,
> > anon_name);
> > + anon_vma_name_put(anon_name);
> >
> > out:
> > /*
> > --
> > 2.35.1.473.g83b2b277ed-goog
>
> --
> Michal Hocko
> SUSE Labs
