Re: [PATCH] idr: fix invalid ptr dereference on item delete

[Roman Kagan]

On Fri, May 11, 2018 at 07:40:26AM +0200, Dmitry Vyukov wrote:
> On Fri, May 11, 2018 at 1:54 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> > On 10/05/2018 21:16, Roman Kagan wrote:
> >> If an IDR contains a single entry at index==0, the underlying radix tree
> >> has a single item in its root node, in which case
> >> __radix_tree_lookup(index!=0) doesn't set its *@nodep argument (in
> >> addition to returning NULL).
> >>
> >> However, the tree itself is not empty, i.e. the tree root doesn't have
> >> IDR_FREE tag.
> >>
> >> As a result, on an attempt to remove an index!=0 entry from such an IDR,
> >> radix_tree_delete_item doesn't return early and calls
> >> __radix_tree_delete with invalid parameters which are then dereferenced.
> >>
> >> Reported-by: syzbot+35666cba7f0a337e2e79@xxxxxxxxxxxxxxxxxxxxxxxxx
> >> Signed-off-by: Roman Kagan <rkagan@xxxxxxxxxxxxx>
> >> ---
> >>  lib/radix-tree.c | 5 +++--
> >>  1 file changed, 3 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/lib/radix-tree.c b/lib/radix-tree.c
> >> index da9e10c827df..10ff1bfae952 100644
> >> --- a/lib/radix-tree.c
> >> +++ b/lib/radix-tree.c
> >> @@ -2040,8 +2040,9 @@ void *radix_tree_delete_item(struct radix_tree_root *root,
> >>       void *entry;
> >>
> >>       entry = __radix_tree_lookup(root, index, &node, &slot);
> >> -     if (!entry && (!is_idr(root) || node_tag_get(root, node, IDR_FREE,
> >> -                                             get_slot_offset(node, slot))))
> >> +     if (!entry && (!is_idr(root) || !node ||
> >> +                    node_tag_get(root, node, IDR_FREE,
> >> +                                 get_slot_offset(node, slot))))
> >>               return NULL;
> >>
> >>       if (item && entry != item)
> >>
> >
> > I cannot really vouch for the patch, but if it is correct it's
> > definitely stuff for stable.  The KVM testcase is only for 4.17-rc but
> > this is a really nasty bug in a core data structure.
> >
> > Cc: stable@xxxxxxxxxxxxxxx
> >
> > Should radix-tree be compilable in userspace, so that we can add unit
> > tests for it?...
> 
> Good point.
> 
> For my education, what/where are the tests that run as user-space code?

Actually there are userspace tests for it under tools/tests/radix-tree,
but I didn't manage to get them to build.  Looks like the recent
introduction of a spin_lock in the radix_tree structure (for XArray
work?) broke them.

Roman.