Re: [PATCH 2/2] maple_tree: Fix a potential memory leak, OOB access, or other unpredictable bug

[Liam R. Howlett]

* Peng Zhang <perlyzhang@xxxxxxxxx> [230410 08:58]:
> 
> 在 2023/4/10 20:43, Liam R. Howlett 写道:
> > * Peng Zhang <zhangpeng.00@xxxxxxxxxxxxx> [230407 00:10]:
> > > In mas_alloc_nodes(), there is such a piece of code:
> > > while (requested) {
> > > 	...
> > > 	node->node_count = 0;
> > > 	...
> > > }
> > You don't need to quote code in your commit message since it is
> > available in the change log or in the file itself.
> Ok, I will change it in the next version.
> > 
> > > "node->node_count = 0" means to initialize the node_count field of the
> > > new node, but the node may not be a new node. It may be a node that
> > > existed before and node_count has a value, setting it to 0 will cause a
> > > memory leak. At this time, mas->alloc->total will be greater than the
> > > actual number of nodes in the linked list, which may cause many other
> > > errors. For example, out-of-bounds access in mas_pop_node(), and
> > > mas_pop_node() may return addresses that should not be used.
> > > Fix it by initializing node_count only for new nodes.
> > > 
> > > Fixes: 54a611b60590 ("Maple Tree: add new data structure")
> > > Signed-off-by: Peng Zhang <zhangpeng.00@xxxxxxxxxxxxx>
> > > Cc: <stable@xxxxxxxxxxxxxxx>
> > > ---
> > >   lib/maple_tree.c | 16 ++++------------
> > >   1 file changed, 4 insertions(+), 12 deletions(-)
> > > 
> > > diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> > > index 65fd861b30e1..9e25b3215803 100644
> > > --- a/lib/maple_tree.c
> > > +++ b/lib/maple_tree.c
> > > @@ -1249,26 +1249,18 @@ static inline void mas_alloc_nodes(struct ma_state *mas, gfp_t gfp)
> > >   	node = mas->alloc;
> > >   	node->request_count = 0;
> > >   	while (requested) {
> > > -		max_req = MAPLE_ALLOC_SLOTS;
> > > -		if (node->node_count) {
> > > -			unsigned int offset = node->node_count;
> > > -
> > > -			slots = (void **)&node->slot[offset];
> > > -			max_req -= offset;
> > > -		} else {
> > > -			slots = (void **)&node->slot;
> > > -		}
> > > -
> > > +		max_req = MAPLE_ALLOC_SLOTS - node->node_count;
> > > +		slots = (void **)&node->slot[node->node_count];
> > Thanks, this is much cleaner.
> > 
> > >   		max_req = min(requested, max_req);
> > >   		count = mt_alloc_bulk(gfp, max_req, slots);
> > >   		if (!count)
> > >   			goto nomem_bulk;
> > > +		if (node->node_count == 0)
> > > +			node->slot[0]->node_count = 0;
> > >   		node->node_count += count;
> > >   		allocated += count;
> > >   		node = node->slot[0];
> > > -		node->node_count = 0;
> > > -		node->request_count = 0;
> > Why are we not clearing request_count anymore?
> Because the node pointed to by the variable "node"
> must not be the head node of the linked list at
> this time, we only need to maintain the information
> of the head node.

Right, at this time it is not the head node, but could it become the
head node with invalid data?  I think it can, because we don't
explicitly set it in mas_pop_node()?

In any case, be sure to mention that you make a change like this in the
change log, like "Drop setting the resquest_count as it is unnecessary
because.." in a new paragraph, so that it is not missed.


> > 
> > >   		requested -= count;
> > >   	}
> > >   	mas->alloc->total = allocated;
> > > -- 
> > > 2.20.1
> > >