Re: [PATCH] bcache: Fix a NULL or wild pointer dereference in btree_gc_rewrite_node()

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Jan 25, 2022 at 12:47:01AM +0800, Zhou Qingyang wrote:
> In btree_gc_rewrite_node(), btree_node_alloc_replacement() is assigned to
> n and return error code or NULL on failure. n is passed to
> bch_btree_node_write_sync() and there is a dereference of it in
> bch_btree_node_write_sync() without checks, which may lead to wild
> pointer dereference or NULL pointer dereference depending on n.
> 
> Fix this bug by adding IS_ERR_OR_NULL check of n.
> 
> This bug was found by a static analyzer.
> 
> Builds with 'make allyesconfig' show no new warnings,
> and our static analyzer no longer warns about this code.
> 
> Fixes: ("bcache: Rework btree cache reserve handling")
> Signed-off-by: Zhou Qingyang <zhou1615@umn.edu>
> ---
> The analysis employs differential checking to identify inconsistent 
> security operations (e.g., checks or kfrees) between two code paths 
> and confirms that the inconsistent operations are not recovered in the
> current function or the callers, so they constitute bugs. 
> 
> Note that, as a bug found by static analysis, it can be a false
> positive or hard to trigger. Multiple researchers have cross-reviewed
> the bug.
> 
>  drivers/md/bcache/btree.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
> index 88c573eeb598..06d42292e86c 100644
> --- a/drivers/md/bcache/btree.c
> +++ b/drivers/md/bcache/btree.c
> @@ -1504,6 +1504,8 @@ static int btree_gc_rewrite_node(struct btree *b, struct btree_op *op,
>  		return 0;
>  
>  	n = btree_node_alloc_replacement(replace, NULL);
> +	if (IS_ERR_OR_NULL(n))
> +		return 0;
>  
>  	/* recheck reserve after allocating replacement node */
>  	if (btree_check_reserve(b, NULL)) {
> -- 
> 2.25.1
> 

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