linux-kernel - Re: [PATCH v2] vfio: Fix uninitialized symbol and potential dereferencing errors in vfio_combine_iova

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Date:   Tue, 19 Sep 2023 12:04:56 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Cong Liu <liucong2@...inos.cn>
Cc:     jgg@...pe.ca, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] vfio: Fix uninitialized symbol and potential
 dereferencing errors in vfio_combine_iova_ranges

On Thu, 14 Sep 2023 17:08:39 +0800
Cong Liu <liucong2@...inos.cn> wrote:

> when compiling with smatch check, the following errors were encountered:
> 
> drivers/vfio/vfio_main.c:957 vfio_combine_iova_ranges() error: uninitialized symbol 'last'.
> drivers/vfio/vfio_main.c:978 vfio_combine_iova_ranges() error: potentially dereferencing uninitialized 'comb_end'.
> drivers/vfio/vfio_main.c:978 vfio_combine_iova_ranges() error: potentially dereferencing uninitialized 'comb_start'.
> 
> this patch fix these error.
> 
> Signed-off-by: Cong Liu <liucong2@...inos.cn>
> ---
>  drivers/vfio/vfio_main.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> index 40732e8ed4c6..96d2f3030ebb 100644
> --- a/drivers/vfio/vfio_main.c
> +++ b/drivers/vfio/vfio_main.c
> @@ -938,14 +938,17 @@ static int vfio_ioctl_device_feature_migration(struct vfio_device *device,
>  void vfio_combine_iova_ranges(struct rb_root_cached *root, u32 cur_nodes,
>  			      u32 req_nodes)
>  {
> -	struct interval_tree_node *prev, *curr, *comb_start, *comb_end;
> +	struct interval_tree_node *prev, *curr;
> +	struct interval_tree_node *comb_start = NULL, *comb_end = NULL;
>  	unsigned long min_gap, curr_gap;
>  
>  	/* Special shortcut when a single range is required */
>  	if (req_nodes == 1) {
> -		unsigned long last;
> +		unsigned long last = 0;
>  
>  		comb_start = interval_tree_iter_first(root, 0, ULONG_MAX);
> +		if (!comb_start)
> +			return;
>  		curr = comb_start;
>  		while (curr) {
>  			last = curr->last;

@last no longer requires initialization with the @comb_start test.

However, all of these are testing for invalid parameters, which I think
we can eliminate if we simply introduce the following at the start of
the function:

        if (!cur_nodes || cur_nodes <= req_nodes ||
            WARN_ON(!req_nodes || !root->rb_root.rb_node))
                return;

At that point we're guaranteed to have any entry for both the above and
below first entry and there must be at least a second entry (or a
driver bug telling us there are more entries than actually exist) for
the next call below.  Thanks,

Alex


> @@ -963,6 +966,10 @@ void vfio_combine_iova_ranges(struct rb_root_cached *root, u32 cur_nodes,
>  		prev = NULL;
>  		min_gap = ULONG_MAX;
>  		curr = interval_tree_iter_first(root, 0, ULONG_MAX);
> +		if (!curr) {
> +			/* No more ranges to combine */
> +			break;
> +		}
>  		while (curr) {
>  			if (prev) {
>  				curr_gap = curr->start - prev->last;
> @@ -975,6 +982,10 @@ void vfio_combine_iova_ranges(struct rb_root_cached *root, u32 cur_nodes,
>  			prev = curr;
>  			curr = interval_tree_iter_next(curr, 0, ULONG_MAX);
>  		}
> +		if (!comb_start || !comb_end) {
> +			/* No more ranges to combine */
> +			break;
> +		}
>  		comb_start->last = comb_end->last;
>  		interval_tree_remove(comb_end, root);
>  		cur_nodes--;