linux-kernel - Re: [PATCH] squashfs: fix NULL pointer dereference in bio_alloc

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Message-ID: <aCXg9t1QWR5jgpqX@casper.infradead.org>
Date: Thu, 15 May 2025 13:41:26 +0100
From: Matthew Wilcox <willy@...radead.org>
To: caius.zone@...oud.com
Cc: phillip@...ashfs.org.uk, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, Caius Zone <zone@...sast.com>
Subject: Re: [PATCH] squashfs: fix NULL pointer dereference in
 bio_alloc_clone failure path

On Thu, May 15, 2025 at 08:01:54PM +0800, caius.zone@...oud.com wrote:
> diff --git a/fs/squashfs/block.c b/fs/squashfs/block.c
> index 2dc730800f44..b00a71f8933c 100644
> --- a/fs/squashfs/block.c
> +++ b/fs/squashfs/block.c
> @@ -115,6 +115,9 @@ static int squashfs_bio_read_cached(struct bio *fullbio,
>  			struct bio *new = bio_alloc_clone(bdev, fullbio,
>  							  GFP_NOIO, &fs_bio_set);
>  
> +			if (!new)
> +				return -ENOMEM;
> +
>  			if (bio) {
>  				bio_trim(bio, start_idx * PAGE_SECTORS,
>  					 (end_idx - start_idx) * PAGE_SECTORS);

This is definitely wrong.  The check for 'new' being NULL should be
after the submit_bio() otherwise we'll leak the previous bio.

But then we need to not call bio_chain() ... so I think this ends up
looking like:

@@ -115,12 +115,15 @@ static int squashfs_bio_read_cached(struct bio *fullbio,
                        struct bio *new = bio_alloc_clone(bdev, fullbio,
                                                          GFP_NOIO, &fs_bio_set);

+                       if (new)
+                               bio_chain(bio, new);
                        if (bio) {
                                bio_trim(bio, start_idx * PAGE_SECTORS,
                                         (end_idx - start_idx) * PAGE_SECTORS);
-                               bio_chain(bio, new);
                                submit_bio(bio);
                        }
+                       if (!new)
+                               return -ENOMEM;

                        bio = new;
                        start_idx = idx;

... but that might not be right either.  We might need to break out of
the loop, rather than returning and do the submit_bio_wait() followed by
the bio_put().