linux-kernel - Re: [PATCH] fs/squashfs: handle possible null pointer

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Date:   Wed, 22 Dec 2021 01:38:28 +0000
From:   Phillip Lougher <phillip@...ashfs.org.uk>
To:     Peng Hao <flyingpenghao@...il.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs/squashfs: handle possible null pointer

On 21/12/2021 02:03, Peng Hao wrote:
>   in squashfs_fill_super:
> 
>          msblk->decompressor = supported_squashfs_filesystem(
>                          fc,
>                          le16_to_cpu(sblk->s_major),
>                          le16_to_cpu(sblk->s_minor),
>                          le16_to_cpu(sblk->compression));
>          if (msblk->decompressor == NULL)
>                  goto failed_mount;
>          ...
> 
> failed_mount:
> 	...
> 	squashfs_decompressor_destroy(msblk);
> 
> in squashfs_decompressor_destroy:
> 	if (stream) {
>          	msblk->decompressor->free(stream->stream);
> msblk->decompressor is NULL.
> 
> so add a judgment whether a null pointer.

NACK.

The NULL pointer dereference (msblk->decompressor) will not happen 
because stream (msblk->stream) is NULL (thus the code in question will 
not be executed).

At the start of the initialisation phase msblk is zeroed out

	sb->s_fs_info = kzalloc(sizeof(*msblk), GFP_KERNEL);
	if (sb->s_fs_info == NULL) {
		ERROR("Failed to allocate squashfs_sb_info\n");
		return -ENOMEM;
	}
	msblk = sb->s_fs_info;

Making msblk->decompressor and msblk->stream NULL.

msblk->decompressor is allocated first

	msblk->decompressor = supported_squashfs_filesystem(
			fc,
			le16_to_cpu(sblk->s_major),
			le16_to_cpu(sblk->s_minor),
			le16_to_cpu(sblk->compression));
	if (msblk->decompressor == NULL)
		goto failed_mount;

If that succeeds (msblk->decompressor != NULL), then msblk->stream is 
allocated

	msblk->stream = squashfs_decompressor_setup(sb, flags);
	if (IS_ERR(msblk->stream)) {
		err = PTR_ERR(msblk->stream);
		msblk->stream = NULL;
		goto insanity;
	}

Thus, in squashfs_decompressor_destroy() if stream (msblk->stream) is 
not NULL, then msblk->decompressor is also guaranteed to be not NULL.

Likewise if msblk->decompressor is NULL, then msblk->stream will also be 
NULL.

Or to put it another way, the decompressor is only destroyed if it was 
previously created (msblk->stream != NULL), and it will only have been 
created if msblk->decompressor is != NULL.

Phillip