Re: [PATCH] bfs: add sanity check at bfs_fill_super().

[Andrew Morton <akpm@linux-foundation.org>]

On Wed,  9 May 2018 19:35:04 +0900 Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> wrote:

> syzbot is reporting too large memory allocation at bfs_fill_super() [1].
> Since file system image is corrupted such that bfs_sb->s_start == 0,
> bfs_fill_super() is trying to allocate 8MB of continuous memory. Fix this
> by adding a sanity check on bfs_sb->s_start, __GFP_NOWARN and printf().
> 
> [1] https://syzkaller.appspot.com/bug?id=16a87c236b951351374a84c8a32f40edbc034e96
> 
> ...
>
> --- a/fs/bfs/inode.c
> +++ b/fs/bfs/inode.c
> @@ -350,7 +350,8 @@ static int bfs_fill_super(struct super_block *s, void *data, int silent)
>  
>  	s->s_magic = BFS_MAGIC;
>  
> -	if (le32_to_cpu(bfs_sb->s_start) > le32_to_cpu(bfs_sb->s_end)) {
> +	if (le32_to_cpu(bfs_sb->s_start) > le32_to_cpu(bfs_sb->s_end) ||
> +	    le32_to_cpu(bfs_sb->s_start) < BFS_BSIZE) {

hm, how did you figure out that s_start cannot be less than BFS_BSIZE? 
>From this, I guess?

	info->si_lasti = (le32_to_cpu(bfs_sb->s_start) - BFS_BSIZE) /

I wonder if this is the most appropriate way to check.

>  		printf("Superblock is corrupted\n");
>  		goto out1;
>  	}
> @@ -359,9 +360,11 @@ static int bfs_fill_super(struct super_block *s, void *data, int silent)
>  					sizeof(struct bfs_inode)
>  					+ BFS_ROOT_INO - 1;
>  	imap_len = (info->si_lasti / 8) + 1;
> -	info->si_imap = kzalloc(imap_len, GFP_KERNEL);
> -	if (!info->si_imap)
> +	info->si_imap = kzalloc(imap_len, GFP_KERNEL | __GFP_NOWARN);
> +	if (!info->si_imap) {
> +		printf("Cannot allocate %u bytes\n", imap_len);
>  		goto out1;
> +	}

This seems unnecessary - if the kzalloc() fails we'll get the
page-allocation-fauilure warning and a nice backtrace, etc.  Why
suppress all of that and add our custom warning instead?