linux-kernel - Re: [PATCH V2] squashfs: Add length check in squashfs_symlink_read

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Message-ID: <20240801153042.prhbovbuys4zmprv@quack3>
Date: Thu, 1 Aug 2024 17:30:42 +0200
From: Jan Kara <jack@...e.cz>
To: Lizhi Xu <lizhi.xu@...driver.com>
Cc: viro@...iv.linux.org.uk, brauner@...nel.org, jack@...e.cz,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	phillip@...ashfs.org.uk, squashfs-devel@...ts.sourceforge.net,
	syzbot+24ac24ff58dc5b0d26b9@...kaller.appspotmail.com,
	syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH V2] squashfs: Add length check in
 squashfs_symlink_read_folio

On Thu 01-08-24 23:17:40, Lizhi Xu wrote:
> syzbot report KMSAN: uninit-value in pick_link, the root cause is that
> squashfs_symlink_read_folio did not check the length, resulting in folio
> not being initialized and did not return the corresponding error code.
> 
> The incorrect value of length is due to the incorrect value of inode->i_size.
> 
> Reported-and-tested-by: syzbot+24ac24ff58dc5b0d26b9@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=24ac24ff58dc5b0d26b9
> Signed-off-by: Lizhi Xu <lizhi.xu@...driver.com>
> ---
>  fs/squashfs/symlink.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/fs/squashfs/symlink.c b/fs/squashfs/symlink.c
> index 6ef735bd841a..d5fa5165ddd6 100644
> --- a/fs/squashfs/symlink.c
> +++ b/fs/squashfs/symlink.c
> @@ -61,6 +61,12 @@ static int squashfs_symlink_read_folio(struct file *file, struct folio *folio)
>  		}
>  	}
>  
> +	if (length < 0) {

OK, so this would mean that (int)inode->i_size is a negative number.
Possible. Perhaps we should rather better validate i_size of symlinks in
squashfs_read_inode()? Otherwise it would be a whack-a-mole game of
catching places that get confused by bogus i_size...

								Honza

> +		ERROR("Unable to read symlink, wrong length [%d]\n", length);
> +		error = -EINVAL;
> +		goto out;
> +	}
> +
>  	/*
>  	 * Read length bytes from symlink metadata.  Squashfs_read_metadata
>  	 * is not used here because it can sleep and we want to use
> -- 
> 2.43.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR