Re: [PATCH 1/3] ext4: super: Fix spectre gadget in ext4_quota_on

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Fri, Jul 27, 2018 at 04:23:55PM +0000, Jeremy Cline wrote:
> 'type' is a user-controlled value used to index into 's_qf_names', which
> can be used in a Spectre v1 attack. Clamp 'type' to the size of the
> array to avoid a speculative out-of-bounds read.
> 
> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> Cc: stable@vger.kernel.org
> Signed-off-by: Jeremy Cline <jcline@redhat.com>
> ---
>  fs/ext4/super.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 6480e763080f..c04a09b51742 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -40,6 +40,7 @@
>  #include <linux/crc16.h>
>  #include <linux/dax.h>
>  #include <linux/cleancache.h>
> +#include <linux/nospec.h>
>  #include <linux/uaccess.h>
>  #include <linux/iversion.h>
>  
> @@ -5559,6 +5560,7 @@ static int ext4_quota_on(struct super_block *sb, int type, int format_id,
>  	if (path->dentry->d_sb != sb)
>  		return -EXDEV;
>  	/* Journaling quota? */
> +	type = array_index_nospec(type, EXT4_MAXQUOTAS);
>  	if (EXT4_SB(sb)->s_qf_names[type]) {
>  		/* Quotafile not in fs root? */
>  		if (path->dentry->d_parent != sb->s_root)

Generally we try to put the array_index_nospec() close to the bounds
check for which it's trying to prevent speculation past.

In this case, I'd expect the EXT4_MAXQUOTAS bounds check to be in
do_quotactl(), but it seems to be missing:

	if (type >= (XQM_COMMAND(cmd) ? XQM_MAXQUOTAS : MAXQUOTAS))
		return -EINVAL;

Also it looks like XQM_MAXQUOTAS, MAXQUOTAS, and EXT4_MAXQUOTAS all have
the same value (3).  Maybe they can be consolidated to just use
MAXQUOTAS everywhere?  Then the nospec would be simple:

	if (type >= MAXQUOTAS)
		return -EINVAL;
	type = array_index_nospec(type, MAXQUOTAS);

Otherwise I think we may need to disperse the array_index_nospec calls
deeper in the callchain.

-- 
Josh