On 07/27/2018 01:46 PM, Josh Poimboeuf wrote:
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.
Makes sense, that would be much nicer. I looked into the history a bit, EXT4_MAXQUOTAS was adjusted from 2 to 3 in v4.5 so a backport of this to v4.4 or earlier would require a bit of adjustment, but XQM_MAXQUOTAS looks to have been 3 for a very long time.
I'll see about consolidating them and adjusting this patch.
Thanks, Jeremy