From: Jeremy Cline Subject: Re: [PATCH 1/3] ext4: super: Fix spectre gadget in ext4_quota_on Date: Fri, 27 Jul 2018 15:17:14 -0400 Message-ID: References: <20180727162357.30801-1-jcline@redhat.com> <20180727162357.30801-2-jcline@redhat.com> <20180727174654.bnooz26puuo7456w@treble> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Cc: Theodore Ts'o , Andreas Dilger , linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org To: Josh Poimboeuf Return-path: In-Reply-To: <20180727174654.bnooz26puuo7456w@treble> Content-Language: en-US Sender: stable-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org 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 >> Cc: stable@vger.kernel.org >> Signed-off-by: Jeremy Cline >> --- >> 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 >> #include >> #include >> +#include >> #include >> #include >> >> @@ -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