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From:   Andreas Dilger <adilger@dilger.ca>
Message-Id: <06E7E8DE-FA81-4764-B069-885C7963084E@dilger.ca>
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Subject: Re: [PATCH v2] ext4: fix use-after-free race with
 debug_want_extra_isize
Date:   Thu, 18 Apr 2019 13:00:48 -0600
In-Reply-To: <20190418155937.164947-1-brho@google.com>
Cc:     Theodore Ts'o <tytso@mit.edu>,
        syzbot+f584efa0ac7213c226b7@syzkaller.appspotmail.com,
        Jan Kara <jack@suse.cz>, stable@vger.kernel.org,
        linux-ext4 <linux-ext4@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>
To:     Barret Rhoden <brho@google.com>
References: <20190415211945.27343-1-brho@google.com>
 <20190418155937.164947-1-brho@google.com>
Sender: linux-kernel-owner@vger.kernel.org
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On Apr 18, 2019, at 9:59 AM, Barret Rhoden <brho@google.com> wrote:
>=20
> When remounting with debug_want_extra_isize, we were not performing =
the
> same checks that we do during a normal mount.  That allowed us to set =
a
> value for s_want_extra_isize that reached outside the s_inode_size.
>=20
> Reported-by: syzbot+f584efa0ac7213c226b7@syzkaller.appspotmail.com
> Reviewed-by: Jan Kara <jack@suse.cz>
> Signed-off-by: Barret Rhoden <brho@google.com>
> Cc: stable@vger.kernel.org # 4.14.111

Reviewed-by: Andreas Dilger <adilger@dilger.ca>

> ---
>=20
> - Updated tags
>=20
> Thanks for the review!
>=20
> fs/ext4/super.c | 58 +++++++++++++++++++++++++++++--------------------
> 1 file changed, 34 insertions(+), 24 deletions(-)
>=20
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 6ed4eb81e674..184944d4d8d1 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -3513,6 +3513,37 @@ int ext4_calculate_overhead(struct super_block =
*sb)
> 	return 0;
> }
>=20
> +static void ext4_clamp_want_extra_isize(struct super_block *sb)
> +{
> +	struct ext4_sb_info *sbi =3D EXT4_SB(sb);
> +	struct ext4_super_block *es =3D sbi->s_es;
> +
> +	/* determine the minimum size of new large inodes, if present */
> +	if (sbi->s_inode_size > EXT4_GOOD_OLD_INODE_SIZE &&
> +	    sbi->s_want_extra_isize =3D=3D 0) {
> +		sbi->s_want_extra_isize =3D sizeof(struct ext4_inode) -
> +						     =
EXT4_GOOD_OLD_INODE_SIZE;
> +		if (ext4_has_feature_extra_isize(sb)) {
> +			if (sbi->s_want_extra_isize <
> +			    le16_to_cpu(es->s_want_extra_isize))
> +				sbi->s_want_extra_isize =3D
> +					=
le16_to_cpu(es->s_want_extra_isize);
> +			if (sbi->s_want_extra_isize <
> +			    le16_to_cpu(es->s_min_extra_isize))
> +				sbi->s_want_extra_isize =3D
> +					=
le16_to_cpu(es->s_min_extra_isize);
> +		}
> +	}
> +	/* Check if enough inode space is available */
> +	if (EXT4_GOOD_OLD_INODE_SIZE + sbi->s_want_extra_isize >
> +							=
sbi->s_inode_size) {
> +		sbi->s_want_extra_isize =3D sizeof(struct ext4_inode) -
> +						       =
EXT4_GOOD_OLD_INODE_SIZE;
> +		ext4_msg(sb, KERN_INFO,
> +			 "required extra inode space not available");
> +	}
> +}
> +
> static void ext4_set_resv_clusters(struct super_block *sb)
> {
> 	ext4_fsblk_t resv_clusters;
> @@ -4387,30 +4418,7 @@ static int ext4_fill_super(struct super_block =
*sb, void *data, int silent)
> 	} else if (ret)
> 		goto failed_mount4a;
>=20
> -	/* determine the minimum size of new large inodes, if present */
> -	if (sbi->s_inode_size > EXT4_GOOD_OLD_INODE_SIZE &&
> -	    sbi->s_want_extra_isize =3D=3D 0) {
> -		sbi->s_want_extra_isize =3D sizeof(struct ext4_inode) -
> -						     =
EXT4_GOOD_OLD_INODE_SIZE;
> -		if (ext4_has_feature_extra_isize(sb)) {
> -			if (sbi->s_want_extra_isize <
> -			    le16_to_cpu(es->s_want_extra_isize))
> -				sbi->s_want_extra_isize =3D
> -					=
le16_to_cpu(es->s_want_extra_isize);
> -			if (sbi->s_want_extra_isize <
> -			    le16_to_cpu(es->s_min_extra_isize))
> -				sbi->s_want_extra_isize =3D
> -					=
le16_to_cpu(es->s_min_extra_isize);
> -		}
> -	}
> -	/* Check if enough inode space is available */
> -	if (EXT4_GOOD_OLD_INODE_SIZE + sbi->s_want_extra_isize >
> -							=
sbi->s_inode_size) {
> -		sbi->s_want_extra_isize =3D sizeof(struct ext4_inode) -
> -						       =
EXT4_GOOD_OLD_INODE_SIZE;
> -		ext4_msg(sb, KERN_INFO, "required extra inode space not"
> -			 "available");
> -	}
> +	ext4_clamp_want_extra_isize(sb);
>=20
> 	ext4_set_resv_clusters(sb);
>=20
> @@ -5194,6 +5202,8 @@ static int ext4_remount(struct super_block *sb, =
int *flags, char *data)
> 		goto restore_opts;
> 	}
>=20
> +	ext4_clamp_want_extra_isize(sb);
> +
> 	if ((old_opts.s_mount_opt & EXT4_MOUNT_JOURNAL_CHECKSUM) ^
> 	    test_opt(sb, JOURNAL_CHECKSUM)) {
> 		ext4_msg(sb, KERN_ERR, "changing journal_checksum "
> --
> 2.21.0.392.gf8f6787159e-goog
>=20


Cheers, Andreas


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