New subject: [PATCH v2] ext4: fix use-after-free race with debug_want_extra_isize

15 Apr 2019

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.
Reported-by: syzbot+f584efa0ac7213c226b7@syzkaller.appspotmail.com
Signed-off-by: Barret Rhoden brho@google.com
Cc: stable@vger.kernel.org
---
- In the current code, it looks like someone could mount with want_extra_isize
  with some value > 0 but less than the minimums in the s_es.  If that's a bug,
  I can submit a follow-on patch.
- Similarly, on a failed remount, sbi->s_want_extra_isize is changed to the
  remounted value.  I can fix that too if it's a problem.
- Is it OK to remount with a smaller s_want_extra_isize than the previous mount?
  I thought it was, but figured I'd ask while I'm looking at it.
fs/ext4/super.c | 58 +++++++++++++++++++++++++++++--------------------
 1 file changed, 34 insertions(+), 24 deletions(-)

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;
 }
+static void ext4_clamp_want_extra_isize(struct super_block *sb)
+{
+	struct ext4_sb_info *sbi = EXT4_SB(sb);
+	struct ext4_super_block *es = 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 == 0) {
+		sbi->s_want_extra_isize = 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 =
+					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 =
+					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 = 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;
-	/* 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 == 0) {
-		sbi->s_want_extra_isize = 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 =
-					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 =
-					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 = 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);
ext4_set_resv_clusters(sb);
@@ -5194,6 +5202,8 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
    	goto restore_opts;
    }
+	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


    

[PATCH] ext4: fix use-after-free race with debug_want_extra_isize