linux-ext4 - [PATCH] ext2: Add more validity checks for inode counts

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Date:   Tue, 26 Jul 2022 13:23:23 +0200
From:   Jan Kara <jack@...e.cz>
To:     <linux-ext4@...r.kernel.org>
Cc:     Jan Kara <jack@...e.cz>,
        syzbot+d273f7d7f58afd93be48@...kaller.appspotmail.com
Subject: [PATCH] ext2: Add more validity checks for inode counts

Add checks verifying number of inodes stored in the superblock matches
the number computed from number of inodes per group. Also verify we have
at least one block worth of inodes per group. This prevents crashes on
corrupted filesystems.

Reported-by: syzbot+d273f7d7f58afd93be48@...kaller.appspotmail.com
Signed-off-by: Jan Kara <jack@...e.cz>
---
 fs/ext2/super.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

I plan to send this patch to Linus for the coming merge window. It mirrors
the checks already done in ext4.

diff --git a/fs/ext2/super.c b/fs/ext2/super.c
index f6a19f6d9f6d..cdffa2a041af 100644
--- a/fs/ext2/super.c
+++ b/fs/ext2/super.c
@@ -1059,9 +1059,10 @@ static int ext2_fill_super(struct super_block *sb, void *data, int silent)
 			sbi->s_frags_per_group);
 		goto failed_mount;
 	}
-	if (sbi->s_inodes_per_group > sb->s_blocksize * 8) {
+	if (sbi->s_inodes_per_group < sbi->s_inodes_per_block ||
+	    sbi->s_inodes_per_group > sb->s_blocksize * 8) {
 		ext2_msg(sb, KERN_ERR,
-			"error: #inodes per group too big: %lu",
+			"error: invalid #inodes per group: %lu",
 			sbi->s_inodes_per_group);
 		goto failed_mount;
 	}
@@ -1071,6 +1072,13 @@ static int ext2_fill_super(struct super_block *sb, void *data, int silent)
 	sbi->s_groups_count = ((le32_to_cpu(es->s_blocks_count) -
 				le32_to_cpu(es->s_first_data_block) - 1)
 					/ EXT2_BLOCKS_PER_GROUP(sb)) + 1;
+	if ((u64)sbi->s_groups_count * sbi->s_inodes_per_group !=
+	    le32_to_cpu(es->s_inodes_count)) {
+		ext2_msg(sb, KERN_ERR, "error: invalid #inodes: %u vs computed %llu",
+			 le32_to_cpu(es->s_inodes_count),
+			 (u64)sbi->s_groups_count * sbi->s_inodes_per_group);
+		goto failed_mount;
+	}
 	db_count = (sbi->s_groups_count + EXT2_DESC_PER_BLOCK(sb) - 1) /
 		   EXT2_DESC_PER_BLOCK(sb);
 	sbi->s_group_desc = kmalloc_array(db_count,
-- 
2.35.3