When mounting an ext4 filesystem with corrupted s_first_data_block, things
can go very wrong and oops.
Because blocks_count in ext4_fill_super is a u64, and we must use do_div,
the calculation of db_count is done differently than on ext4. If
first_data_block is corrupted such that it is larger than ext4_blocks_count,
for example, then the intermediate blocks_count value may go negative,
but sign-extend to a very large value:
blocks_count = (ext4_blocks_count(es) -
le32_to_cpu(es->s_first_data_block) +
EXT4_BLOCKS_PER_GROUP(sb) - 1);
This is then assigned to s_groups_count which is an unsigned long:
sbi->s_groups_count = blocks_count;
This may result in a value of 0xFFFFFFFF which is then used to compute
db_count:
db_count = (sbi->s_groups_count + EXT4_DESC_PER_BLOCK(sb) - 1) /
EXT4_DESC_PER_BLOCK(sb);
and in this case db_count will wind up as 0 because the addition overflows
32 bits. This in turn causes the kmalloc for group_desc to be of 0 size:
sbi->s_group_desc = kmalloc(db_count * sizeof (struct buffer_head *),
GFP_KERNEL);
and eventually in ext4_check_descriptors, dereferencing
sbi->s_group_desc[desc_block] will result in a NULL pointer dereference.
The simplest test seems to be to sanity check s_first_data_block,
EXT4_BLOCKS_PER_GROUP, and ext4_blocks_count values to be sure
their combination won't result in a bad intermediate value for
blocks_count. We could just check for db_count == 0, but
catching it at the root cause seems like it provides more info.
Signed-off-by: Eric Sandeen <[email protected]>
---
Index: linux-2.6.24-rc1/fs/ext4/super.c
===================================================================
--- linux-2.6.24-rc1.orig/fs/ext4/super.c
+++ linux-2.6.24-rc1/fs/ext4/super.c
@@ -2047,6 +2047,17 @@ static int ext4_fill_super (struct super
if (EXT4_BLOCKS_PER_GROUP(sb) == 0)
goto cantfind_ext4;
+
+ /* ensure blocks_count calculation below doesn't sign-extend */
+ if (ext4_blocks_count(es) + EXT4_BLOCKS_PER_GROUP(sb) <
+ le32_to_cpu(es->s_first_data_block) + 1) {
+ printk(KERN_WARNING "EXT4-fs: bad geometry: block count %llu, "
+ "first data block %u, blocks per group %lu\n",
+ ext4_blocks_count(es),
+ le32_to_cpu(es->s_first_data_block),
+ EXT4_BLOCKS_PER_GROUP(sb));
+ goto failed_mount;
+ }
blocks_count = (ext4_blocks_count(es) -
le32_to_cpu(es->s_first_data_block) +
EXT4_BLOCKS_PER_GROUP(sb) - 1);
Eric Sandeen wrote:
> When mounting an ext4 filesystem with corrupted s_first_data_block, things
> can go very wrong and oops.
>
> Because blocks_count in ext4_fill_super is a u64, and we must use do_div,
> the calculation of db_count is done differently than on ext4.
Urgh... "than on ext3"
-Eric