On Fri, Jul 01, 2016 at 03:06:41PM +0200, Vegard Nossum wrote:
> Hi,
>
> I've found that sbi->s_es->s_reserved_gdt_blocks is not validated before
> being used, so e.g. a value of 25600 will overflow the buffer head and
> corrupt random kernel memory (I've observed 20+ different stack traces
> due to this bug! many of them long after the code has finished).
This function merely initializes a bitmap block once ext4 decides to start
using the block group, which could be a long time after mount...
> The following patch fixes it for me:
>
> diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
> index 3020fd7..1ea5054 100644
> --- a/fs/ext4/balloc.c
> +++ b/fs/ext4/balloc.c
> @@ -208,6 +208,9 @@ static int ext4_init_block_bitmap(struct super_block
> *sb,
> memset(bh->b_data, 0, sb->s_blocksize);
>
> bit_max = ext4_num_base_meta_clusters(sb, block_group);
> + if ((bit_max >> 3) >= bh->b_size)
> + return -EFSCORRUPTED;
> +
> for (bit = 0; bit < bit_max; bit++)
> ext4_set_bit(bit, bh->b_data);
>
> However, I think there are potentially more bugs later in this function
> where offsets are not validated so it needs to be reviewed carefully.
>
> Another question is whether we should do the validation earlier, e.g. in
> ext4_fill_super(). I'm not too familiar with the code, but maybe
> something like the attached patch would be better? It does seem to fix
> the issue as well.
...whereas superblock fields such as s_reserved_gdt_blocks ought to be
checked (and -EFSCORRUPTED'ed) at mount time. Since we know that BG 0
has everything (superblock, old school gdt tables, inodes), maybe we
should make mount check that ext4_num_base_meta_clusters() >
NBBY * fs->block_size?
(Kinda surprised ext4 doesn't already do this...)
--D
>
>
> Vegard
> From efcee80eb78816a4d495224ffc624adf04217044 Mon Sep 17 00:00:00 2001
> From: Vegard Nossum <vegard.nossum@xxxxxxxxxx>
> Date: Fri, 1 Jul 2016 15:03:39 +0200
> Subject: [PATCH] ext4: validate number of base meta clusters in group
>
> ---
> fs/ext4/balloc.c | 4 +---
> fs/ext4/ext4.h | 2 ++
> fs/ext4/super.c | 10 ++++++++++
> 3 files changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
> index 3020fd7..ec03f01 100644
> --- a/fs/ext4/balloc.c
> +++ b/fs/ext4/balloc.c
> @@ -22,8 +22,6 @@
>
> #include <trace/events/ext4.h>
>
> -static unsigned ext4_num_base_meta_clusters(struct super_block *sb,
> - ext4_group_t block_group);
> /*
> * balloc.c contains the blocks allocation and deallocation routines
> */
> @@ -817,7 +815,7 @@ unsigned long ext4_bg_num_gdb(struct super_block *sb, ext4_group_t group)
> * This function returns the number of file system metadata clusters at
> * the beginning of a block group, including the reserved gdt blocks.
> */
> -static unsigned ext4_num_base_meta_clusters(struct super_block *sb,
> +unsigned ext4_num_base_meta_clusters(struct super_block *sb,
> ext4_group_t block_group)
> {
> struct ext4_sb_info *sbi = EXT4_SB(sb);
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index b84aa1ca..e492b0b 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -2270,6 +2270,8 @@ extern ext4_grpblk_t ext4_block_group_offset(struct super_block *sb,
> extern int ext4_bg_has_super(struct super_block *sb, ext4_group_t group);
> extern unsigned long ext4_bg_num_gdb(struct super_block *sb,
> ext4_group_t group);
> +extern unsigned ext4_num_base_meta_clusters(struct super_block *sb,
> + ext4_group_t block_group);
> extern ext4_fsblk_t ext4_new_meta_blocks(handle_t *handle, struct inode *inode,
> ext4_fsblk_t goal,
> unsigned int flags,
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 563555e..b5a9d28 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -3641,6 +3641,16 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
> goto failed_mount;
> }
> sbi->s_groups_count = blocks_count;
> +
> + for (i = 0; i < sbi->s_groups_count; ++i) {
> + int bit_max = ext4_num_base_meta_clusters(sb, i);
> + if ((bit_max >> 3) >= sb->s_blocksize) {
> + ext4_msg(sb, KERN_WARNING, "meta cluster base for "
> + "group %u exceeds block size", i);
> + goto failed_mount;
> + }
> + }
> +
> sbi->s_blockfile_groups = min_t(ext4_group_t, sbi->s_groups_count,
> (EXT4_MAX_BLOCK_FILE_PHYS / EXT4_BLOCKS_PER_GROUP(sb)));
> db_count = (sbi->s_groups_count + EXT4_DESC_PER_BLOCK(sb) - 1) /
> --
> 1.9.1
>
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