Re: [PATCH 2/2] ext4: add inode table check in __ext4_get_inode_loc to aovid possible infinite loop

Jan Kara
2022-08-18T17:24:13Z

On Thu 18-08-22 20:13:53, Ritesh Harjani (IBM) wrote:
> On 22/08/17 04:31PM, Jan Kara wrote:
> > On Wed 17-08-22 21:27:01, Baokun Li wrote:
> > > In do_writepages, if the value returned by ext4_writepages is "-ENOMEM"
> > > and "wbc->sync_mode == WB_SYNC_ALL", retry until the condition is not met.
> > >
> > > In __ext4_get_inode_loc, if the bh returned by sb_getblk is NULL,
> > > the function returns -ENOMEM.
> > >
> > > In __getblk_slow, if the return value of grow_buffers is less than 0,
> > > the function returns NULL.
> > >
> > > When the three processes are connected in series like the following stack,
> > > an infinite loop may occur:
> > >
> > > do_writepages <--- keep retrying
> > > ext4_writepages
> > > mpage_map_and_submit_extent
> > > mpage_map_one_extent
> > > ext4_map_blocks
> > > ext4_ext_map_blocks
> > > ext4_ext_handle_unwritten_extents
> > > ext4_ext_convert_to_initialized
> > > ext4_split_extent
> > > ext4_split_extent_at
> > > __ext4_ext_dirty
> > > __ext4_mark_inode_dirty
> > > ext4_reserve_inode_write
> > > ext4_get_inode_loc
> > > __ext4_get_inode_loc <--- return -ENOMEM
> > > sb_getblk
> > > __getblk_gfp
> > > __getblk_slow <--- return NULL
> > > grow_buffers
> > > grow_dev_page <--- return -ENXIO
> > > ret = (block < end_block) ? 1 : -ENXIO;
> > >
> > > In this issue, bg_inode_table_hi is overwritten as an incorrect value.
> > > As a result, `block < end_block` cannot be met in grow_dev_page.
> > > Therefore, __ext4_get_inode_loc always returns '-ENOMEM' and do_writepages
> > > keeps retrying. As a result, the writeback process is in the D state due
> > > to an infinite loop.
> > >
> > > Add a check on inode table block in the __ext4_get_inode_loc function by
> > > referring to ext4_read_inode_bitmap to avoid this infinite loop.
> > >
> > > Signed-off-by: Baokun Li <libaokun1@huawei.com>
> >
> > Thanks for the fixes. Normally, we check that inode table is fine in
> > ext4_check_descriptors() (and those checks are much stricter) so it seems
> > unnecessary to check it again here. I understand that in your case it was
> > resize that corrupted the group descriptor after the filesystem was mounted
> > which is nasty but there's much more metadata that can be corrupted like
> > this and it's infeasible to check each metadata block before we use it.
> >
> > IMHO a proper fix to this class of issues would be for sb_getblk() to
> > return proper error so that we can distinguish ENOMEM from other errors.
> > But that will be a larger undertaking...
> >
>
> Hi Jan,
>
> How about adding a wrapper around sb_getblk() which will do the basic block
> bound checks for ext4. Then we can carefully convert all the callers of
> sb_getblk() in ext4 to call ext4_sb_getblk().
>
> ext4_sb_getblk() will then return either of below -
> 1. ERR_PTR(-EFSCORRUPTED)
> 2. NULL
> 3. struct buffer_head*
>
> It's caller can then implement the proper error handling.
>
> Folding a small patch to implement the simple bound check. Is this the right
> approach?

Yep, looks sensible to me. Maybe I'd just make ext4_sb_getblk() return bh
or ERR_PTR so something like ERR_PTR(-EFSCORRUPTED), ERR_PTR(-ENXIO), or bh
pointer.

Honza

>
> From: "Ritesh Harjani (IBM)" <ritesh.list@gmail.com>
> Date: Thu, 18 Aug 2022 07:53:58 -0500
> Subject: [RFC] ext4: Add ext4_sb_getblk() wrapper for block bounds checking
>
> We might need more bounds checking on the block before calling sb_getblk().
> This helper does that and if it is not valid then returns ERR_PTR(-EFSCORRUPTED)
> Later we will need to carefully convert the callers to use ext4_sb_getblk()
> instead of sb_getblk().
> For e.g. __ext4_get_inode_loc()
>
> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
> ---
> fs/ext4/block_validity.c | 4 +---
> fs/ext4/ext4.h | 12 ++++++++++++
> fs/ext4/super.c | 9 +++++++++
> 3 files changed, 22 insertions(+), 3 deletions(-)
>
> diff --git a/fs/ext4/block_validity.c b/fs/ext4/block_validity.c
> index 5504f72bbbbe..69347fab7c38 100644
> --- a/fs/ext4/block_validity.c
> +++ b/fs/ext4/block_validity.c
> @@ -301,9 +301,7 @@ int ext4_sb_block_valid(struct super_block *sb, struct inode *inode,
> struct rb_node *n;
> int ret = 1;
>
> - if ((start_blk <= le32_to_cpu(sbi->s_es->s_first_data_block)) ||
> - (start_blk + count < start_blk) ||
> - (start_blk + count > ext4_blocks_count(sbi->s_es)))
> + if (!ext4_sb_block_valid_fastcheck(sbi->s_es, start_blk, count))
> return 0;
>
> /*
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 9bca5565547b..79d0e45185d3 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -3072,6 +3072,8 @@ extern struct buffer_head *ext4_sb_bread(struct super_block *sb,
> sector_t block, blk_opf_t op_flags);
> extern struct buffer_head *ext4_sb_bread_unmovable(struct super_block *sb,
> sector_t block);
> +extern struct buffer_head *ext4_sb_getblk(struct super_block *sb,
> + sector_t block);
> extern void ext4_read_bh_nowait(struct buffer_head *bh, blk_opf_t op_flags,
> bh_end_io_t *end_io);
> extern int ext4_read_bh(struct buffer_head *bh, blk_opf_t op_flags,
> @@ -3358,6 +3360,16 @@ static inline unsigned int ext4_flex_bg_size(struct ext4_sb_info *sbi)
> return 1 << sbi->s_log_groups_per_flex;
> }
>
> +static inline bool ext4_sb_block_valid_fastcheck(struct ext4_super_block *es,
> + sector_t start_blk, unsigned int count)
> +{
> + if ((start_blk <= le32_to_cpu(es->s_first_data_block)) ||
> + (start_blk + count < start_blk) ||
> + (start_blk + count > ext4_blocks_count(es)))
> + return false;
> + return true;
> +}
> +
> #define ext4_std_error(sb, errno) \
> do { \
> if ((errno)) \
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 9a66abcca1a8..5b29272ad9a9 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -269,6 +269,15 @@ void ext4_sb_breadahead_unmovable(struct super_block *sb, sector_t block)
> }
> }
>
> +struct buffer_head *ext4_sb_getblk(struct super_block *sb, sector_t block)
> +{
> + struct ext4_super_block *es = EXT4_SB(sb)->s_es;
> +
> + if (!ext4_sb_block_valid_fastcheck(es, block, 1))
> + return ERR_PTR(-EFSCORRUPTED);
> + return sb_getblk(sb, block);
> +}
> +
> static int ext4_verify_csum_type(struct super_block *sb,
> struct ext4_super_block *es)
> {
> --
> 2.25.1
>
> -ritesh
>
>
>
> > Honza
> >
> > > ---
> > > fs/ext4/inode.c | 10 +++++++++-
> > > 1 file changed, 9 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > > index 601214453c3a..5e171879fa23 100644
> > > --- a/fs/ext4/inode.c
> > > +++ b/fs/ext4/inode.c
> > > @@ -4466,9 +4466,17 @@ static int __ext4_get_inode_loc(struct super_block *sb, unsigned long ino,
> > > inodes_per_block = EXT4_SB(sb)->s_inodes_per_block;
> > > inode_offset = ((ino - 1) %
> > > EXT4_INODES_PER_GROUP(sb));
> > > - block = ext4_inode_table(sb, gdp) + (inode_offset / inodes_per_block);
> > > iloc->offset = (inode_offset % inodes_per_block) * EXT4_INODE_SIZE(sb);
> > >
> > > + block = ext4_inode_table(sb, gdp);
> > > + if ((block <= le32_to_cpu(EXT4_SB(sb)->s_es->s_first_data_block)) ||
> > > + (block >= ext4_blocks_count(EXT4_SB(sb)->s_es))) {
> > > + ext4_error(sb, "Invalid inode table block %llu in "
> > > + "block_group %u", block, iloc->block_group);
> > > + return -EFSCORRUPTED;
> > > + }
> > > + block += (inode_offset / inodes_per_block);
> > > +
> > > bh = sb_getblk(sb, block);
> > > if (unlikely(!bh))
> > > return -ENOMEM;
> > > --
> > > 2.31.1
> > >
> > --
> > Jan Kara <jack@suse.com>
> > SUSE Labs, CR
--
Jan Kara <jack@suse.com>
SUSE Labs, CR