Re: [PATCH v3 1/1] ext4: Mark buffer new if it is unwritten to avoid stale data exposure

Ritesh Harjani
2023-09-19T08:16:37Z

Ojaswin Mujoo <ojaswin@linux.ibm.com> writes:

> ** Short Version **
>
> In ext4 with dioread_nolock, we could have a scenario where the bh returned by
> get_blocks (ext4_get_block_unwritten()) in __block_write_begin_int() has
> UNWRITTEN and MAPPED flag set. Since such a bh does not have NEW flag set we
> never zero out the range of bh that is not under write, causing whatever stale
> data is present in the folio at that time to be written out to disk. To fix this
> mark the buffer as new, in case it is unwritten, in ext4_get_block_unwritten().
>
> ** Long Version **
>
> The issue mentioned above was resulting in two different bugs:
>
> 1. On block size < page size case in ext4, generic/269 was reliably
> failing with dioread_nolock. The state of the write was as follows:
>
> * The write was extending i_size.
> * The last block of the file was fallocated and had an unwritten extent
> * We were near ENOSPC and hence we were switching to non-delayed alloc
> allocation.
>
> In this case, the back trace that triggers the bug is as follows:
>
> ext4_da_write_begin()
> /* switch to nodelalloc due to low space */
> ext4_write_begin()
> ext4_should_dioread_nolock() // true since mount flags still have delalloc
> __block_write_begin(..., ext4_get_block_unwritten)
> __block_write_begin_int()
> for(each buffer head in page) {
> /* first iteration, this is bh1 which contains i_size */
> if (!buffer_mapped)
> get_block() /* returns bh with only UNWRITTEN and MAPPED */
> /* second iteration, bh2 */
> if (!buffer_mapped)
> get_block() /* we fail here, could be ENOSPC */
> }
> if (err)
> /*
> * this would zero out all new buffers and mark them uptodate.
> * Since bh1 was never marked new, we skip it here which causes
> * the bug later.
> */
> folio_zero_new_buffers();
> /* ext4_wrte_begin() error handling */
> ext4_truncate_failed_write()
> ext4_truncate()
> ext4_block_truncate_page()
> __ext4_block_zero_page_range()
> if(!buffer_uptodate())
> ext4_read_bh_lock()
> ext4_read_bh() -> ... ext4_submit_bh_wbc()
> BUG_ON(buffer_unwritten(bh)); /* !!! */
>
> 2. The second issue is stale data exposure with page size >= blocksize
> with dioread_nolock. The conditions needed for it to happen are same as
> the previous issue ie dioread_nolock around ENOSPC condition. The issue
> is also similar where in __block_write_begin_int() when we call
> ext4_get_block_unwritten() on the buffer_head and the underlying extent
> is unwritten, we get an unwritten and mapped buffer head. Since it is
> not new, we never zero out the partial range which is not under write,
> thus writing stale data to disk. This can be easily observed with the
> following reproducer:
>
> fallocate -l 4k testfile
> xfs_io -c "pwrite 2k 2k" testfile
> # hexdump output will have stale data in from byte 0 to 2k in testfile
> hexdump -C testfile
>
> NOTE: To trigger this, we need dioread_nolock enabled and write happening via
> ext4_write_begin(), which is usually used when we have -o nodealloc. Since
> dioread_nolock is disabled with nodelalloc, the only alternate way to call
> ext4_write_begin() is to ensure that delayed alloc switches to nodelalloc ie
> ext4_da_write_begin() calls ext4_write_begin(). This will usually happen when
> ext4 is almost full like the way generic/269 was triggering it in Issue 1 above.
> This might make the issue harder to hit. Hence, for reliable replication, I used
> the below patch to temporarily allow dioread_nolock with nodelalloc and then
> mount the disk with -o nodealloc,dioread_nolock. With this you can hit the stale
> data issue 100% of times:
>
> @@ -508,8 +508,8 @@ static inline int ext4_should_dioread_nolock(struct inode *inode)
> if (ext4_should_journal_data(inode))
> return 0;
> /* temporary fix to prevent generic/422 test failures */
> - if (!test_opt(inode->i_sb, DELALLOC))
> - return 0;
> + // if (!test_opt(inode->i_sb, DELALLOC))
> + // return 0;
> return 1;
> }
>
> After applying this patch to mark buffer as NEW, both the above issues are
> fixed.
>
> Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
> Reviewed-by: Jan Kara <jack@suse.cz>
> ---
> fs/ext4/inode.c | 14 +++++++++++++-
> 1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 6c490f05e2ba..8b286a800193 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -789,10 +789,22 @@ int ext4_get_block(struct inode *inode, sector_t iblock,
> int ext4_get_block_unwritten(struct inode *inode, sector_t iblock,
> struct buffer_head *bh_result, int create)
> {
> + int ret = 0;
> +
> ext4_debug("ext4_get_block_unwritten: inode %lu, create flag %d\n",
> inode->i_ino, create);
> - return _ext4_get_block(inode, iblock, bh_result,
> + ret = _ext4_get_block(inode, iblock, bh_result,
> EXT4_GET_BLOCKS_CREATE_UNWRIT_EXT);
> +
> + /*
> + * If the buffer is marked unwritten, mark it as new to make sure it is
> + * zeroed out correctly in case of partial writes. Otherwise, there is
> + * a chance of stale data getting exposed.
> + */
> + if (ret == 0 && buffer_unwritten(bh_result))
> + set_buffer_new(bh_result);
> +
> + return ret;
> }

Doing above was my first thought too when we were discussing
the issue internally. But I am glad the discussions happened with Jan to
understand the right reason behing MAP_NEW flag.

While at it we also noticed that the ext4_map_blocks() returns different
map->m_flags set for the same extent type when passed with different flags
in it's argument :). So it quickly become confusing. Thanks for sorting
out the unknowns.

This patch looks good to me too! Feel free to add -

Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>