linux-ext4 - Re: Data exposure on IO error

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Date:   Sat, 1 Aug 2020 10:32:53 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Ext4 <linux-ext4@...r.kernel.org>, rebello.anthony@...il.com
Subject: Re: Data exposure on IO error

On Sat, Aug 1, 2020 at 1:59 AM Jan Kara <jack@...e.cz> wrote:
>
> Hello!
>
> In bug 207729, Anthony reported a bug that can actually lead to a stale
> data exposure on IO error. The problem is relatively simple: Suppose we
> do:
>
>   fd = open("file", O_WRONLY | O_CREAT | O_TRUNC, 0644);
>   write(fd, buf, 4096);
>   fsync(fd);
>
> And IO error happens when fsync writes the block of "file". The IO error
> gets properly reported to userspace but otherwise the filesystem keeps
> running. So the transaction creating "file" and allocating block to it can
> commit. Then when page cache of "file" gets evicted, the user can read
> stale block contents (provided the IO error was just temporary or involving
> only writes).
>
> Now I understand in face of IO errors the behavior is really undefined but
> potential exposure of stale data seems worse than strictly necessary. Also
> if we run in data=ordered mode, especially if also data_err=abort is set,
> user would rightfully expect that the filesystem gets aborted when such IO
> error happens but that's not the case. Generally data_err=abort seems a bit
> misnamed (and the manpage is wrong about this mount option) since what it
> really does is that if jbd2 thread encounters error when writing back
> ordered data, the filesystem is aborted. However the ordered data can be
> written back by other processes as well and in that case the error is just
> lost / reported to userspace but the filesystem doesn't get aborted.
>
> As I was thinking about it, it seems to me that in data=ordered mode, we
> should just always abort the filesystem when writeback of newly allocated
> block fails to avoid the stale data exposure mentioned above. And then, we
> could just deprecate data_err= mount option because it wouldn't be any
> useful anymore... What do people think?
>

It sounds worse than strictly necessary.

In what way is that use case different from writing into a punched hole
in the middle of the file and getting an IO error on writeback?

It looks like ext4 already goes into a great deal of trouble to handle
extent conversion to init at io end.

So couldn't the described case be handled as a private case of
filling a hole at the end of the file?

Am I missing something beyond the fact that traditionally, extending
a file enjoyed the protection of i_disksize, so did not need to worry
about unwritten extents?

Thanks,
Amir.