On Mon, Sep 18, 2017 at 8:11 PM, Darrick J. Wong
<darrick.wong@xxxxxxxxxx> wrote:
> On Fri, Sep 15, 2017 at 03:40:24PM +0300, Amir Goldstein wrote:
>> On Wed, Aug 30, 2017 at 4:38 PM, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
>> > When calling into _xfs_log_force{,_lsn}() with a pointer
>> > to log_flushed variable, log_flushed will be set to 1 if:
>> > 1. xlog_sync() is called to flush the active log buffer
>> > AND/OR
>> > 2. xlog_wait() is called to wait on a syncing log buffers
>> >
>> > xfs_file_fsync() checks the value of log_flushed after
>> > _xfs_log_force_lsn() call to optimize away an explicit
>> > PREFLUSH request to the data block device after writing
>> > out all the file's pages to disk.
>> >
>> > This optimization is incorrect in the following sequence of events:
>> >
>> > Task A Task B
>> > -------------------------------------------------------
>> > xfs_file_fsync()
>> > _xfs_log_force_lsn()
>> > xlog_sync()
>> > [submit PREFLUSH]
>> > xfs_file_fsync()
>> > file_write_and_wait_range()
>> > [submit WRITE X]
>> > [endio WRITE X]
>> > _xfs_log_force_lsn()
>> > xlog_wait()
>> > [endio PREFLUSH]
>> >
>> > The write X is not guarantied to be on persistent storage
>> > when PREFLUSH request in completed, because write A was submitted
>> > after the PREFLUSH request, but xfs_file_fsync() of task A will
>> > be notified of log_flushed=1 and will skip explicit flush.
>> >
>> > If the system crashes after fsync of task A, write X may not be
>> > present on disk after reboot.
>> >
>> > This bug was discovered and demonstrated using Josef Bacik's
>> > dm-log-writes target, which can be used to record block io operations
>> > and then replay a subset of these operations onto the target device.
>> > The test goes something like this:
>> > - Use fsx to execute ops of a file and record ops on log device
>> > - Every now and then fsync the file, store md5 of file and mark
>> > the location in the log
>> > - Then replay log onto device for each mark, mount fs and compare
>> > md5 of file to stored value
>> >
>> > Cc: Christoph Hellwig <hch@xxxxxx>
>> > Cc: Josef Bacik <jbacik@xxxxxx>
>> > Cc: <stable@xxxxxxxxxxxxxxx>
>> > Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
>> > ---
>> >
>> > Christoph, Dave,
>> >
>> > It's hard to believe, but I think the reported bug has been around
>> > since 2005 f538d4da8d52 ("[XFS] write barrier support"), but I did
>> > not try to test old kernels.
>>
>> Forgot to tag commit message with:
>> Fixes: f538d4da8d52 ("[XFS] write barrier support")
>>
>> Maybe the tag could be added when applying to recent stables,
>> so distros and older downstream stables can see the tag.
>>
>> The disclosure of the security bug fix (commit b31ff3cdf5) made me wonder
>> if possible data loss bug should also be disclosed in some distros forum?
>> I bet some users would care more about the latter than the former.
>> Coincidentally, both data loss and security bugs fix the same commit..
>
> Yes the the patch ought to get sent on to stable w/ fixes tag. One
> would hope that the distros will pick up the stable fixes from there.
Greg, for your consideration, please add
Fixes: f538d4da8d52 ("[XFS] write barrier support")
If not pushed yet.
> That said, it's been in the kernel for 12 years without widespread
> complaints about corruption, so I'm not sure this warrants public
> disclosure via CVE/Phoronix vs. just fixing it.
>
I'm not sure either.
My intuition tells me that the chances of hitting the data loss bug
given a power failure are not slim, but the chances of users knowing
about the data loss are slim.
Meaning, the chances of users complaining:
"I swear, I did fsync, lost power, and after boot data was not there" are slim
and the chances of developers believing the report on hear say are
even slimmer.
Oh well. I was just wondering...
Amir.
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