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Date:   Wed, 23 Feb 2022 22:50:09 -0500
From:   "Theodore Ts'o" <tytso@mit.edu>
To:     Dave Chinner <david@fromorbit.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        John Hubbard <jhubbard@nvidia.com>,
        Lee Jones <lee.jones@linaro.org>, linux-ext4@vger.kernel.org,
        Christoph Hellwig <hch@lst.de>,
        Dave Chinner <dchinner@redhat.com>,
        Goldwyn Rodrigues <rgoldwyn@suse.com>,
        "Darrick J . Wong" <darrick.wong@oracle.com>,
        Bob Peterson <rpeterso@redhat.com>,
        Damien Le Moal <damien.lemoal@wdc.com>,
        Andreas Gruenbacher <agruenba@redhat.com>,
        Ritesh Harjani <riteshh@linux.ibm.com>,
        Johannes Thumshirn <jth@kernel.org>, linux-xfs@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, cluster-devel@redhat.com,
        linux-kernel@vger.kernel.org
Subject: Re: [REPORT] kernel BUG at fs/ext4/inode.c:2620 - page_buffers()
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On Thu, Feb 24, 2022 at 12:48:42PM +1100, Dave Chinner wrote:
> > Fair enough; on the other hand, we could also view this as making ext4
> > more robust against buggy code in other subsystems, and while other
> > file systems may be losing user data if they are actually trying to do
> > remote memory access to file-backed memory, apparently other file
> > systems aren't noticing and so they're not crashing.
> 
> Oh, we've noticed them, no question about that.  We've got bug
> reports going back years for systems being crashed, triggering BUGs
> and/or corrupting data on both XFS and ext4 filesystems due to users
> trying to run RDMA applications with file backed pages.

Is this issue causing XFS to crash?  I didn't know that.

I tried the Syzbot reproducer with XFS mounted, and it didn't trigger
any crashes.  I'm sure data was getting corrupted, but I figured I
should bring ext4 to the XFS level of "at least we're not reliably
killing the kernel".

On ext4, an unprivileged process can use process_vm_writev(2) to crash
the system.  I don't know how quickly we can get a fix into mm/gup.c,
but if some other kernel path tries calling set_page_dirty() on a
file-backed page without first asking permission from the file system,
it seems to be nice if the file system doesn't BUG() --- as near as I
can tell, xfs isn't crashing in this case, but ext4 is.

					- Ted