Re: [PATCH -v2] ext4: don't BUG if kernel subsystems dirty pages without asking ext4 first

[Eric Biggers]

On Fri, Feb 25, 2022 at 02:24:35PM -0500, Theodore Ts'o wrote:
> [un]pin_user_pages_remote is dirtying pages without properly warning
> the file system in advance (or faulting in the file data if the page
> is not yet in the page cache).  This was noted by Jan Kara in 2018[1]
> and more recently has resulted in bug reports by Syzbot in various
> Android kernels[2].
> 
> This is technically a bug in the mm/gup.c codepath, but arguably ext4
> is fragile in that a buggy get_user_pages() implementation causes ext4
> to crash, where as other file systems are not crashing (although in
> some cases the user data will be lost since gup code is not properly
> informing the file system to potentially allocate blocks or reserve
> space when writing into a sparse portion of file).  I suspect in real
> life it is rare that people are using RDMA into file-backed memory,
> which is why no one has complained to ext4 developers except fuzzing
> programs.
> 
> So instead of crashing with a BUG, issue a warning (since there may be
> potential data loss) and just mark the page as clean to avoid
> unprivileged denial of service attacks until the problem can be
> properly fixed.  More discussion and background can be found in the
> thread starting at [2].
> 
> [1] https://www.spinics.net/lists/linux-mm/msg142700.html

Can you use a lore link
(https://lore.kernel.org/linux-mm/20180103100430.GE4911@xxxxxxxxxxxxxx/T/#u)?

> +			/*
> +			 * Should never happen but for buggy code in
> +			 * other subsystemsa that call

subsystemsa => subsystems

> +			 * set_page_dirty() without properly warning
> +			 * the file system first.  See [1] for more
> +			 * information.
> +			 *
> +			 * [1] https://www.spinics.net/lists/linux-mm/msg142700.html

Likewise, lore link here.

- Eric