Re: [PATCH -v4] ext4: don't BUG if kernel subsystems dirty pages without asking ext4 first

[Lee Jones]

On Wed, 02 Mar 2022, Theodore Ts'o wrote:

> [un]pin_user_pages_remote is dirtying pages without properly warning
> the file system in advance.  A related race was noted by Jan Kara in
> 2018[1]; however, more recently instead of it being a very hard-to-hit
> race, it could be reliably triggered by process_vm_writev(2) which was
> discovered by Syzbot[2].
> 
> This is technically a bug in mm/gup.c, but arguably ext4 is fragile in
> that if some other kernel subsystem dirty pages without properly
> notifying the file system using page_mkwrite(), ext4 will BUG, while
> other file systems will not BUG (although data will still be lost).
> 
> 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://lore.kernel.org/linux-mm/20180103100430.GE4911@xxxxxxxxxxxxxx
> [2] https://lore.kernel.org/r/Yg0m6IjcNmfaSokM@xxxxxxxxxx
> 
> Reported-by: syzbot+d59332e2db681cf18f0318a06e994ebbb529a8db@xxxxxxxxxxxxxxxxxxxxxxxxx
> Reported-by: Lee Jones <lee.jones@xxxxxxxxxx>
> Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
> Cc: stable@xxxxxxxxxx
> ---
> v4 - only changes to the commit description to eliminate some inaccuracies
>      and clarify the text.
> 
>  fs/ext4/inode.c | 27 ++++++++++++++++++++++++++-
>  1 file changed, 26 insertions(+), 1 deletion(-)

Thanks a bunch for sticking with this Ted.

I've been following along with great interest.

Sadly I am not in a position to provide a review.

Just wanted to pop by and say thank you.

-- 
Lee Jones [李琼斯]
Principal Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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