ext4: don't BUG if someone dirty pages without asking ext4 first

In the Linux kernel, the following vulnerability has been resolved:

ext4: don't BUG if someone dirty pages without asking ext4 first

[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/[email protected]

[2] https://lore.kernel.org/r/[email protected]