Re: current->journal_info got nested! (was Re: [syzbot] [xfs?] [ext4?] general protection fault in jbd2__journal_start)

[Matthew Wilcox]

On Tue, Jan 30, 2024 at 11:58:22PM -0500, Theodore Ts'o wrote:
> Hmm, could XFS pre-fault target memory buffer for the bulkstat output
> before starting its transaction?  Alternatively, ext4 could do a save
> of current->journal_info before starting to process the page fault,
> and restore it when it is done.  Both of these seem a bit hacky, and
> the question is indeed, are there other avenues that might cause the
> transaction context nesting, such that a more general solution is
> called for?

I'd suggest that saving off current->journal_info is risky because
it might cover a real problem where you've taken a pagefault inside
a transaction (eg ext4 faulting while in the middle of a transaction on
the same filesystem that contains the faulting file).

Seems to me that we shouldn't be writing to userspace while in the
middle of a transaction.  We could even assert that in copy_to_user()?