linux-ext4 - Re: current->journal_info got nested! (was Re: [syzbot] [xfs?] [ext4?] general protection fault in jbd2__journal

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Date: Tue, 30 Jan 2024 21:47:41 -0800
From: Christoph Hellwig <hch@...radead.org>
To: Matthew Wilcox <willy@...radead.org>
Cc: Theodore Ts'o <tytso@....edu>, Dave Chinner <david@...morbit.com>,
	syzbot <syzbot+cdee56dbcdf0096ef605@...kaller.appspotmail.com>,
	adilger.kernel@...ger.ca, chandan.babu@...cle.com, jack@...e.com,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
	syzkaller-bugs@...glegroups.com
Subject: Re: current->journal_info got nested! (was Re: [syzbot] [xfs?]
 [ext4?] general protection fault in jbd2__journal_start)

On Wed, Jan 31, 2024 at 05:20:10AM +0000, Matthew Wilcox wrote:
> 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).

Agreed.

> 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()?

That sounds useful, but also rather expensive.