linux-kernel - Re: [PATCH] mm/filemap: fix NULL pointer dereference in do_read_cache

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Message-ID: <aRySpQbNuw3Y5DN-@casper.infradead.org>
Date: Tue, 18 Nov 2025 15:37:09 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Christoph Hellwig <hch@...radead.org>
Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>,
	"Darrick J. Wong" <djwong@...nel.org>,
	SHAURYA RANE <ssrane_b23@...vjti.ac.in>, akpm@...ux-foundation.org,
	shakeel.butt@...ux.dev, eddyz87@...il.com, andrii@...nel.org,
	ast@...nel.org, linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-kernel-mentees@...ts.linux.dev,
	skhan@...uxfoundation.org, david.hunter.linux@...il.com,
	khalid@...nel.org,
	syzbot+09b7d050e4806540153d@...kaller.appspotmail.com,
	bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH] mm/filemap: fix NULL pointer dereference in
 do_read_cache_folio()

On Tue, Nov 18, 2025 at 05:03:24AM -0800, Christoph Hellwig wrote:
> On Mon, Nov 17, 2025 at 10:45:31AM -0800, Andrii Nakryiko wrote:
> > As I replied on another email, ideally we'd have some low-level file
> > reading interface where we wouldn't have to know about secretmem, or
> > XFS+DAX, or whatever other unusual combination of conditions where
> > exposed internal APIs like filemap_get_folio() + read_cache_folio()
> > can crash.
> 
> The problem is that you did something totally insane and it kinda works
> most of the time.

... on 64-bit systems.  The HIGHMEM handling is screwed up too.

> But bpf or any other file system consumer has
> absolutely not business poking into the page cache to start with.

Agreed.

> And I'm really pissed off that you wrote and merged this code without
> ever bothering to talk to a FS or MM person who have immediately told
> you so.  Let's just rip out this buildid junk for now and restart
> because the problem isn't actually that easy.

Oh, they did talk to fs & mm people originally and were told NO, so they
sneaked it in through the BPF tree.

https://lore.kernel.org/all/20230316170149.4106586-1-jolsa@kernel.org/

> > The only real limitation is that we'd like to be able to control
> > whether we are ok sleeping or not, as this code can be called from
> > pretty much anywhere BPF might run, which includes NMI context.
> > 
> > Would this kiocb_read() approach work under those circumstances?
> 
> No.  IOCB_NOWAIT is just a hint to avoid blocking function calls.
> It is not guarantee and a guarantee is basically impossible.

I'm not sure I'd go that far -- I think we're pretty good about not
sleeping when IOCB_NOWAIT is specified and any remaining places can
be fixed up.

But I am inclined to rip out the buildid code, just because the
authors have been so rude.