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

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Message-ID: <aReUv1kVACh3UKv-@casper.infradead.org>
Date: Fri, 14 Nov 2025 20:44:47 +0000
From: Matthew Wilcox <willy@...radead.org>
To: ssrane_b23@...vjti.ac.in
Cc: 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
Subject: Re: [PATCH] mm/filemap: fix NULL pointer dereference in
 do_read_cache_folio()

On Sat, Nov 15, 2025 at 01:07:29AM +0530, ssrane_b23@...vjti.ac.in wrote:
> When read_cache_folio() is called with a NULL filler function on a
> mapping that does not implement read_folio, a NULL pointer
> dereference occurs in filemap_read_folio().
> 
> The crash occurs when:
> 
> build_id_parse() is called on a VMA backed by a file from a
> filesystem that does not implement ->read_folio() (e.g. procfs,
> sysfs, or other virtual filesystems).

Not a fan of this approach, to be honest.  This should be caught at
a higher level.  In __build_id_parse(), there's already a check:

        /* only works for page backed storage  */
        if (!vma->vm_file)
                return -EINVAL;

which is funny because the comment is correct, but the code is not.
I suspect the right answer is to add right after it:

+	if (vma->vm_file->f_mapping->a_ops == &empty_aops)
+		return -EINVAL;

Want to test that out?