linux-kernel - Re: [PATCH] comedi: Flush partial mappings in error case

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Message-ID: <CAG48ez2_86QrYnDe5xKxbngqF7OBQ3=HDF55bDP0hKvbasNCQQ@mail.gmail.com>
Date: Tue, 15 Oct 2024 20:26:58 +0200
From: Jann Horn <jannh@...gle.com>
To: Ian Abbott <abbotti@....co.uk>
Cc: H Hartley Sweeten <hsweeten@...ionengravers.com>, linux-kernel@...r.kernel.org, 
	stable@...r.kernel.org
Subject: Re: [PATCH] comedi: Flush partial mappings in error case

On Tue, Oct 15, 2024 at 12:19 PM Ian Abbott <abbotti@....co.uk> wrote:
> On 14/10/2024 21:50, Jann Horn wrote:
> > If some remap_pfn_range() calls succeeded before one failed, we still have
> > buffer pages mapped into the userspace page tables when we drop the buffer
> > reference with comedi_buf_map_put(bm). The userspace mappings are only
> > cleaned up later in the mmap error path.
> >
> > Fix it by explicitly flushing all mappings in our VMA on the comedi_mmap()
> > error path.
> >
> > See commit 79a61cc3fc04 ("mm: avoid leaving partial pfn mappings around in
> > error case").
> >
> > Cc: stable@...r.kernel.org
>
> Your patched version won't compile before 6.1 so you may want to
> indicate that in the Cc line.

Ah, thanks for pointing that out - I can just use zap_vma_ptes()
instead, which is available in older kernels, that way it will be more
concise and the backport will be easier.

> > Fixes: ed9eccbe8970 ("Staging: add comedi core")
> > Signed-off-by: Jann Horn <jannh@...gle.com>
> > ---
> > Note: compile-tested only; I don't actually have comedi hardware, and I
> > don't know anything about comedi.
> > ---
> >   drivers/comedi/comedi_fops.c | 9 +++++++++
> >   1 file changed, 9 insertions(+)
> >
> > diff --git a/drivers/comedi/comedi_fops.c b/drivers/comedi/comedi_fops.c
> > index 1b481731df96..0e573df8646f 100644
> > --- a/drivers/comedi/comedi_fops.c
> > +++ b/drivers/comedi/comedi_fops.c
> > @@ -2414,6 +2414,15 @@ static int comedi_mmap(struct file *file, struct vm_area_struct *vma)
> >               vma->vm_private_data = bm;
> >
> >               vma->vm_ops->open(vma);
> > +     } else {
> > +             /*
> > +              * Leaving behind a partial mapping of a buffer we're about to
> > +              * drop is unsafe, see remap_pfn_range_notrack().
> > +              * We need to zap the range here ourselves instead of relying
> > +              * on the automatic zapping in remap_pfn_range() because we call
> > +              * remap_pfn_range() in a loop.
> > +              */
> > +             zap_page_range_single(vma, vma->vm_start, size, NULL);
> >       }
> >
> >   done:
> >
> > ---
> > base-commit: 6485cf5ea253d40d507cd71253c9568c5470cd27
> > change-id: 20241014-comedi-tlb-400246505961
>
> I guess this doesn't need to be done for the path that calls
> dma_mmap_coherent() since that does not do any range splitting. Would it
> be better to move it into the branch that calls remap_pfn_range() in a loop?

Sure, I'll move it up into the branch.

> Note that I have no commit access to pulled-from repositories.  Greg-KH
> usually commits them on one of his repos, so could you Cc him too?  Thanks.

Ack, will do.