linux-kernel - Re: [PATCH] iio: accel: fxls8962af: Fix use after free in fxls8962af_fifo

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Message-ID: <x6lmsxsz6njt22z23l3nbetlstkwn4jk5ohgtpyd23idwleeg5@szatvfu4drjj>
Date: Mon, 02 Jun 2025 10:50:14 +0000
From: Sean Nyekjaer <sean@...nix.com>
To: Jonathan Cameron <jic23@...nel.org>
Cc: David Lechner <dlechner@...libre.com>, Nuno Sá <nuno.sa@...log.com>, Andy Shevchenko <andy@...nel.org>, Jonathan Cameron <Jonathan.Cameron@...wei.com>, linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] iio: accel: fxls8962af: Fix use after free in fxls8962af_fifo_flush

Hi Jonathan,

On Sat, May 31, 2025 at 05:53:02PM +0100, Jonathan Cameron wrote:
> On Sat, 24 May 2025 12:34:09 +0200
> Sean Nyekjaer <sean@...nix.com> wrote:
> 
> > fxls8962af_fifo_flush() uses indio_dev->active_scan_mask (with
> > iio_for_each_active_channel()) without making sure the indio_dev
> > stays in buffer mode.
> > There is a race if indio_dev exits buffer mode in the middle of the
> > interrupt that flushes the fifo. Fix this by calling
> > iio_device_claim_buffer_mode() to ensure indio_dev can't exit buffer
> > mode during the flush.
> >
> > Unable to handle kernel NULL pointer dereference at virtual address 00000000 when read
> > [...]
> > _find_first_bit_le from fxls8962af_fifo_flush+0x17c/0x290
> > fxls8962af_fifo_flush from fxls8962af_interrupt+0x80/0x178
> > fxls8962af_interrupt from irq_thread_fn+0x1c/0x7c
> > irq_thread_fn from irq_thread+0x110/0x1f4
> > irq_thread from kthread+0xe0/0xfc
> > kthread from ret_from_fork+0x14/0x2c
> >
> > Fixes: 79e3a5bdd9ef ("iio: accel: fxls8962af: add hw buffered sampling")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Sean Nyekjaer <sean@...nix.com>
> 
> That's nasty and a case I'd never thought about.  Most of the
> races around disabling end up with an extra sample or two which then gets
> dropped because there are no buffers enabled.
> 
> We need to consider the active scan mask as part of the buffer state.
> So effectively taking mlock if we enter this code will delay the state
> transition (and change of active_scan_mask until after this interrupt is done).
> 
> If David's synchronize_irq() is enough maybe that's a lighter weight path?

I agree if David's proposal is sufficient, I can try it.
It's something we have seen once in some unrelated testing, so it's
quite hard to reproduce :/

/Sean