Re: [dm-devel] [PATCH v2 1/6] device-mapper: Check that target specs are sufficiently aligned

[Demi Marie Obenour <demi@invisiblethingslab.com>]

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On Thu, Jun 22, 2023 at 07:29:52PM +0200, Mikulas Patocka wrote:
> 
> 
> On Sat, 3 Jun 2023, Demi Marie Obenour wrote:
> 
> > Otherwise subsequent code will dereference a misaligned
> > `struct dm_target_spec *`, which is undefined behavior.
> > 
> > Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
> > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > Cc: stable@vger.kernel.org
> > ---
> >  drivers/md/dm-ioctl.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> > 
> > diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c
> > index cc77cf3d410921432eb0c62cdede7d55b9aa674a..34fa74c6a70db8aa67aaba3f6a2fc4f38ef736bc 100644
> > --- a/drivers/md/dm-ioctl.c
> > +++ b/drivers/md/dm-ioctl.c
> > @@ -1394,6 +1394,13 @@ static inline fmode_t get_mode(struct dm_ioctl *param)
> >  static int next_target(struct dm_target_spec *last, uint32_t next, void *end,
> >  		       struct dm_target_spec **spec, char **target_params)
> >  {
> > +	static_assert(_Alignof(struct dm_target_spec) <= 8,
> > +		      "struct dm_target_spec has excessive alignment requirements");
> > +	if (next % 8) {
> > +		DMERR("Next target spec (offset %u) is not 8-byte aligned", next);
> > +		return -EINVAL;
> > +	}
> > +
> >  	*spec = (struct dm_target_spec *) ((unsigned char *) last + next);
> >  	*target_params = (char *) (*spec + 1);
> >  
> > -- 
> > Sincerely,
> > Demi Marie Obenour (she/her/hers)
> > Invisible Things Lab
> 
> Hi
> 
> Some architectures (such as 32-bit x86) specify that the alignment of 
> 64-bit integers is only 4-byte. This could in theory break old userspace 
> code that only uses 4-byte alignment. I would change "next % 8" to "next % 
> __alignof__(struct dm_target_spec)".

That’s fine, provided that the rest of the code is okay with 4-byte
alignment.

> I think that there is no need to backport this patch series to the stable 
> kernels because the bugs that it fixes may only be exploited by the user 
> with CAP_SYS_ADMIN privilege. So, there is no security or reliability 
> problem being fixed.

I agree that there is no reliability problem, but with kernel lockdown
root → kernel is a security boundary, so fixes for memory unsafety
problems should still be backported IMO.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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