Re: [PATCH] binderfs: Fix the maximum minor value in binderfs_binder_device_create() and binderfs_binder_ctl_create()

[Christophe JAILLET]

Le 29/03/2022 à 13:20, Christian Brauner a écrit :
> On Sun, Mar 27, 2022 at 01:18:17PM +0200, Christophe JAILLET wrote:
> > ida_alloc_max(..., max, ...) returns values from 0 to max, inclusive.
> >
> > So, BINDERFS_MAX_MINOR is a valid value for 'minor'.
> >
> > BINDERFS_MAX_MINOR is '1U << MINORBITS' and we have:
> > 	#define MKDEV(ma,mi)	(((ma) << MINORBITS) | (mi))
> >
> > So, When this value is used in MKDEV() and it will overflow.
> >
> > Fixes: 3ad20fe393b3 ("binder: implement binderfs")
> > Signed-off-by: Christophe JAILLET <christophe.jaillet@xxxxxxxxxx>
> > ---
> > This patch is completely speculative.
> >
> > The 'BINDERFS_MAX_MINOR_CAPPED - 1' is here only for symmetry with the
> > BINDERFS_MAX_MINOR case. I'm not sure at all that is is needed and, more
> > importantly, that it is correct.
>
> Hm, since we're "removing" one alloctable device for the initial ipc
> namespace, I think we need the -1 for the capped value.
>
> Though I wonder if the simpler fix wouldn't just be to:
>
> #define BINDERFS_MAX_MINOR MINORMASK
>
> since include/linux/kdev_t.h has:
>
> #define MINORBITS	20
> #define MINORMASK	((1U << MINORBITS) - 1)

Hi,
I mostly agree with you, but I don't have a strong opinion on the other 
uses of BINDERFS_MAX_MINOR.

The ones related to 'max' values looks good to me, but I don't know the 
implication for the one used in binderfs_make_inode() and in 
init_binderfs().

I won't be able to help further here.

CJ