linux-kernel - Re: [PATCH] binderfs: Fix the maximum minor value in binderfs_binder_device_create() and binderfs_binder_ctl

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Message-ID: <5a5fec66-f8af-7e6e-5afe-97e4eb21c51a@wanadoo.fr>
Date:   Thu, 31 Mar 2022 22:35:37 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Christian Brauner <brauner@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Hridya Valsaraju <hridya@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        Todd Kjos <tkjos@...gle.com>
Subject: Re: [PATCH] binderfs: Fix the maximum minor value in
 binderfs_binder_device_create() and binderfs_binder_ctl_create()

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@...adoo.fr>
>> ---
>> 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