linux-kernel - Re: [PATCH v3 3/6] sysctl: Add flags to support min/max range clamping

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Date:   Thu, 8 Mar 2018 14:35:32 -0500
From:   Waiman Long <longman@...hat.com>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v3 3/6] sysctl: Add flags to support min/max range
 clamping

On 03/08/2018 12:57 PM, Luis R. Rodriguez wrote:
> On Thu, Mar 08, 2018 at 05:51:09PM +0000, Luis R. Rodriguez wrote:
>> On Thu, Mar 01, 2018 at 01:31:17PM -0800, Andrew Morton wrote:
>>> On Thu,  1 Mar 2018 12:43:37 -0500 Waiman Long <longman@...hat.com> wrote:
>>>
>>>> When minimum/maximum values are specified for a sysctl parameter in
>>>> the ctl_table structure with proc_dointvec_minmax() handler, update
>>>> to that parameter will fail with error if the given value is outside
>>>> of the required range.
>>>>
>>>> There are use cases where it may be better to clamp the value of
>>>> the sysctl parameter to the given range without failing the update,
>>>> especially if the users are not aware of the actual range limits.
>>>> Reading the value back after the update will now be a good practice
>>>> to see if the provided value exceeds the range limits.
>>>>
>>>> To provide this less restrictive form of range checking, a new flags
>>>> field is added to the ctl_table structure. The new field is a 16-bit
>>>> value that just fits into the hole left by the 16-bit umode_t field
>>>> without increasing the size of the structure.
>>>>
>>>> When the CTL_FLAGS_CLAMP_RANGE flag is set in the ctl_table entry,
>>>> any update from the userspace will be clamped to the given range
>>>> without error.
>>>>
>>>> ...
>>>>
>>>> --- a/include/linux/sysctl.h
>>>> +++ b/include/linux/sysctl.h
>>>> @@ -116,6 +116,7 @@ struct ctl_table
>>>>  	void *data;
>>>>  	int maxlen;
>>>>  	umode_t mode;
>>>> +	uint16_t flags;
>>> It would be nice to make this have type `enum ctl_table_flags', but I
>>> guess there's then no reliable way of forcing it to be 16-bit.
>>>
>>> I guess this is the best we can do...
>>>
>> We can add this to the enum:
>>
>> enum ctl_table_flags {                                                                                                                                                                       
>>        CTL_FLAGS_CLAMP_RANGE           = BIT(0),                                                                                                                                             
>> +	__CTL_FLAGS_CLAMP_MAX          = BIT(16),
>> }; 
>>
>>
>> Then also:
>>
>> #define CTL_TABLE_FLAGS_ALL	((BIT(__CTL_FLAGS_CLAMP_MAX + 1))-1)
>>
>> at the end of the definition, then a helper which can be used during
>> parsing:
>>
>> static int check_ctl_table_flags(u16 flags)
>> {
>> 	if (flags & ~(CTL_TABLE_FLAGS_ALL))
>> 		return -ERANGE;
>> 	return 0;
>> }
>>
>> Waiman please evaluate and add.
> Also, I guess we have ... max bit used and max allowed (16) really, where one is the
> max allowed bit field given current definitions, the other is the max flag possible
> setting in the future. We might as well go with the smaller one, which is the current
> max, so it can just be
>
> enum ctl_table_flags {
> 	CTL_FLAGS_CLAMP_RANGE	 = BIT(0),
> 	__CTL_FLAGS_CLAMP_MAX    = BIT(1),
> };
>
>
> #define CTL_TABLE_FLAGS_ALL	((BIT(__CTL_FLAGS_CLAMP_MAX))-1)
>
> That way we just check against the actual max defined, now the max allowed on
> the entire flag setting.
>
>   Luis

Yes, I can certainly add check to see if the flags are out of range.
However, I would like to know your opinion of what to do when an invalid
flag bit is set. Do we just print a warning in the ring buffer or fail
the registration of the ctl table?

Cheers,
Longman