Re: [PATCH] platform/x86: thinkpad_acpi: Prefer struct_size over open coded arithmetic

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 9/20/21 7:58 AM, Kees Cook wrote:
> On Sat, Sep 18, 2021 at 05:05:00PM +0200, Len Baker wrote:
>> As noted in the "Deprecated Interfaces, Language Features, Attributes,
>> and Conventions" documentation [1], size calculations (especially
>> multiplication) should not be performed in memory allocator (or similar)
>> function arguments due to the risk of them overflowing. This could lead
>> to values wrapping around and a smaller allocation being made than the
>> caller was expecting. Using those allocations could lead to linear
>> overflows of heap memory and other misbehaviors.
>>
>> So, switch to flexible array member in the struct attribute_set_obj and
>> refactor the code accordingly to use the struct_size() helper instead of
>> the argument "size + count * size" in the kzalloc() function.
>>
>> [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments
>>
>> Signed-off-by: Len Baker <len.baker@gmx.com>
>> ---
>>  drivers/platform/x86/thinkpad_acpi.c | 8 +++-----
>>  1 file changed, 3 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/platform/x86/thinkpad_acpi.c b/drivers/platform/x86/thinkpad_acpi.c
>> index 50ff04c84650..ed0b01ead796 100644
>> --- a/drivers/platform/x86/thinkpad_acpi.c
>> +++ b/drivers/platform/x86/thinkpad_acpi.c
>> @@ -1008,7 +1008,7 @@ struct attribute_set {
>>
>>  struct attribute_set_obj {
>>  	struct attribute_set s;
>> -	struct attribute *a;
>> +	struct attribute *a[];
>>  } __attribute__((packed));
> 
> Whoa. I have so many questions... :)
> 
>>
>>  static struct attribute_set *create_attr_set(unsigned int max_members,
>> @@ -1020,13 +1020,11 @@ static struct attribute_set *create_attr_set(unsigned int max_members,
>>  		return NULL;
>>
>>  	/* Allocates space for implicit NULL at the end too */
>> -	sobj = kzalloc(sizeof(struct attribute_set_obj) +
>> -		    max_members * sizeof(struct attribute *),
>> -		    GFP_KERNEL);
>> +	sobj = kzalloc(struct_size(sobj, a, max_members + 1), GFP_KERNEL);
> 
> Whoa, this needs a lot more detail in the changelog if this is actually
> correct. The original code doesn't seem to match the comment? (Where is
> the +1?) So is this also a bug-fix?

Kees, at first I thought you were spot-on with this comment, but the
truth is more subtle. struct attribute_set_obj was:

struct attribute_set_obj {
        struct attribute_set s;
        struct attribute *a;
} __attribute__((packed));

Another way of looking at this, which makes things more clear is as:

struct attribute_set_obj {
        struct attribute_set s;
        struct attribute *a[1];
} __attribute__((packed));

So the sizeof(struct attribute_set_obj) in the original kzalloc call
included room for 1 "extra" pointer which is reserved for the terminating
NULL pointer.

Changing the struct to:

struct attribute_set_obj {
        struct attribute_set s;
        struct attribute *a[];
} __attribute__((packed));

Is equivalent to changing it to:

struct attribute_set_obj {
        struct attribute_set s;
        struct attribute *a[0];
} __attribute__((packed));

So the change in the struct declaration reduces the sizeof(struct attribute_set_obj)
by the size of 1 pointer, making the +1 necessary.

So AFAICT there is actually no functional change here.

Still I will hold off merging this until we agree on this :)

> (I see the caller uses +2? Why? It seems to be using each of hotkey_attributes,
> plus 1 more attr, plus a final NULL?)

The +2 is actually for 2 extra attributes (making the total number
of extra attributes +3 because the sizeof(struct attribute_set_obj)
already includes 1 extra). 

FWIW these 2 extra attributes are for devices with a
a physical rfkill on/off switch and for the device being
a convertible capable of reporting laptop- vs tablet-mode.

>>  	if (!sobj)
>>  		return NULL;
>>  	sobj->s.max_members = max_members;
>> -	sobj->s.group.attrs = &sobj->a;
>> +	sobj->s.group.attrs = sobj->a;
>>  	sobj->s.group.name = name;
> 
> The caller also never sets a name?

attribute_group.name may be NULL, I don't know
of (m)any drivers which actual set this to non NULL.

> Why is struct attribute_set_obj marked as __packed?

I have no clue, this seems completely unnecessary.

Len Baker can you submit a separate patch removing the useless
__packed ?

Regards,

Hans