Date:   Mon, 22 Aug 2022 16:05:49 +0200
From:   Andrzej Hajda <andrzej.hajda@...el.com>
To:     Kees Cook <keescook@...omium.org>,
        Andi Shyti <andi.shyti@...ux.intel.com>
Cc:     thomas.hellstrom@...ux.intel.com,
        Gwan-gyeong Mun <gwan-gyeong.mun@...el.com>,
        jani.nikula@...el.com, intel-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        chris@...is-wilson.co.uk, airlied@...ux.ie,
        linux-hardening@...r.kernel.org, matthew.auld@...el.com,
        mchehab@...nel.org, nirmoy.das@...el.com
Subject: Re: [PATCH v7 1/8] overflow: Move and add few utility macros into
 overflow

On 18.08.2022 02:12, Kees Cook wrote:
> On Thu, Aug 18, 2022 at 01:07:29AM +0200, Andi Shyti wrote:
>> Hi Kees,
>>
>> would you mind taking a look at this patch?
> 
> Hi! Thanks for the heads-up!
> 
>>
>> Thanks,
>> Andi
>>
>> On Tue, Aug 16, 2022 at 06:35:18PM +0900, Gwan-gyeong Mun wrote:
>>> It moves overflows_type utility macro into overflow header from i915_utils
>>> header. The overflows_type can be used to catch the truncation between data
>>> types. And it adds safe_conversion() macro which performs a type conversion
>>> (cast) of an source value into a new variable, checking that the
>>> destination is large enough to hold the source value. And the functionality
>>> of overflows_type has been improved to handle the signbit.
>>> The is_unsigned_type macro has been added to check the sign bit of the
>>> built-in type.
>>>
>>> v3: Add is_type_unsigned() macro (Mauro)
>>>      Modify overflows_type() macro to consider signed data types (Mauro)
>>>      Fix the problem that safe_conversion() macro always returns true
>>> v4: Fix kernel-doc markups
>>> v6: Move macro addition location so that it can be used by other than drm
>>>      subsystem (Jani, Mauro, Andi)
>>>      Change is_type_unsigned to is_unsigned_type to have the same name form
>>>      as is_signed_type macro
>>>
>>> Signed-off-by: Gwan-gyeong Mun <gwan-gyeong.mun@...el.com>
>>> Cc: Thomas Hellström <thomas.hellstrom@...ux.intel.com>
>>> Cc: Matthew Auld <matthew.auld@...el.com>
>>> Cc: Nirmoy Das <nirmoy.das@...el.com>
>>> Cc: Jani Nikula <jani.nikula@...el.com>
>>> Cc: Andi Shyti <andi.shyti@...ux.intel.com>
>>> Reviewed-by: Mauro Carvalho Chehab <mchehab@...nel.org> (v5)
>>> ---
>>>   drivers/gpu/drm/i915/i915_utils.h |  5 +--
>>>   include/linux/overflow.h          | 54 +++++++++++++++++++++++++++++++
>>>   2 files changed, 55 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/i915/i915_utils.h b/drivers/gpu/drm/i915/i915_utils.h
>>> index c10d68cdc3ca..eb0ded23fa9c 100644
>>> --- a/drivers/gpu/drm/i915/i915_utils.h
>>> +++ b/drivers/gpu/drm/i915/i915_utils.h
>>> @@ -32,6 +32,7 @@
>>>   #include <linux/types.h>
>>>   #include <linux/workqueue.h>
>>>   #include <linux/sched/clock.h>
>>> +#include <linux/overflow.h>
>>>   
>>>   #ifdef CONFIG_X86
>>>   #include <asm/hypervisor.h>
>>> @@ -111,10 +112,6 @@ bool i915_error_injected(void);
>>>   #define range_overflows_end_t(type, start, size, max) \
>>>   	range_overflows_end((type)(start), (type)(size), (type)(max))
>>>   
>>> -/* Note we don't consider signbits :| */
>>> -#define overflows_type(x, T) \
>>> -	(sizeof(x) > sizeof(T) && (x) >> BITS_PER_TYPE(T))
>>> -
>>>   #define ptr_mask_bits(ptr, n) ({					\
>>>   	unsigned long __v = (unsigned long)(ptr);			\
>>>   	(typeof(ptr))(__v & -BIT(n));					\
>>> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
>>> index f1221d11f8e5..462a03454377 100644
>>> --- a/include/linux/overflow.h
>>> +++ b/include/linux/overflow.h
>>> @@ -35,6 +35,60 @@
>>>   #define type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T)))
>>>   #define type_min(T) ((T)((T)-type_max(T)-(T)1))
>>>   
>>> +/**
>>> + * is_unsigned_type - helper for checking data type which is an unsigned data
>>> + * type or not
>>> + * @x: The data type to check
>>> + *
>>> + * Returns:
>>> + * True if the data type is an unsigned data type, false otherwise.
>>> + */
>>> +#define is_unsigned_type(x) ((typeof(x))-1 >= (typeof(x))0)
> 
> I'd rather not have separate logic for this. Instead, I'd like it to be:
> 
> #define is_unsigned_type(x) (!is_signed_type(x))
> 
>>> +
>>> +/**
>>> + * overflows_type - helper for checking the truncation between data types
>>> + * @x: Source for overflow type comparison
>>> + * @T: Destination for overflow type comparison
>>> + *
>>> + * It compares the values and size of each data type between the first and
>>> + * second argument to check whether truncation can occur when assigning the
>>> + * first argument to the variable of the second argument.
>>> + * Source and Destination can be used with or without sign bit.
>>> + * Composite data structures such as union and structure are not considered.
>>> + * Enum data types are not considered.
>>> + * Floating point data types are not considered.
>>> + *
>>> + * Returns:
>>> + * True if truncation can occur, false otherwise.
>>> + */
>>> +#define overflows_type(x, T) \
>>> +	(is_unsigned_type(x) ? \
>>> +		is_unsigned_type(T) ? \
>>> +			(sizeof(x) > sizeof(T) && (x) >> BITS_PER_TYPE(T)) ? 1 : 0 \
>>> +			: (sizeof(x) >= sizeof(T) && (x) >> (BITS_PER_TYPE(T) - 1)) ? 1 : 0 \
>>> +	: is_unsigned_type(T) ? \
>>> +		((x) < 0) ? 1 : (sizeof(x) > sizeof(T) && (x) >> BITS_PER_TYPE(T)) ? 1 : 0 \
>>> +		: (sizeof(x) > sizeof(T)) ? \
>>> +			((x) < 0) ? (((x) * -1) >> BITS_PER_TYPE(T)) ? 1 : 0 \
>>> +				: ((x) >> BITS_PER_TYPE(T)) ? 1 : 0 \
>>> +			: 0)
> 
> Like the other, I'd much rather this was rephrased in terms of the
> existing macros (e.g. type_min()/type_max().)


I am not sure how it could be rephrased with type_(min|max), but I guess 
the shortest could be sth like:

#define overflows_type(x, T) __builtin_add_overflow_p(x, (typeof(T))0, 
(typeof(T))0)

Regards
Andrzej


> 
>>> +
>>> +/**
>>> + * safe_conversion - perform a type conversion (cast) of an source value into
>>> + * a new variable, checking that the destination is large enough to hold the
>>> + * source value.
>>> + * @ptr: Destination pointer address
>>> + * @value: Source value
>>> + *
>>> + * Returns:
>>> + * If the value would overflow the destination, it returns false.
>>> + */
>>> +#define safe_conversion(ptr, value) ({ \
>>> +	typeof(value) __v = (value); \
>>> +	typeof(ptr) __ptr = (ptr); \
>>> +	overflows_type(__v, *__ptr) ? 0 : ((*__ptr = (typeof(*__ptr))__v), 1); \
>>> +})
> 
> I try to avoid "safe" as an adjective for interface names, since it
> doesn't really answer "safe from what?" This looks more like "assign, but
> zero when out of bounds". And it can be built from existing macros here:
> 
> 	if (check_add_overflow(0, value, ptr))
> 		*ptr = 0;
> 
> I actually want to push back on this a bit, because there can still be
> logic bugs built around this kind of primitive. Shouldn't out-of-bounds
> assignments be seen as a direct failure? I would think this would be
> sufficient:
> 
> #define check_assign(value, ptr)	check_add_overflow(0, value, ptr)
> 
> And callers would do:
> 
> 	if (check_assign(value, &var))
> 		return -EINVAL;
> 
> etc.
> 
> 

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