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Date:   Fri, 9 Sep 2022 19:52:41 +0900
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Subject: Re: [Intel-gfx] [PATCH v9 1/8] overflow: Move and add few utility
 macros into overflow
Content-Language: en-US
To:     Andrzej Hajda <andrzej.hajda@intel.com>,
        Kees Cook <keescook@chromium.org>
CC:     <thomas.hellstrom@linux.intel.com>, <jani.nikula@intel.com>,
        "Nick Desaulniers" <ndesaulniers@google.com>,
        <intel-gfx@lists.freedesktop.org>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        <linux-kernel@vger.kernel.org>, <dri-devel@lists.freedesktop.org>,
        <chris@chris-wilson.co.uk>, <airlied@linux.ie>,
        <matthew.auld@intel.com>, <daniel@ffwll.ch>,
        <intel-gfx-trybot@lists.freedesktop.org>, <mchehab@kernel.org>,
        <nirmoy.das@intel.com>
References: <20220824084514.2261614-1-gwan-gyeong.mun@intel.com>
 <20220824084514.2261614-2-gwan-gyeong.mun@intel.com>
 <202208250848.1C77B9C38@keescook>
 <1d8cc163-2435-42bc-83c8-62648d3c5ada@intel.com>
From:   Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
In-Reply-To: <1d8cc163-2435-42bc-83c8-62648d3c5ada@intel.com>
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On 8/26/22 10:44 PM, Andrzej Hajda wrote:
> On 25.08.2022 18:47, Kees Cook wrote:
>> On Wed, Aug 24, 2022 at 05:45:07PM +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 truncaion (overflow)
>>> between different data types. And it adds check_assign() macro which
>>> performs an assigning source value into destination ptr along with an
>>> overflow check. overflow_type macro has been improved to handle the 
>>> signbit
>>> by gcc's built-in overflow check function. And it adds overflows_ptr()
>>> helper macro for checking the overflows between a value and a pointer
>>> type/value.
>>>
>>> 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
>>> v8: Add check_assign() and remove safe_conversion() (Kees)
>>>      Fix overflows_type() to use gcc's built-in overflow function 
>>> (Andrzej)
>>>      Add overflows_ptr() to allow overflow checking when assigning a 
>>> value
>>>      into a pointer variable (G.G.)
>>> v9: Fix overflows_type() to use __builtin_add_overflow() instead of
>>>      __builtin_add_overflow_p() (Andrzej)
>>>      Fix overflows_ptr() to use overflows_type() with the unsigned 
>>> long type
>>>      (Andrzej)
>>>
>>> Signed-off-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
>>> Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
>>> Cc: Matthew Auld <matthew.auld@intel.com>
>>> Cc: Nirmoy Das <nirmoy.das@intel.com>
>>> Cc: Jani Nikula <jani.nikula@intel.com>
>>> Cc: Andi Shyti <andi.shyti@linux.intel.com>
>>> Cc: Andrzej Hajda <andrzej.hajda@intel.com>
>>> Cc: Mauro Carvalho Chehab <mauro.chehab@linux.intel.com>
>>> Cc: Kees Cook <keescook@chromium.org>
>>> Reviewed-by: Mauro Carvalho Chehab <mchehab@kernel.org> (v5)
>>> ---
>>>   drivers/gpu/drm/i915/i915_user_extensions.c |  3 +-
>>>   drivers/gpu/drm/i915/i915_utils.h           |  5 +-
>>>   include/linux/overflow.h                    | 62 +++++++++++++++++++++
>>>   3 files changed, 64 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/i915/i915_user_extensions.c 
>>> b/drivers/gpu/drm/i915/i915_user_extensions.c
>>> index c822d0aafd2d..6f6b5b910968 100644
>>> --- a/drivers/gpu/drm/i915/i915_user_extensions.c
>>> +++ b/drivers/gpu/drm/i915/i915_user_extensions.c
>>> @@ -50,8 +50,7 @@ int i915_user_extensions(struct i915_user_extension 
>>> __user *ext,
>>>           if (err)
>>>               return err;
>>> -        if (get_user(next, &ext->next_extension) ||
>>> -            overflows_type(next, ext))
>>> +        if (get_user(next, &ext->next_extension) || 
>>> overflows_ptr(next))
>>>               return -EFAULT;
>>>           ext = u64_to_user_ptr(next);
>>
>> I continue to dislike the layers of macros and specialization here.
>> This is just a fancy version of check_assign():
>>
>>     if (get_user(next, &ext->next_extension) || check_assign(next, &ext))
>>         return -EFAULT;
>>
>> However, the __builtin_*_overflow() family only wants to work on
>> integral types, so this needs to be slightly expanded:
>>
>>     uintptr_t kptr;
>>     ...
>>     if (get_user(next, &ext->next_extension) || check_assign(next, 
>> &kptr))
>>         return -EFAULT;
>>
>>     ext = (void * __user)kptr;
>>
>> But, it does seem like the actual problem here is that u64_to_user_ptr()
>> is not performing the checking? It's used heavily in the drm code.
>>
>> Is a check_assign_user_ptr() needed?
>>
>>> 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..6af9df1d67a1 100644
>>> --- a/include/linux/overflow.h
>>> +++ b/include/linux/overflow.h
>>> @@ -52,6 +52,68 @@ static inline bool __must_check 
>>> __must_check_overflow(bool overflow)
>>>       return unlikely(overflow);
>>>   }
>>> +/**
>>> + * overflows_type - helper for checking the overflows between data 
>>> types or
>>> + *                  values
>>> + *
>>> + * @x: Source value or data type for overflow check
>>> + * @T: Destination value or data type for overflow check
>>> + *
>>> + * It compares the values or data type between the first and second 
>>> argument to
>>> + * check whether overflow can occur when assigning the first 
>>> argument to the
>>> + * variable of the second argument. Source and Destination can be 
>>> singned or
>>> + * unsigned data types. Source and Destination can be different data 
>>> types.
>>> + *
>>> + * Returns:
>>> + * True if overflow can occur, false otherwise.
>>> + */
>>> +#define overflows_type(x, T) __must_check_overflow(({    \
>>> +    typeof(T) v = 0;                \
>>> +    __builtin_add_overflow((x), v, &v);        \
>>> +}))
>>
>> I'd like to avoid "externalizing" these kinds of checks when the better
>> path is to catch the issue at operation type (add, sub, mul, assign).
>> Looking at existing users, I see stuff like:
>>
>>                  if (overflows_type(item.query_id - 1, unsigned long))
>>                          return -EINVAL;
>>
>>                  func_idx = item.query_id - 1;
>>
>> This requires too much open-coded checking, IMO. It's better as:
>>
>>         if (check_assign(item.query_id - 1, &func_idx))
>>             return -EINVAL;
>>
>> or other similar:
>>
>>          if (overflows_type(user->slice_mask, context->slice_mask) ||
>>     ...
>>          context->slice_mask = user->slice_mask;
>>
>> and some that don't make sense to me. Why check argument types? Is this
>> trying to avoid implicit type conversions?
>>
>> So, if it's _really_ needed, I can live with adding overflows_type().
>>
>>> +
>>> +/**
>>> + * overflows_ptr - helper for checking the occurrence of overflows 
>>> when a value
>>> + *                 assigns to  the pointer data type
>>> + *
>>> + * @x: Source value for overflow check
>>> + *
>>> + * gcc's built-in overflow check functions don't support checking 
>>> between the
>>> + * pointer type and non-pointer type. And ILP32 and LP64 have the 
>>> same bit size
>>> + * between long and pointer. Therefore it internally compares the 
>>> source value
>>> + * and unsigned long data type for checking overflow.
>>> + *
>>> + * Returns:
>>> + * True if overflow can occur, false otherwise.
>>> + */
>>> +#define overflows_ptr(x) __must_check_overflow(overflows_type(x, 
>>> unsigned long))
>>
>> I'd rather not have this -- it's just a specialized use of
>> overflows_type(), and only used in 1 place.
>>
>>> +
>>> +/**
>>> + * check_assign - perform an assigning source value into destination 
>>> ptr along
>>> + *                with an overflow check.
>>> + *
>>> + * @value: Source value
>>> + * @ptr: Destination pointer address, If the pointer type is not used,
>>> + *       a warning message is output during build.
>>> + *
>>> + * It checks internally the ptr is a pointer type. And it uses gcc's 
>>> built-in
>>> + * overflow check function.
>>> + * It does not use the check_*() wrapper functions, but directly 
>>> uses gcc's
>>> + * built-in overflow check function so that it can be used even when
>>> + * the type of value and the type pointed to by ptr are different 
>>> without build
>>> + * warning messages.
>>
>> This is a good point: the check_{add,sub,mul}_overflow() helpers
>> currently require all the params be the same type, which rather limits
>> their usage. Perhaps this can be weakened now that we're not using[1]
>> the fallback logic any more? (Separate patch.)
>>
>>> + *
>>> + * Returns:
>>> + * If the value would overflow the destination, it returns true. If 
>>> not return
>>> + * false. When overflow does not occur, the assigning into ptr from 
>>> value
>>> + * succeeds. It follows the return policy as other 
>>> check_*_overflow() functions
>>> + * return non-zero as a failure.
>>> + */
>>> +#define check_assign(value, ptr) __must_check_overflow(({    \
>>> +    typecheck_pointer(ptr);         \
>>> +    __builtin_add_overflow(0, value, ptr);    \
>>> +}))
>>
>> But yes, this looks correct. I really like it. :)
> 
> 
> One more thing, I suspect __builtin_add_overflow checks already if ptr 
> is pointer, so typecheck_pointer seems redundant.
> 
thanks for check in detail.
I'll remove redundant code and send new version.

> Regards
> Andrzej
> 
> 
>>
>>> +
>>>   /*
>>>    * For simplicity and code hygiene, the fallback code below insists on
>>>    * a, b and *d having the same type (similar to the min() and max()
>>> -- 
>>> 2.37.1
>>>
>>
>> -Kees
>>
>> [1] 4eb6bd55cfb2 ("compiler.h: drop fallback overflow checkers")
>>
>