Revert "riscv: Define TASK_SIZE_MAX for __access_ok()"

[PATCH] Revert "riscv: Define TASK_SIZE_MAX for __access_ok()"

Posted by Nam Cao 3 months, 3 weeks ago

This reverts commit ad5643cf2f69 ("riscv: Define TASK_SIZE_MAX for
__access_ok()").

This commit changes TASK_SIZE_MAX to be LONG_MAX to optimize access_ok(),
because the previous TASK_SIZE_MAX (default to TASK_SIZE) requires some
computation.

The reasoning was that all user addresses are less than LONG_MAX, and all
kernel addresses are greater than LONG_MAX. Therefore access_ok() can
filter kernel addresses.

Addresses between TASK_SIZE and LONG_MAX are not valid user addresses, but
access_ok() let them pass. That was thought to be okay, because they are
not valid addresses at hardware level.

Unfortunately, one case is missed: get_user_pages_fast() happily accepts
addresses between TASK_SIZE and LONG_MAX. futex(), for instance, uses
get_user_pages_fast(). This causes the problem reported by Robert [1].

Therefore, revert this commit. TASK_SIZE_MAX is changed to the default:
TASK_SIZE.

This unfortunately reduces performance, because TASK_SIZE is more expensive
to compute compared to LONG_MAX. But correctness first, we can think about
optimization later, if required.

Reported-by: <rtm@csail.mit.edu>
Closes: https://lore.kernel.org/linux-riscv/77605.1750245028@localhost/
Signed-off-by: Nam Cao <namcao@linutronix.de>
Cc: stable@vger.kernel.org
---
 arch/riscv/include/asm/pgtable.h | 1 -
 1 file changed, 1 deletion(-)

diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
index 438ce7df24c39..5bd5aae60d536 100644
--- a/arch/riscv/include/asm/pgtable.h
+++ b/arch/riscv/include/asm/pgtable.h
@@ -1075,7 +1075,6 @@ static inline pte_t pte_swp_clear_exclusive(pte_t pte)
  */
 #ifdef CONFIG_64BIT
 #define TASK_SIZE_64	(PGDIR_SIZE * PTRS_PER_PGD / 2)
-#define TASK_SIZE_MAX	LONG_MAX
 
 #ifdef CONFIG_COMPAT
 #define TASK_SIZE_32	(_AC(0x80000000, UL) - PAGE_SIZE)
-- 
2.39.5

Re: [PATCH] Revert "riscv: Define TASK_SIZE_MAX for __access_ok()"

Posted by Alexandre Ghiti 3 months, 2 weeks ago

Hi Nam,

On 6/19/25 17:58, Nam Cao wrote:
> This reverts commit ad5643cf2f69 ("riscv: Define TASK_SIZE_MAX for
> __access_ok()").
>
> This commit changes TASK_SIZE_MAX to be LONG_MAX to optimize access_ok(),
> because the previous TASK_SIZE_MAX (default to TASK_SIZE) requires some
> computation.
>
> The reasoning was that all user addresses are less than LONG_MAX, and all
> kernel addresses are greater than LONG_MAX. Therefore access_ok() can
> filter kernel addresses.
>
> Addresses between TASK_SIZE and LONG_MAX are not valid user addresses, but
> access_ok() let them pass. That was thought to be okay, because they are
> not valid addresses at hardware level.
>
> Unfortunately, one case is missed: get_user_pages_fast() happily accepts
> addresses between TASK_SIZE and LONG_MAX. futex(), for instance, uses
> get_user_pages_fast(). This causes the problem reported by Robert [1].
>
> Therefore, revert this commit. TASK_SIZE_MAX is changed to the default:
> TASK_SIZE.
>
> This unfortunately reduces performance, because TASK_SIZE is more expensive
> to compute compared to LONG_MAX. But correctness first, we can think about
> optimization later, if required.
>
> Reported-by: <rtm@csail.mit.edu>
> Closes: https://lore.kernel.org/linux-riscv/77605.1750245028@localhost/
> Signed-off-by: Nam Cao <namcao@linutronix.de>
> Cc: stable@vger.kernel.org
> ---
>   arch/riscv/include/asm/pgtable.h | 1 -
>   1 file changed, 1 deletion(-)
>
> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
> index 438ce7df24c39..5bd5aae60d536 100644
> --- a/arch/riscv/include/asm/pgtable.h
> +++ b/arch/riscv/include/asm/pgtable.h
> @@ -1075,7 +1075,6 @@ static inline pte_t pte_swp_clear_exclusive(pte_t pte)
>    */
>   #ifdef CONFIG_64BIT
>   #define TASK_SIZE_64	(PGDIR_SIZE * PTRS_PER_PGD / 2)
> -#define TASK_SIZE_MAX	LONG_MAX
>   
>   #ifdef CONFIG_COMPAT
>   #define TASK_SIZE_32	(_AC(0x80000000, UL) - PAGE_SIZE)


I agree with this revert, the next step is to implement the same 
optimization using alternatives (like x86 does).

Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>

It should land into -fixes.

Thanks,

Alex

Re: [PATCH] Revert "riscv: Define TASK_SIZE_MAX for __access_ok()"

Posted by Palmer Dabbelt 3 months, 2 weeks ago

On Mon, 23 Jun 2025 02:15:48 PDT (-0700), Alexandre Ghiti wrote:
> Hi Nam,
>
> On 6/19/25 17:58, Nam Cao wrote:
>> This reverts commit ad5643cf2f69 ("riscv: Define TASK_SIZE_MAX for
>> __access_ok()").
>>
>> This commit changes TASK_SIZE_MAX to be LONG_MAX to optimize access_ok(),
>> because the previous TASK_SIZE_MAX (default to TASK_SIZE) requires some
>> computation.
>>
>> The reasoning was that all user addresses are less than LONG_MAX, and all
>> kernel addresses are greater than LONG_MAX. Therefore access_ok() can
>> filter kernel addresses.
>>
>> Addresses between TASK_SIZE and LONG_MAX are not valid user addresses, but
>> access_ok() let them pass. That was thought to be okay, because they are
>> not valid addresses at hardware level.
>>
>> Unfortunately, one case is missed: get_user_pages_fast() happily accepts
>> addresses between TASK_SIZE and LONG_MAX. futex(), for instance, uses
>> get_user_pages_fast(). This causes the problem reported by Robert [1].
>>
>> Therefore, revert this commit. TASK_SIZE_MAX is changed to the default:
>> TASK_SIZE.
>>
>> This unfortunately reduces performance, because TASK_SIZE is more expensive
>> to compute compared to LONG_MAX. But correctness first, we can think about
>> optimization later, if required.
>>
>> Reported-by: <rtm@csail.mit.edu>
>> Closes: https://lore.kernel.org/linux-riscv/77605.1750245028@localhost/
>> Signed-off-by: Nam Cao <namcao@linutronix.de>
>> Cc: stable@vger.kernel.org
>> ---
>>   arch/riscv/include/asm/pgtable.h | 1 -
>>   1 file changed, 1 deletion(-)
>>
>> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
>> index 438ce7df24c39..5bd5aae60d536 100644
>> --- a/arch/riscv/include/asm/pgtable.h
>> +++ b/arch/riscv/include/asm/pgtable.h
>> @@ -1075,7 +1075,6 @@ static inline pte_t pte_swp_clear_exclusive(pte_t pte)
>>    */
>>   #ifdef CONFIG_64BIT
>>   #define TASK_SIZE_64	(PGDIR_SIZE * PTRS_PER_PGD / 2)
>> -#define TASK_SIZE_MAX	LONG_MAX
>>
>>   #ifdef CONFIG_COMPAT
>>   #define TASK_SIZE_32	(_AC(0x80000000, UL) - PAGE_SIZE)
>
>
> I agree with this revert, the next step is to implement the same
> optimization using alternatives (like x86 does).
>
> Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
>
> It should land into -fixes.

It's not clear if you're picking it up?  I will, it's hitting the 
tester now...

>
> Thanks,
>
> Alex