linux-kernel - Re: [PATCH] Revert "riscv: Define TASK_SIZE_MAX for __access

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Message-ID: <027ef1a9-6a5c-4dba-8816-159411739b71@ghiti.fr>
Date: Mon, 23 Jun 2025 11:15:48 +0200
From: Alexandre Ghiti <alex@...ti.fr>
To: Nam Cao <namcao@...utronix.de>, Paul Walmsley <paul.walmsley@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc: rtm@...il.mit.edu, stable@...r.kernel.org
Subject: Re: [PATCH] Revert "riscv: Define TASK_SIZE_MAX for __access_ok()"

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@...il.mit.edu>
> Closes: https://lore.kernel.org/linux-riscv/77605.1750245028@localhost/
> Signed-off-by: Nam Cao <namcao@...utronix.de>
> Cc: stable@...r.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@...osinc.com>

It should land into -fixes.

Thanks,

Alex