Re: [PATCH v2] RISC-V: Fix FIXMAP area corruption on RV32 systems

[Paul Walmsley]

Hello Anup,

On Mon, 19 Aug 2019, Anup Patel wrote:

> Currently, various virtual memory areas of Linux RISC-V are organized
> in increasing order of their virtual addresses is as follows:
> 1. User space area (This is lowest area and starts at 0x0)
> 2. FIXMAP area
> 3. VMALLOC area
> 4. Kernel area (This is highest area and starts at PAGE_OFFSET)
> 
> The maximum size of user space aread is represented by TASK_SIZE.
> 
> On RV32 systems, TASK_SIZE is defined as VMALLOC_START which causes the
> user space area to overlap the FIXMAP area. This allows user space apps
> to potentially corrupt the FIXMAP area and kernel OF APIs will crash
> whenever they access corrupted FDT in the FIXMAP area.
> 
> On RV64 systems, TASK_SIZE is set to fixed 256GB and no other areas
> happen to overlap so we don't see any FIXMAP area corruptions.
> 
> This patch fixes FIXMAP area corruption on RV32 systems by setting
> TASK_SIZE to FIXADDR_START. 

This part -- the TASK_SIZE change -- makes sense to me.  

However, the patch also changes FIXADDR_SIZE to be defined in terms of 
page table-related constants.  Previously, FIXADDR_SIZE was based on 
__end_of_fixed_addresses, as it is for most other architectures. The part 
of the patch that changes FIXADDR_SIZE seems unrelated to the actual fix.

If that's indeed the case -- that the change to FIXADDR_SIZE is unrelated 
from the fix -- could you please split that into a separate patch, with a 
description of the rationale?  I think I understand why you're proposing 
it, but it seems odd to explicitly connect it to page table-related 
constants, rather than the contents of "enum fixed_addresses", and I'm 
reluctant to merge that part of this patch without a bit more discussion.


> We also move FIXADDR_TOP, FIXADDR_SIZE, and FIXADDR_START defines to 
> asm/pgtable.h so that we can avoid cyclic header includes.
> 
> Signed-off-by: Anup Patel <anup.patel@xxxxxxx>
> Tested-by: Alistair Francis <alistair.francis@xxxxxxx>
> Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> ---
> Changes since v1:
> - Drop braces from "#define FIXADDR_TOP"
> ---
>  arch/riscv/include/asm/fixmap.h  |  4 ----
>  arch/riscv/include/asm/pgtable.h | 12 ++++++++++--
>  2 files changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/fixmap.h b/arch/riscv/include/asm/fixmap.h
> index 9c66033c3a54..161f28d04a07 100644
> --- a/arch/riscv/include/asm/fixmap.h
> +++ b/arch/riscv/include/asm/fixmap.h
> @@ -30,10 +30,6 @@ enum fixed_addresses {
>  	__end_of_fixed_addresses
>  };
> 
> -#define FIXADDR_SIZE		(__end_of_fixed_addresses * PAGE_SIZE)
> -#define FIXADDR_TOP		(VMALLOC_START)
> -#define FIXADDR_START		(FIXADDR_TOP - FIXADDR_SIZE)
> -
>  #define FIXMAP_PAGE_IO		PAGE_KERNEL
> 
>  #define __early_set_fixmap	__set_fixmap
> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
> index a364aba23d55..c24a083b3e12 100644
> --- a/arch/riscv/include/asm/pgtable.h
> +++ b/arch/riscv/include/asm/pgtable.h
> @@ -420,14 +420,22 @@ static inline void pgtable_cache_init(void)
>  #define VMALLOC_END      (PAGE_OFFSET - 1)
>  #define VMALLOC_START    (PAGE_OFFSET - VMALLOC_SIZE)
> 
> +#define FIXADDR_TOP      VMALLOC_START
> +#ifdef CONFIG_64BIT
> +#define FIXADDR_SIZE     PMD_SIZE
> +#else
> +#define FIXADDR_SIZE     PGDIR_SIZE
> +#endif
> +#define FIXADDR_START    (FIXADDR_TOP - FIXADDR_SIZE)
> +
>  /*
> - * Task size is 0x4000000000 for RV64 or 0xb800000 for RV32.
> + * Task size is 0x4000000000 for RV64 or 0x9fc00000 for RV32.
>   * Note that PGDIR_SIZE must evenly divide TASK_SIZE.
>   */
>  #ifdef CONFIG_64BIT
>  #define TASK_SIZE (PGDIR_SIZE * PTRS_PER_PGD / 2)
>  #else
> -#define TASK_SIZE VMALLOC_START
> +#define TASK_SIZE FIXADDR_START
>  #endif
> 
>  #include <asm-generic/pgtable.h>
> --
> 2.17.1
> 


- Paul