Re: [PATCH] riscv: fix overlap of allocated page and PTR_ERR

[Björn Töpel]

Nam Cao <namcao@xxxxxxxxxxxxx> writes:

> On riscv32, it is possible for the last page in virtual address space
> (0xfffff000) to be allocated. This page overlaps with PTR_ERR, so that
> shouldn't happen.
>
> There is already some code to ensure memblock won't allocate the last page.
> However, buddy allocator is left unchecked.
>
> Fix this by reserving physical memory that would be mapped at virtual
> addresses greater than 0xfffff000.
>
> Reported-by: Björn Töpel <bjorn@xxxxxxxxxx>
> Closes: https://lore.kernel.org/linux-riscv/878r1ibpdn.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: 76d2a0493a17 ("RISC-V: Init and Halt Code")
> Signed-off-by: Nam Cao <namcao@xxxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>


Thanks for picking it up again, Nam.

This passes my test:
Tested-by: Björn Töpel <bjorn@xxxxxxxxxxxx>

> ---
>  arch/riscv/mm/init.c | 21 +++++++++++----------
>  1 file changed, 11 insertions(+), 10 deletions(-)
>
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index 968761843203..7c985435b3fc 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -235,18 +235,19 @@ static void __init setup_bootmem(void)
>  		kernel_map.va_pa_offset = PAGE_OFFSET - phys_ram_base;
>  
>  	/*
> -	 * memblock allocator is not aware of the fact that last 4K bytes of
> -	 * the addressable memory can not be mapped because of IS_ERR_VALUE
> -	 * macro. Make sure that last 4k bytes are not usable by memblock
> -	 * if end of dram is equal to maximum addressable memory.  For 64-bit
> -	 * kernel, this problem can't happen here as the end of the virtual
> -	 * address space is occupied by the kernel mapping then this check must
> -	 * be done as soon as the kernel mapping base address is determined.
> +	 * Reserve physical address space that would be mapped to virtual
> +	 * addresses greater than (void *)(-PAGE_SIZE) because:
> +	 *  - This memory would overlap with ERR_PTR
> +	 *  - This memory belongs to high memory, which is not supported
> +	 *
> +	 * This is not applicable to 64-bit kernel, because virtual addresses
> +	 * after (void *)(-PAGE_SIZE) are not linearly mapped: they are
> +	 * occupied by kernel mapping. Also it is unrealistic for high memory
> +	 * to exist on 64-bit platforms.
>  	 */
>  	if (!IS_ENABLED(CONFIG_64BIT)) {
> -		max_mapped_addr = __pa(~(ulong)0);
> -		if (max_mapped_addr == (phys_ram_end - 1))
> -			memblock_set_current_limit(max_mapped_addr - 4096);
> +		max_mapped_addr = __va_to_pa_nodebug(-PAGE_SIZE);
> +		memblock_reserve(max_mapped_addr, (phys_addr_t)-max_mapped_addr);

Nit (and only if you respin for some reason): Move max_mapped_addr into
the if-clause, or simply get rid of it (all on one line).

Regardless:
Reviewed-by: Björn Töpel <bjorn@xxxxxxxxxxxx>


Björn