[PATCH] init: fix allocated page overlapping with PTR_ERR

[Nam Cao]

On 2024-04-18 Nam Cao wrote:
> There is nothing preventing kernel memory allocators from allocating a
> page that overlaps with PTR_ERR(), except for architecture-specific
> code that setup memblock.
> 
> It was discovered that RISCV architecture doesn't setup memblock
> corectly, leading to a page overlapping with PTR_ERR() being allocated,
> and subsequently crashing the kernel (link in Close: )
> 
> The reported crash has nothing to do with PTR_ERR(): the last page
> (at address 0xfffff000) being allocated leads to an unexpected
> arithmetic overflow in ext4; but still, this page shouldn't be
> allocated in the first place.
> 
> Because PTR_ERR() is an architecture-independent thing, we shouldn't
> ask every single architecture to set this up. There may be other
> architectures beside RISCV that have the same problem.
> 
> Fix this one and for all by reserving the physical memory page that
> may be mapped to the last virtual memory page as part of low memory.
> 
> Unfortunately, this means if there is actual memory at this reserved
> location, that memory will become inaccessible. However, if this page
> is not reserved, it can only be accessed as high memory, so this
> doesn't matter if high memory is not supported. Even if high memory is
> supported, it is still only one page.
> 
> Closes: https://lore.kernel.org/linux-riscv/878r1ibpdn.fsf@all.your.base.are.belong.to.us
> Signed-off-by: Nam Cao <namcao at linutronix.de>
> Cc: <stable at vger.kernel.org> # all versions

Sorry, forgot to add:
Reported-by: Björn Töpel <bjorn at kernel.org>

> ---
>  init/main.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/init/main.c b/init/main.c
> index 881f6230ee59..f8d2793c4641 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -900,6 +900,7 @@ void start_kernel(void)
>  	page_address_init();
>  	pr_notice("%s", linux_banner);
>  	early_security_init();
> +	memblock_reserve(__pa(-PAGE_SIZE), PAGE_SIZE); /* reserve last page for ERR_PTR */
>  	setup_arch(&command_line);
>  	setup_boot_config();
>  	setup_command_line(command_line);