Re: [tip:x86/urgent] x86/boot/compressed/64: Fix boot on machines with broken E820 table

[Gustavo A. R. Silva]

Hi all,

On 8/19/19 9:16 AM, tip-bot for Kirill A. Shutemov wrote:
[..]
> 
> diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
> index 5f2d03067ae5..2faddeb0398a 100644
> --- a/arch/x86/boot/compressed/pgtable_64.c
> +++ b/arch/x86/boot/compressed/pgtable_64.c
> @@ -72,6 +72,8 @@ static unsigned long find_trampoline_placement(void)
>  
>  	/* Find the first usable memory region under bios_start. */
>  	for (i = boot_params->e820_entries - 1; i >= 0; i--) {
> +		unsigned long new;
> +
>  		entry = &boot_params->e820_table[i];
>  
>  		/* Skip all entries above bios_start. */
> @@ -84,15 +86,20 @@ static unsigned long find_trampoline_placement(void)
>  
>  		/* Adjust bios_start to the end of the entry if needed. */
>  		if (bios_start > entry->addr + entry->size)

Notice that if this condition happens to be false, we end up with an
uninitialized variable *new*.

What would be the right value to assign to *new* at declaration under
this condition?

> -			bios_start = entry->addr + entry->size;
> +			new = entry->addr + entry->size;
>  
>  		/* Keep bios_start page-aligned. */
> -		bios_start = round_down(bios_start, PAGE_SIZE);
> +		new = round_down(new, PAGE_SIZE);
>  
>  		/* Skip the entry if it's too small. */
> -		if (bios_start - TRAMPOLINE_32BIT_SIZE < entry->addr)
> +		if (new - TRAMPOLINE_32BIT_SIZE < entry->addr)
>  			continue;
>  
> +		/* Protect against underflow. */
> +		if (new - TRAMPOLINE_32BIT_SIZE > bios_start)
> +			break;
> +
> +		bios_start = new;
>  		break;
>  	}
>  
> 

--
Gustavo