Re: [Xen-devel] [PATCH v2 2/6] relocator: Do not use memory region if its starta is smaller than size

[Andrei Borzenkov <arvidjaar@xxxxxxxxx>]

On Mon, Jul 20, 2015 at 5:35 PM, Daniel Kiper <daniel.kiper@xxxxxxxxxx> wrote:
> malloc_in_range() should not use memory region if its starta is smaller
> than size. Otherwise target wraps around and points to region which is
> usually not a RAM, e.g.:
>
> loader/multiboot.c:93: segment 0: paddr=0x800000, memsz=0x3f800000, 
> vaddr=0x800000
> lib/relocator.c:1241: min_addr = 0x0, max_addr = 0xffffffffffffffff, target = 
> 0x800000
> lib/relocator.c:434: trying to allocate in 0x800000-0xffffffffffffffff 
> aligned 0x1 size 0x3f800000
> lib/relocator.c:434: trying to allocate in 0x0-0x800000 aligned 0x1 size 
> 0x3f800000
> lib/relocator.c:434: trying to allocate in 0x0-0xffffffffffffffff aligned 0x1 
> size 0x3f800000
> lib/relocator.c:1188: allocated: 0xffffffffc07fffff+0x3f800000
> lib/relocator.c:1277: allocated 0xffffffffc07fffff/0x800000
>
> Signed-off-by: Daniel Kiper <daniel.kiper@xxxxxxxxxx>
> ---
>  grub-core/lib/relocator.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/grub-core/lib/relocator.c b/grub-core/lib/relocator.c
> index f759c7f..4eee0c5 100644
> --- a/grub-core/lib/relocator.c
> +++ b/grub-core/lib/relocator.c
> @@ -748,7 +748,7 @@ malloc_in_range (struct grub_relocator *rel,
>               /* Found an usable address.  */
>               goto found;
>           }
> -       if (isinsidebefore && !isinsideafter && !from_low_priv)
> +       if (isinsidebefore && !isinsideafter && !from_low_priv && starta >= 
> size)

That's too late, we need to check end of region on previous iteration.
Consider region of 128 bytes, requested size 129 and alignment 256.
Than starta still ends up high in memory.

>           {
>             target = starta - size;
>             if (target > end - size)
> --
> 1.7.10.4
>
>
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