Re: [PATCH] efi: fix boundary checking in efi_high_alloc()

[Mark Rutland]

Hi Matt,

> From 1e7295b5d4c5226a160a9167e61b581e388f7f9a Mon Sep 17 00:00:00 2001
> From: Yinghai Lu <yinghai@xxxxxxxxxx>
> Date: Thu, 19 Feb 2015 20:18:03 -0800
> Subject: [PATCH] efi/libstub: Fix boundary checking in efi_high_alloc()
> 
> While adding support loading kernel and initrd above 4G to grub2 in legacy
> mode, I was referring to efi_high_alloc().
> That will allocate buffer for kernel and then initrd, and initrd will
> use kernel buffer start as limit.
> 
> During testing found two buffers will be overlapped when initrd size is
> very big like 400M.
> 
> It turns out efi_high_alloc() boundary checking is not right.
> end - size will be the new start, and should not compare new
> start with max, we need to make sure end is smaller than max.
> 
> [ Basically, with the current efi_high_alloc() code it's possible to
>   allocate memory above 'max', because efi_high_alloc() doesn't check
>   that the tail of the allocation is below 'max'.
> 
>   If you have an EFI memory map with a single entry that looks like so,
> 
>    [0xc0000000-0xc0004000]
> 
>   And want to allocate 0x3000 bytes below 0xc0003000 the current code
>   will allocate [0xc0001000-0xc0004000], not [0xc0000000-0xc0003000]
>   like you would expect. - Matt ]
> 
> Signed-off-by: Yinghai Lu <yinghai@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>

I've convinced myself that the new logic is sound, and with this patch
applied atop of v4.0-rc1 I don't see regressions on the platforms I have
access to. So:

Reviewed-by: Mark Rutland <mark.rutland@xxxxxxx>
Tested-by: Mark Rutland <mark.rutland@xxxxxxx>

Ard, Leif:

On a related note, I think that the logic for deciding where to place
the kernel and DTB isn't quite right. The kernel needs to be in the same
naturally-aligned 512M region as the DTB in order to be able to map it,
but the kernel could get relocated above the max address we'll consider
for the DTB if there isn't sufficient space for the kernel between
dram_base and dram_base + 512M.

We should try to use the fixmap to map the DTB so it can be located
anywhere in physical memory. That will make things easier for the stub
and other loaders.

Thanks,
Mark.

> Signed-off-by: Matt Fleming <matt.fleming@xxxxxxxxx>
> ---
>  drivers/firmware/efi/libstub/efi-stub-helper.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/firmware/efi/libstub/efi-stub-helper.c b/drivers/firmware/efi/libstub/efi-stub-helper.c
> index 9bd9fbb5bea8..c927bccd92bd 100644
> --- a/drivers/firmware/efi/libstub/efi-stub-helper.c
> +++ b/drivers/firmware/efi/libstub/efi-stub-helper.c
> @@ -170,12 +170,12 @@ again:
>  		start = desc->phys_addr;
>  		end = start + desc->num_pages * (1UL << EFI_PAGE_SHIFT);
>  
> -		if ((start + size) > end || (start + size) > max)
> -			continue;
> -
> -		if (end - size > max)
> +		if (end > max)
>  			end = max;
>  
> +		if ((start + size) > end)
> +			continue;
> +
>  		if (round_down(end - size, align) < start)
>  			continue;
>  
> -- 
> 1.9.3
> 
> -- 
> Matt Fleming, Intel Open Source Technology Center
> --
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