Re: [Qemu-devel] [PATCH v4] loader: Fix misaligned member access

[David Gibson]

On Tue, Apr 24, 2018 at 07:21:03PM -0300, Philippe Mathieu-Daudé wrote:
> The libfdt does not guarantee than fdt_getprop() returns a pointer
> aligned to the property size.
> 
> Assuming the base of the fdt is aligned, a 32-bit property returns
> a 32-bit aligned pointer. This is however not guaranteed for 64-bit
> properties, where 64-bit loads might trigger unaligned access.
> 
> Fix the 64-bit access using the ldst (host) API, which uses a local
> copy on the stack, thus guaranteeing a safe aligned access.
> 
> This fixes the following ASan warning:
> 
>   $ qemu-system-mips64el -M boston -kernel vmlinux.gz.itb -nographic
>   hw/core/loader-fit.c:108:17: runtime error: load of misaligned address 
> 0x7f95cd7e4264 for type 'fdt64_t', which requires 8 byte alignment
>   0x7f95cd7e4264: note: pointer points here
>     00 00 00 3e ff ff ff ff  80 7d 2a c0 00 00 00 01  68 61 73 68 40 30 00 00 
>  00 00 00 03 00 00 00 14
>                 ^
> 
> Reported-by: AddressSanitizer
> Suggested-by: Peter Maydell <address@hidden>
> Signed-off-by: Philippe Mathieu-Daudé <address@hidden>

Reviewed-by: David Gibson <address@hidden>

I'll look at adding a hlper for this to libfdt, but no promises on timeframe.

> ---
> v4: do not change the 32-bit access, use ldq_he_p() for the 64-bit access
> v3: do not use memcpy(), incorrectly change ldl_he_p()
> v2: do not change the 32-bit access, use memcpy(), add comments (David Gibson)
> v1: use memcpy()
> 
>  hw/core/loader-fit.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/core/loader-fit.c b/hw/core/loader-fit.c
> index 0c4a7207f4..ed4140061b 100644
> --- a/hw/core/loader-fit.c
> +++ b/hw/core/loader-fit.c
> @@ -102,10 +102,17 @@ static int fit_image_addr(const void *itb, int img, 
> const char *name,
>  
>      switch (len) {
>      case 4:
> +        /* Assuming the base of the fdt is aligned, then fdt_getprop()
> +         * returns 32-bit aligned properties, so this load is guaranteed
> +         * to be 32-bit aligned.
> +         */
>          *addr = fdt32_to_cpu(*(fdt32_t *)prop);
>          return 0;
>      case 8:
> -        *addr = fdt64_to_cpu(*(fdt64_t *)prop);
> +        /* Since the property is not guaranteed to be 64-bit aligned,
> +         * use ldq_he_p()'s stack to avoid an unaligned load.
> +         */
> +        *addr = fdt64_to_cpu(ldq_he_p(prop));
>          return 0;
>      default:
>          error_printf("invalid %s address length %d\n", name, len);

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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