Re: [PATCH] xen/arm: Fix UBSAN failure in start_xen()

[Julien Grall <julien@xxxxxxx>]

Hi Michal,

On 08/02/2024 10:43, Michal Orzel wrote:
> When running Xen on arm32, in scenario where Xen is loaded at an address
> such as boot_phys_offset >= 2GB, UBSAN reports the following:
>
> (XEN) UBSAN: Undefined behaviour in arch/arm/setup.c:739:58
> (XEN) pointer operation underflowed 00200000 to 86800000
> (XEN) Xen WARN at common/ubsan/ubsan.c:172
> (XEN) ----[ Xen-4.19-unstable  arm32  debug=y ubsan=y  Not tainted ]----
> ...
> (XEN) Xen call trace:
> (XEN)    [<0031b4c0>] ubsan.c#ubsan_epilogue+0x18/0xf0 (PC)
> (XEN)    [<0031d134>] __ubsan_handle_pointer_overflow+0xb8/0xd4 (LR)
> (XEN)    [<0031d134>] __ubsan_handle_pointer_overflow+0xb8/0xd4
> (XEN)    [<004d15a8>] start_xen+0xe0/0xbe0
> (XEN)    [<0020007c>] head.o#primary_switched+0x4/0x30
>
> The failure is reported for the following line:
> (paddr_t)(uintptr_t)(_start + boot_phys_offset)
>
> This occurs because the compiler treats (ptr + size) with size bigger than
> PTRDIFF_MAX as undefined behavior. To address this, switch to macro
> virt_to_maddr(), given the future plans to eliminate boot_phys_offset.
>
> Signed-off-by: Michal Orzel <michal.orzel@xxxxxxx>
> ---
>   xen/arch/arm/setup.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/xen/arch/arm/setup.c b/xen/arch/arm/setup.c
> index 7e28f62d09f1..424744ad5e1a 100644
> --- a/xen/arch/arm/setup.c
> +++ b/xen/arch/arm/setup.c
> @@ -736,7 +736,7 @@ void asmlinkage __init start_xen(unsigned long 
> boot_phys_offset,
>   
>       /* Register Xen's load address as a boot module. */
>       xen_bootmodule = add_boot_module(BOOTMOD_XEN,
> -                             (paddr_t)(uintptr_t)(_start + boot_phys_offset),
> +                             virt_to_maddr(_start),
The two lines can now be merged. So I have done it while committing.

Also, Stefano, I think this wants to be backported.

Cheers,

--
Julien Grall