Re: [PATCH mips-fixes] MIPS: relocatable: fix possible boot hangup with KASLR enabled

[Nathan Chancellor <natechancellor@gmail.com>]

On Sun, Jan 10, 2021 at 02:21:05PM +0000, Alexander Lobakin wrote:
> LLVM-built Linux triggered a boot hangup with KASLR enabled.
> 
> arch/mips/kernel/relocate.c:get_random_boot() uses linux_banner,
> which is a string constant, as a random seed, but accesses it
> as an array of unsigned long (in rotate_xor()).
> When the address of linux_banner is not aligned to sizeof(long),
> such access emits unaligned access exception and hangs the kernel.
> 
> Use PTR_ALIGN() to align input address to sizeof(long) and also
> align down the input length to prevent possible access-beyond-end.
> 
> Fixes: 405bc8fd12f5 ("MIPS: Kernel: Implement KASLR using CONFIG_RELOCATABLE")
> Cc: stable@vger.kernel.org # 4.7+
> Signed-off-by: Alexander Lobakin <alobakin@pm.me>

Apologies for not being familiar enough with the issue to give a review
but I did reproduce the hang that the commit mentions with
malta_kvm_guest_defconfig + CONFIG_RELOCATABLE=y +
CONFIG_RANDOMIZE_BASE=y and this patch does resolve it so:

Tested-by: Nathan Chancellor <natechancellor@gmail.com>

> ---
>  arch/mips/kernel/relocate.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/mips/kernel/relocate.c b/arch/mips/kernel/relocate.c
> index 47aeb3350a76..0e365b7c742d 100644
> --- a/arch/mips/kernel/relocate.c
> +++ b/arch/mips/kernel/relocate.c
> @@ -187,8 +187,14 @@ static int __init relocate_exception_table(long offset)
>  static inline __init unsigned long rotate_xor(unsigned long hash,
>  					      const void *area, size_t size)
>  {
> -	size_t i;
> -	unsigned long *ptr = (unsigned long *)area;
> +	const typeof(hash) *ptr = PTR_ALIGN(area, sizeof(hash));
> +	size_t diff, i;
> +
> +	diff = (void *)ptr - area;
> +	if (unlikely(size < diff + sizeof(hash)))
> +		return hash;
> +
> +	size = ALIGN_DOWN(size - diff, sizeof(hash));
>  
>  	for (i = 0; i < size / sizeof(hash); i++) {
>  		/* Rotate by odd number of bits and XOR. */
> -- 
> 2.30.0
> 
>