Re: [PATCH v3] RISC-V: Check clint_time_val before use

[Palmer Dabbelt]

On Sat, 26 Sep 2020 22:39:16 PDT (-0700), Anup Patel wrote:
> The NoMMU kernel is broken for QEMU virt machine from Linux-5.9-rc6
> because clint_time_val is used even before CLINT driver is probed
> at following places:
> 1. rand_initialize() calls get_cycles() which in-turn uses
>    clint_time_val
> 2. boot_init_stack_canary() calls get_cycles() which in-turn
>    uses clint_time_val
>
> The issue#1 (above) is fixed by providing custom random_get_entropy()
> for RISC-V NoMMU kernel. For issue#2 (above), we remove dependency of
> boot_init_stack_canary() on get_cycles() and this is aligned with the
> boot_init_stack_canary() implementations of ARM, ARM64 and MIPS kernel.
>
> Fixes: d5be89a8d118 ("RISC-V: Resurrect the MMIO timer implementation
> for M-mode systems")
> Signed-off-by: Palmer Dabbelt <palmerdabbelt@xxxxxxxxxx>
> Signed-off-by: Anup Patel <anup.patel@xxxxxxx>
> ---
> Changes since v2:
>  - Take different approach and provide custom random_get_entropy() for
>    RISC-V NoMMU kernel
>  - Remove dependency of boot_init_stack_canary() on get_cycles()
>  - Hopefully we don't require to set clint_time_val = NULL in CLINT
>    driver with a different approach to fix.
> Changes since v1:
>  - Explicitly initialize clint_time_val to NULL in CLINT driver to
>    avoid hang on Kendryte K210
> ---
>  arch/riscv/include/asm/stackprotector.h |  4 ----
>  arch/riscv/include/asm/timex.h          | 13 +++++++++++++
>  2 files changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/arch/riscv/include/asm/stackprotector.h b/arch/riscv/include/asm/stackprotector.h
> index d95f7b2a7f37..5962f8891f06 100644
> --- a/arch/riscv/include/asm/stackprotector.h
> +++ b/arch/riscv/include/asm/stackprotector.h
> @@ -5,7 +5,6 @@
>
>  #include <linux/random.h>
>  #include <linux/version.h>
> -#include <asm/timex.h>
>
>  extern unsigned long __stack_chk_guard;
>
> @@ -18,12 +17,9 @@ extern unsigned long __stack_chk_guard;
>  static __always_inline void boot_init_stack_canary(void)
>  {
>  	unsigned long canary;
> -	unsigned long tsc;
>
>  	/* Try to get a semi random initial value. */
>  	get_random_bytes(&canary, sizeof(canary));
> -	tsc = get_cycles();
> -	canary += tsc + (tsc << BITS_PER_LONG/2);
>  	canary ^= LINUX_VERSION_CODE;
>  	canary &= CANARY_MASK;
>
> diff --git a/arch/riscv/include/asm/timex.h b/arch/riscv/include/asm/timex.h
> index 7f659dda0032..ab104905d4db 100644
> --- a/arch/riscv/include/asm/timex.h
> +++ b/arch/riscv/include/asm/timex.h
> @@ -33,6 +33,19 @@ static inline u32 get_cycles_hi(void)
>  #define get_cycles_hi get_cycles_hi
>  #endif /* CONFIG_64BIT */
>
> +/*
> + * Much like MIPS, we may not have a viable counter to use at an early point
> + * in the boot process. Unfortunately we don't have a fallback, so instead
> + * we just return 0.
> + */
> +static inline unsigned long random_get_entropy(void)
> +{
> +	if (unlikely(clint_time_val == NULL))
> +		return 0;
> +	return get_cycles();
> +}
> +#define random_get_entropy()	random_get_entropy()
> +
>  #else /* CONFIG_RISCV_M_MODE */
>
>  static inline cycles_t get_cycles(void)

Reviewed-by: Palmer Dabbelt <palmerdabbelt@xxxxxxxxxx>

I'm going to wait for Damien to chime in about the NULL initialization boot
failure, though, as I'm a bit worried something else was going on.

Thanks!