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

[Damien Le Moal <Damien.LeMoal@wdc.com>]

On Sat, 2020-09-26 at 22:46 -0700, Palmer Dabbelt wrote:
> 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@google.com>
> > Signed-off-by: Anup Patel <anup.patel@wdc.com>
> > ---
> > 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@google.com>
> 
> 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!

For Kendryte, no problems. Boots correctly.

Tested-by: Damien Le Moal <damien.lemoal@wdc.com>



-- 
Damien Le Moal
Western Digital
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