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SubjectRe: [PATCH v3] RISC-V: Check clint_time_val before use
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On Sat, 26 Sep 2020 22:52:19 PDT (-0700), Damien Le Moal wrote:
> On Sun, 2020-09-27 at 11:09 +0530, 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)
>
> Did not reply to the patch... So again for Kendryte:
>
> Tested-by: Damien Le Moal <damien.lemoal@wdc.com>

Thanks. I've put this on for-next, right after the patch I just posted to add
EXPORT_SYMBOL(clint_time_val) as it turns out random_get_entropy() is used by a
driver.

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