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Subject[PATCH v3] RISC-V: Check clint_time_val before use
Date
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)
--
2.25.1
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