From: Jisheng Zhang jszhang@kernel.org
[ Upstream commit 7e1864332fbc1b993659eab7974da9fe8bf8c128 ]
Currently, when detecting vmap stack overflow, riscv firstly switches to the so called shadow stack, then use this shadow stack to call the get_overflow_stack() to get the overflow stack. However, there's a race here if two or more harts use the same shadow stack at the same time.
To solve this race, we introduce spin_shadow_stack atomic var, which will be swap between its own address and 0 in atomic way, when the var is set, it means the shadow_stack is being used; when the var is cleared, it means the shadow_stack isn't being used.
Fixes: 31da94c25aea ("riscv: add VMAP_STACK overflow detection") Signed-off-by: Jisheng Zhang jszhang@kernel.org Suggested-by: Guo Ren guoren@kernel.org Reviewed-by: Guo Ren guoren@kernel.org Link: https://lore.kernel.org/r/20221030124517.2370-1-jszhang@kernel.org [Palmer: Add AQ to the swap, and also some comments.] Signed-off-by: Palmer Dabbelt palmer@rivosinc.com Signed-off-by: Sasha Levin sashal@kernel.org --- arch/riscv/include/asm/asm.h | 1 + arch/riscv/kernel/entry.S | 13 +++++++++++++ arch/riscv/kernel/traps.c | 18 ++++++++++++++++++ 3 files changed, 32 insertions(+)
diff --git a/arch/riscv/include/asm/asm.h b/arch/riscv/include/asm/asm.h index 618d7c5af1a2..e15a1c9f1cf8 100644 --- a/arch/riscv/include/asm/asm.h +++ b/arch/riscv/include/asm/asm.h @@ -23,6 +23,7 @@ #define REG_L __REG_SEL(ld, lw) #define REG_S __REG_SEL(sd, sw) #define REG_SC __REG_SEL(sc.d, sc.w) +#define REG_AMOSWAP_AQ __REG_SEL(amoswap.d.aq, amoswap.w.aq) #define REG_ASM __REG_SEL(.dword, .word) #define SZREG __REG_SEL(8, 4) #define LGREG __REG_SEL(3, 2) diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S index 7e52ad5d61ad..5ca2860cc06c 100644 --- a/arch/riscv/kernel/entry.S +++ b/arch/riscv/kernel/entry.S @@ -387,6 +387,19 @@ handle_syscall_trace_exit:
#ifdef CONFIG_VMAP_STACK handle_kernel_stack_overflow: + /* + * Takes the psuedo-spinlock for the shadow stack, in case multiple + * harts are concurrently overflowing their kernel stacks. We could + * store any value here, but since we're overflowing the kernel stack + * already we only have SP to use as a scratch register. So we just + * swap in the address of the spinlock, as that's definately non-zero. + * + * Pairs with a store_release in handle_bad_stack(). + */ +1: la sp, spin_shadow_stack + REG_AMOSWAP_AQ sp, sp, (sp) + bnez sp, 1b + la sp, shadow_stack addi sp, sp, SHADOW_OVERFLOW_STACK_SIZE
diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c index 8c58aa5d2b36..2f4cd85fb651 100644 --- a/arch/riscv/kernel/traps.c +++ b/arch/riscv/kernel/traps.c @@ -218,11 +218,29 @@ asmlinkage unsigned long get_overflow_stack(void) OVERFLOW_STACK_SIZE; }
+/* + * A pseudo spinlock to protect the shadow stack from being used by multiple + * harts concurrently. This isn't a real spinlock because the lock side must + * be taken without a valid stack and only a single register, it's only taken + * while in the process of panicing anyway so the performance and error + * checking a proper spinlock gives us doesn't matter. + */ +unsigned long spin_shadow_stack; + asmlinkage void handle_bad_stack(struct pt_regs *regs) { unsigned long tsk_stk = (unsigned long)current->stack; unsigned long ovf_stk = (unsigned long)this_cpu_ptr(overflow_stack);
+ /* + * We're done with the shadow stack by this point, as we're on the + * overflow stack. Tell any other concurrent overflowing harts that + * they can proceed with panicing by releasing the pseudo-spinlock. + * + * This pairs with an amoswap.aq in handle_kernel_stack_overflow. + */ + smp_store_release(&spin_shadow_stack, 0); + console_verbose();
pr_emerg("Insufficient stack space to handle exception!\n");