[PATCH 6.0 105/124] riscv: fix race when vmap stack overflow

Greg Kroah-Hartman
2022-12-05T19:27:25Z

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 1b471ff73178..816e753de636 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 b9eda3fcbd6d..186abd146eaf 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -404,6 +404,19 @@ 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 635e6ec26938..6e8822446069 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");
--
2.35.1