Patch "riscv: fix race when vmap stack overflow" has been added to the 5.15-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    riscv: fix race when vmap stack overflow

to the 5.15-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     riscv-fix-race-when-vmap-stack-overflow.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 129f6b6589da3224c9052370bf89d08281c97728
Author: Jisheng Zhang <jszhang@xxxxxxxxxx>
Date:   Sun Oct 30 20:45:17 2022 +0800

    riscv: fix race when vmap stack overflow
    
    [ 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@xxxxxxxxxx>
    Suggested-by: Guo Ren <guoren@xxxxxxxxxx>
    Reviewed-by: Guo Ren <guoren@xxxxxxxxxx>
    Link: https://lore.kernel.org/r/20221030124517.2370-1-jszhang@xxxxxxxxxx
    [Palmer: Add AQ to the swap, and also some comments.]
    Signed-off-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

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");