Unverified Commit 39cefc5f authored by Palmer Dabbelt's avatar Palmer Dabbelt
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RISC-V: Fix a race condition during kernel stack overflow 

This fixes a concrete bug but is also the basis for some cleanup work,
so I'm merging it based on the offending commit in order to minimize
future conflicts.

* commit '7e186433':
  riscv: fix race when vmap stack overflow
parents d556a9ae 7e186433

arch/riscv/include/asm/asm.h

+1 −0
Original line number Diff line number Diff line
@@ -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)

arch/riscv/kernel/entry.S

+13 −0
Original line number Diff line number Diff line
@@ -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

arch/riscv/kernel/traps.c

+18 −0
Original line number Diff line number Diff line
@@ -221,11 +221,29 @@ 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");