Re: [PATCH v2] powerpc: Fix irq_soft_mask_set() and irq_soft_mask_return() with sanitizer

["Nicholas Piggin" <npiggin@gmail.com>]

On Wed Aug 24, 2022 at 2:39 AM AEST, Christophe Leroy wrote:
> In ppc, compiler based sanitizer will generate instrument instructions
> around statement WRITE_ONCE(local_paca->irq_soft_mask, mask):
>
>    0xc000000000295cb0 <+0>:	addis   r2,r12,774
>    0xc000000000295cb4 <+4>:	addi    r2,r2,16464
>    0xc000000000295cb8 <+8>:	mflr    r0
>    0xc000000000295cbc <+12>:	bl      0xc00000000008bb4c <mcount>
>    0xc000000000295cc0 <+16>:	mflr    r0
>    0xc000000000295cc4 <+20>:	std     r31,-8(r1)
>    0xc000000000295cc8 <+24>:	addi    r3,r13,2354
>    0xc000000000295ccc <+28>:	mr      r31,r13
>    0xc000000000295cd0 <+32>:	std     r0,16(r1)
>    0xc000000000295cd4 <+36>:	stdu    r1,-48(r1)
>    0xc000000000295cd8 <+40>:	bl      0xc000000000609b98 <__asan_store1+8>
>    0xc000000000295cdc <+44>:	nop
>    0xc000000000295ce0 <+48>:	li      r9,1
>    0xc000000000295ce4 <+52>:	stb     r9,2354(r31)
>    0xc000000000295ce8 <+56>:	addi    r1,r1,48
>    0xc000000000295cec <+60>:	ld      r0,16(r1)
>    0xc000000000295cf0 <+64>:	ld      r31,-8(r1)
>    0xc000000000295cf4 <+68>:	mtlr    r0
>
> If there is a context switch before "stb     r9,2354(r31)", r31 may
> not equal to r13, in such case, irq soft mask will not work.
>
> The same problem occurs in irq_soft_mask_return() with
> READ_ONCE(local_paca->irq_soft_mask).

WRITE_ONCE doesn't require address generation to be atomic with the
store so this is a bug without sanitizer too. I have seen gcc put r13
into a nvgpr before.

READ_ONCE maybe could be argued is safe in this case because data
could be stale when you use it anyway, but pointless and risky
in some cases (imagine cpu offline -> store poison value to irq soft
mask.

> This patch partially reverts commit ef5b570d3700 ("powerpc/irq: Don't
> open code irq_soft_mask helpers") with a more modern inline assembly.
>
> Reported-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
> Fixes: ef5b570d3700 ("powerpc/irq: Don't open code irq_soft_mask helpers")
> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> ---
> v2: Use =m constraint for stb instead of m constraint
> ---
>  arch/powerpc/include/asm/hw_irq.h | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/hw_irq.h b/arch/powerpc/include/asm/hw_irq.h
> index 26ede09c521d..815420988ef3 100644
> --- a/arch/powerpc/include/asm/hw_irq.h
> +++ b/arch/powerpc/include/asm/hw_irq.h
> @@ -113,7 +113,11 @@ static inline void __hard_RI_enable(void)
>  
>  static inline notrace unsigned long irq_soft_mask_return(void)
>  {
> -	return READ_ONCE(local_paca->irq_soft_mask);
> +	unsigned long flags;
> +
> +	asm volatile("lbz%X1 %0,%1" : "=r" (flags) : "m" (local_paca->irq_soft_mask));
> +
> +	return flags;
>  }
>  
>  /*
> @@ -140,8 +144,7 @@ static inline notrace void irq_soft_mask_set(unsigned long mask)
>  	if (IS_ENABLED(CONFIG_PPC_IRQ_SOFT_MASK_DEBUG))
>  		WARN_ON(mask && !(mask & IRQS_DISABLED));
>  
> -	WRITE_ONCE(local_paca->irq_soft_mask, mask);
> -	barrier();
> +	asm volatile("stb%X0 %1,%0" : "=m" (local_paca->irq_soft_mask) : "r" (mask) : "memory");

This is still slightly concerning to me. Is there any guarantee that the
compiler would not use a different sequence for the address here?

Maybe explicit r13 is required.

Thanks,
Nick