Re: [PATCH 2/2] powerpc/64s/interrupt: Check and fix srr_valid without crashing

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 22/06/2021 à 08:04, Nicholas Piggin a écrit :
> The PPC_RFI_SRR_DEBUG check added by patch "powerpc/64s: avoid reloading
> (H)SRR registers if they are still valid" has a few deficiencies. It
> does not fix the actual problem, it's not enabled by default, and it
> causes a program check interrupt which can cause more difficulties.
> 
> However there are a lot of paths which may clobber SRRs or change return
> regs, and difficult to have a high confidence that all paths are covered
> without wider testing.
> 
> Add a relatively low overhead always-enabled check that catches most
> such cases, reports once, and fixes it so the kernel can continue.
> 
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>   arch/powerpc/kernel/interrupt.c | 58 +++++++++++++++++++++++++++++++++
>   1 file changed, 58 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
> index 05fa3ae56e25..5920a3e8d1d5 100644
> --- a/arch/powerpc/kernel/interrupt.c
> +++ b/arch/powerpc/kernel/interrupt.c
> @@ -231,6 +231,56 @@ static notrace void booke_load_dbcr0(void)
>   #endif
>   }
>   
> +#include <linux/sched/debug.h> /* for show_regs */
> +static void check_return_regs_valid(struct pt_regs *regs)
> +{
> +#ifdef CONFIG_PPC_BOOK3S_64
> +	static bool warned = false;
> +
> +	if (regs->trap == 0x980 || regs->trap == 0xe00 || regs->trap == 0xe20 ||
> +	    regs->trap == 0xe40 || regs->trap == 0xe60 || regs->trap == 0xe80 ||
> +	    regs->trap == 0xea0 || regs->trap == 0xf80 || regs->trap == 0x1200 ||
> +	    regs->trap == 0x1500 || regs->trap == 0x1600 || regs->trap == 0x1800) {

Can you use names defined in asm/interrupt.h instead of raw values ?
Some are already there, others can be added.


> +		if (local_paca->hsrr_valid) {
> +			unsigned long hsrr0 = mfspr(SPRN_HSRR0);
> +			unsigned long hsrr1 = mfspr(SPRN_HSRR1);
> +
> +			if (hsrr0 == regs->nip && hsrr1 == regs->msr)
> +				return;
> +
> +			if (!warned) {
> +				warned = true;
> +				printk("HSRR0 was: %lx should be: %lx\n",
> +					hsrr0, regs->nip);
> +				printk("HSRR1 was: %lx should be: %lx\n",
> +					hsrr1, regs->msr);
> +				show_regs(regs);
> +			}
> +			local_paca->hsrr_valid = 0; /* fixup */
> +		}
> +
> +	} else if (regs->trap != 0x3000) {
> +		if (local_paca->srr_valid) {
> +			unsigned long srr0 = mfspr(SPRN_SRR0);
> +			unsigned long srr1 = mfspr(SPRN_SRR1);
> +
> +			if (srr0 == regs->nip && srr1 == regs->msr)
> +				return;
> +
> +			if (!warned) {
> +				warned = true;
> +				printk("SRR0 was: %lx should be: %lx\n",
> +					srr0, regs->nip);
> +				printk("SRR1 was: %lx should be: %lx\n",
> +					srr1, regs->msr);
> +				show_regs(regs);
> +			}
> +			local_paca->srr_valid = 0; /* fixup */
> +		}
> +	}
> +#endif
> +}
> +
>   /*
>    * This should be called after a syscall returns, with r3 the return value
>    * from the syscall. If this function returns non-zero, the system call
> @@ -327,6 +377,8 @@ notrace unsigned long syscall_exit_prepare(unsigned long r3,
>   		}
>   	}
>   
> +	check_return_regs_valid(regs);
> +
>   	user_enter_irqoff();
>   
>   	/* scv need not set RI=0 because SRRs are not used */
> @@ -405,6 +457,8 @@ notrace unsigned long interrupt_exit_user_prepare(struct pt_regs *regs)
>   		}
>   	}
>   
> +	check_return_regs_valid(regs);
> +
>   	user_enter_irqoff();
>   
>   	if (unlikely(!__prep_irq_for_enabled_exit(true))) {
> @@ -469,9 +523,13 @@ notrace unsigned long interrupt_exit_kernel_prepare(struct pt_regs *regs)
>   			}
>   		}
>   
> +		check_return_regs_valid(regs);
> +
>   		if (unlikely(!prep_irq_for_enabled_exit(true, !irqs_disabled_flags(flags))))
>   			goto again;
>   	} else {
> +		check_return_regs_valid(regs);
> +
>   		/* Returning to a kernel context with local irqs disabled. */
>   		__hard_EE_RI_disable();
>   #ifdef CONFIG_PPC64
>