Re: [PATCH] MIPS: BMIPS: Reserve exception base to prevent corruption

[Serge Semin]

On Mon, Mar 01, 2021 at 08:19:38PM -0800, Florian Fainelli wrote:
> BMIPS is one of the few platforms that do change the exception base.
> After commit 2dcb39645441 ("memblock: do not start bottom-up allocations
> with kernel_end") we started seeing BMIPS boards fail to boot with the
> built-in FDT being corrupted.
> 
> Before the cited commit, early allocations would be in the [kernel_end,
> RAM_END] range, but after commit they would be within [RAM_START +
> PAGE_SIZE, RAM_END].
> 
> The custom exception base handler that is installed by
> bmips_ebase_setup() done for BMIPS5000 CPUs ends-up trampling on the
> memory region allocated by unflatten_and_copy_device_tree() thus
> corrupting the FDT used by the kernel.
> 
> To fix this, we need to perform an early reservation of the custom
> exception that is going to be installed and this needs to happen at
> plat_mem_setup() time to ensure that unflatten_and_copy_device_tree()
> finds a space that is suitable, away from reserved memory.
> 
> Huge thanks to Serget for analysing and proposing a solution to this
> issue.
> 
> Fixes: Fixes: 2dcb39645441 ("memblock: do not start bottom-up allocations with kernel_end")

> Debugged-by: Serge Semin <Sergey.Semin@xxxxxxxxxxxxxxxxxxxx>
> Reported-by: Kamal Dasu <kdasu.kdev@xxxxxxxxx>

I'd change the order of these two tags... 

> Signed-off-by: Florian Fainelli <f.fainelli@xxxxxxxxx>
> ---

> Thomas,
> 
> This is intended as a stop-gap solution for 5.12-rc1 and to be picked up
> by the stable team for 5.11. We should find a safer way to avoid these
> problems for 5.13 maybe.

Thomas, could you join the discussion? If we had a more clever
solution to reserve the exceptions table for each possibly affected
platform this patch could have been omitted.

> 
>  arch/mips/bmips/setup.c       | 22 ++++++++++++++++++++++
>  arch/mips/include/asm/traps.h |  2 ++
>  2 files changed, 24 insertions(+)
> 
> diff --git a/arch/mips/bmips/setup.c b/arch/mips/bmips/setup.c
> index 31bcfa4e08b9..0088bd45b892 100644
> --- a/arch/mips/bmips/setup.c
> +++ b/arch/mips/bmips/setup.c
> @@ -149,6 +149,26 @@ void __init plat_time_init(void)
>  	mips_hpt_frequency = freq;
>  }
>  
> +static void __init bmips_ebase_reserve(void)
> +{
> +	phys_addr_t base, size = VECTORSPACING * 64;
> +
> +	switch (current_cpu_type()) {
> +	default:
> +	case CPU_BMIPS4350:
> +		return;
> +	case CPU_BMIPS3300:
> +	case CPU_BMIPS4380:
> +		base = 0x0400;
> +		break;
> +	case CPU_BMIPS5000:
> +		base = 0x1000;
> +		break;
> +	}
> +
> +	memblock_reserve(base, size);
> +}
> +
>  void __init plat_mem_setup(void)
>  {
>  	void *dtb;
> @@ -169,6 +189,8 @@ void __init plat_mem_setup(void)
>  
>  	__dt_setup_arch(dtb);
>  
> +	bmips_ebase_reserve();
> +
>  	for (q = bmips_quirk_list; q->quirk_fn; q++) {
>  		if (of_flat_dt_is_compatible(of_get_flat_dt_root(),
>  					     q->compatible)) {
> diff --git a/arch/mips/include/asm/traps.h b/arch/mips/include/asm/traps.h
> index 6aa8f126a43d..0ba6bb7f9618 100644
> --- a/arch/mips/include/asm/traps.h
> +++ b/arch/mips/include/asm/traps.h
> @@ -14,6 +14,8 @@
>  #define MIPS_BE_FIXUP	1		/* return to the fixup code */
>  #define MIPS_BE_FATAL	2		/* treat as an unrecoverable error */
>  

> +#define VECTORSPACING 0x100	/* for EI/VI mode */

What about the same macro declared in arch/mips/kernel/traps.c? I'd suggest
to remove it from there and explicitly #include this header file into
the arch/mips/bmips/setup.c file.

-Sergey

> +
>  extern void (*board_be_init)(void);
>  extern int (*board_be_handler)(struct pt_regs *regs, int is_fixup);
>  
> -- 
> 2.25.1
>