Re: [PATCH v2 3/8] ARM: mmu: use reserve mem entries to modify maps

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

On 09.08.21 20:30, Sascha Hauer wrote:
> On Thu, Aug 05, 2021 at 04:06:42PM +0200, Ahmad Fatoum wrote:
>> On 03.08.21 11:44, Rouven Czerwinski wrote:
>>> Use the information from the reserved memory entries to modify the
>>> mapping of memory regions to mark them as uncachable and not-executable.
>>> This also prevents the processor from speculating into these regions,
>>> preventing hard to debug scenarios where boots fail for unknown reasons.
>>>
>>> Signed-off-by: Rouven Czerwinski <r.czerwinski@pengutronix.de>
>>> ---
>>>  arch/arm/cpu/mmu.c | 34 ++++++++++++++++++++++++++++++++--
>>>  1 file changed, 32 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/arch/arm/cpu/mmu.c b/arch/arm/cpu/mmu.c
>>> index 6388e1bf14..73f4cf5b36 100644
>>> --- a/arch/arm/cpu/mmu.c
>>> +++ b/arch/arm/cpu/mmu.c
>>> @@ -17,6 +17,7 @@
>>>  #include <memory.h>
>>>  #include <asm/system_info.h>
>>>  #include <asm/sections.h>
>>> +#include <of.h>
>>>  
>>>  #include "mmu.h"
>>>  
>>> @@ -407,6 +408,36 @@ static void vectors_init(void)
>>>  	create_vector_table(ARM_LOW_VECTORS);
>>>  }
>>>  
>>> +static void create_sections_with_intersect(struct memory_bank *bank)
>>> +{
>>> +	struct of_reserve_map *res_map;
>>> +	unsigned long j_end;
>>> +	unsigned long end;
>>> +	unsigned long j;
>>> +	u32 pmd_flags;
>>> +	int i;
>>> +
>>> +	res_map = of_get_reserve_map();
>>> +	if (!res_map)
>>> +		return;
>>
>> If there is no reserve map, you should still map the banks cached.
>> So this early exit is wrong.
>>
>>> +
>>> +	end = bank->start + bank->size - 1;
>>> +
>>> +	for (j = bank->start; j < end; j += PGDIR_SIZE) {
>>> +		pmd_flags = PMD_SECT_DEF_CACHED;
>>> +		j_end = j + PGDIR_SIZE - 1;
>>> +
>>> +		for (i = 0; i < res_map->num_entries; i++) {
>>> +			if ((BIT(i) & res_map->xn) &&
>>> +			    j_end >= res_map->start[i] &&
>>> +			    j_end <= res_map->end[i])
>>> +				pmd_flags = PMD_SECT_DEF_UNCACHED | PMD_SECT_XN;
>>> +		}
>>> +
>>> +		create_sections(ttb, j, j_end, pmd_flags);
>>> +	}
>>
>> Do we need the nested loop? Can't we create sections cached first
>> and then iterate once over the reserved map, deduce the sections containing
>> start and end and map those uncached?
> 
> Wouldn't that open a window to speculate into the areas we want to avoid
> speculating into in the first place?

Right. Forgot that MMU is enabled here.

> 
> Sascha
> 


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