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

[Sascha Hauer <sha@pengutronix.de>]

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?

Sascha

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