Re: [PATCH] efi: get_memory_map: add sufficient slack for memory descriptors

[Mark Rutland <mark.rutland-5wv7dgnIgG8@public.gmane.org>]

On Thu, Feb 12, 2015 at 10:39:46AM +0000, Ard Biesheuvel wrote:
> On 12 February 2015 at 18:22, Mark Rutland <mark.rutland-5wv7dgnIgG8@public.gmane.org> wrote:
> > On Thu, Feb 12, 2015 at 05:24:19AM +0000, Ard Biesheuvel wrote:
> >> As it turns out, when allocating room for the UEFI memory map using
> >> UEFI's AllocatePool (), it may result in two new memory map entries
> >> being created, for instance, when using Tianocore's preallocated region
> >> feature. For example, the following region
> >>
> >> 0x00005ead5000-0x00005ebfffff [Conventional Memory|   |  |  |  |   |WB|WT|WC|UC]
> >>
> >> may be split like this
> >>
> >> 0x00005ead5000-0x00005eae2fff [Conventional Memory|   |  |  |  |   |WB|WT|WC|UC]
> >> 0x00005eae3000-0x00005eae4fff [Loader Data        |   |  |  |  |   |WB|WT|WC|UC]
> >> 0x00005eae5000-0x00005ebfffff [Conventional Memory|   |  |  |  |   |WB|WT|WC|UC]
> >>
> >> if the preallocated Loader Data region was chosen to be right in the
> >> middle of the original free space.
> >>
> >> After patch d1a8d66b9177 ("efi/libstub: Call get_memory_map() to
> >> obtain map and desc sizes"), this is not being dealt with correctly
> >> anymore, as the existing logic to allocate room for a single additional
> >> entry has become insufficient.
> >>
> >> So instead, add room for two additional entries instead.
> >>
> >> Fixes: d1a8d66b9177 ("efi/libstub: Call get_memory_map() to obtain map and desc sizes")
> >> Signed-off-by: Ard Biesheuvel <ard.biesheuvel-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
> >> ---
> >>  drivers/firmware/efi/libstub/efi-stub-helper.c | 6 +++---
> >>  1 file changed, 3 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/drivers/firmware/efi/libstub/efi-stub-helper.c b/drivers/firmware/efi/libstub/efi-stub-helper.c
> >> index af5d63c7cc53..ca0b07ed3b14 100644
> >> --- a/drivers/firmware/efi/libstub/efi-stub-helper.c
> >> +++ b/drivers/firmware/efi/libstub/efi-stub-helper.c
> >> @@ -84,10 +84,10 @@ efi_status_t efi_get_memory_map(efi_system_table_t *sys_table_arg,
> >>               return EFI_LOAD_ERROR;
> >>
> >>       /*
> >> -      * Add an additional efi_memory_desc_t because we're doing an
> >> -      * allocation which may be in a new descriptor region.
> >> +      * Add room for two additional efi_memory_desc_t entries because we're
> >> +      * doing an allocation which may be in a new descriptor region.
> >
> > It might be worth noting that a existing regions can be
> > split/reorganised here, otherwise it's a little difficult to deduce from
> > the comment why to regions are needed.
> >
> 
> OK
> 
> >>        */
> >> -     *map_size += *desc_size;
> >> +     *map_size += *desc_size * 2;
> >
> > Can we forsee any cases where we might need more than two additional
> > descs? Is it perhaps adding a little more slack now?
> >
> 
> I don't see how doing a single allocation could result in a single
> free region to be split into more than 1 occupied region + 2 free
> regions.
> So no, I don't think it is ...

Fair enough. If we're surprised later we can always fix things up.

With the comment fix up:

Acked-by: Mark Rutland <mark.rutland-5wv7dgnIgG8@public.gmane.org>

Thanks,
Mark.