Re: [PATCH] x86/boot/KASLR: exclude EFI_BOOT_SERVICES_{CODE|DATA} from KASLR's choice

Naoya Horiguchi
2017-07-06T09:37:13Z

On Thu, Jul 06, 2017 at 05:18:09PM +0800, Baoquan He wrote:
> Hi Naoya Horiguchi,
>
> Thanks for making this!
>
> On 07/06/17 at 08:31am, Naoya Horiguchi wrote:
> > Hi Baoquan, everyone,
> >
> > I'm also interested in KASLR/EFI related issue (but not the same issue
> > with yours, so I separated the thread.)
> >
> > This patch is based on Baoquan's recent patches[1], adding more code
> > on the new function process_efi_entry().
> > If it's OK, could you queue this onto your tree/series?
>
> This is interesting. So you are suggesting that we should try to avoid
> those EFI_BOOT_SERVICES_{CODE|DATA} efi regions as long as efi map
> regions are available, meanwhile try to locate kernel inside mirrored
> regions if existed. I do know the efi work around, so it seems reasonable
> to me, I can add it when repost. Or you can post after mine has been
> merged.

Thank you for the positive response, Baoquan.

>
> A little adjustment, please see the inline comment.
> >
> > [1] "[PATCH v3 0/2] x86/boot/KASLR: Restrict kernel to be randomized"
> > https://lkml.org/lkml/2017/7/5/98
> >
> > Thanks,
> > Naoya Horiguchi
> > ---
> > From: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> > Date: Thu, 6 Jul 2017 16:40:52 +0900
> > Subject: [PATCH] x86/boot/KASLR: exclude EFI_BOOT_SERVICES_{CODE|DATA} from
> > KASLR's choice
> >
> > KASLR chooses kernel location from E820_TYPE_RAM regions by walking over
> > e820 entries now. E820_TYPE_RAM includes EFI_BOOT_SERVICES_CODE and
> > EFI_BOOT_SERVICES_DATA, so those regions can be the target. According to
> > UEFI spec, all memory regions marked as EfiBootServicesCode and
> > EfiBootServicesData are available for free memory after the first call
> > of ExitBootServices(). So such regions should be usable for kernel on
> > spec basis.
> >
> > In x86, however, we have some workaround for broken firmware, where we
> > keep such regions reserved until SetVirtualAddressMap() is done.
> > See the following code in should_map_region():
> >
> > static bool should_map_region(efi_memory_desc_t *md)
> > {
> > ...
> > /*
> > * Map boot services regions as a workaround for buggy
> > * firmware that accesses them even when they shouldn't.
> > *
> > * See efi_{reserve,free}_boot_services().
> > */
> > if (md->type == EFI_BOOT_SERVICES_CODE ||
> > md->type == EFI_BOOT_SERVICES_DATA)
> > return false;
> >
> > This workaround suppressed a boot crash, but potential issues still
> > remain because no one prevents the regions from overlapping with kernel
> > image by KASLR.
> >
> > So let's make sure that EFI_BOOT_SERVICES_{CODE|DATA} regions are never
> > chosen as kernel memory for the workaround to work fine.
> >
> > Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> > ---
> > arch/x86/boot/compressed/kaslr.c | 41 +++++++++++++++++++++++++++++++---------
> > 1 file changed, 32 insertions(+), 9 deletions(-)
> >
> > diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> > index 94f08fd375ae..f43fed0441a6 100644
> > --- a/arch/x86/boot/compressed/kaslr.c
> > +++ b/arch/x86/boot/compressed/kaslr.c
> > @@ -563,7 +563,8 @@ static void process_mem_region(struct mem_vector *entry,
> > /* Marks if efi mirror regions have been found and handled. */
> > static bool efi_mirror_found;
> >
> > -static void process_efi_entry(unsigned long minimum, unsigned long image_size)
> > +/* Returns true if we really enter efi memmap walk, otherwise returns false. */
> > +static bool process_efi_entry(unsigned long minimum, unsigned long image_size)
> > {
> > struct efi_info *e = &boot_params->efi_info;
> > struct mem_vector region;
> > @@ -577,13 +578,13 @@ static void process_efi_entry(unsigned long minimum, unsigned long image_size)
> > signature = (char *)&boot_params->efi_info.efi_loader_signature;
> > if (strncmp(signature, EFI32_LOADER_SIGNATURE, 4) &&
> > strncmp(signature, EFI64_LOADER_SIGNATURE, 4))
> > - return;
> > + return false;
> >
> > #ifdef CONFIG_X86_32
> > /* Can't handle data above 4GB at this time */
> > if (e->efi_memmap_hi) {
> > warn("Memory map is above 4GB, EFI should be disabled.\n");
> > - return;
> > + return false;
> > }
> > pmap = e->efi_memmap;
> > #else
> > @@ -593,13 +594,36 @@ static void process_efi_entry(unsigned long minimum, unsigned long image_size)
> > nr_desc = e->efi_memmap_size / e->efi_memdesc_size;
> > for (i = 0; i < nr_desc; i++) {
> > md = (efi_memory_desc_t *)(pmap + (i * e->efi_memdesc_size));
> > - if (md->attribute & EFI_MEMORY_MORE_RELIABLE) {
> > - region.start = md->phys_addr;
> > - region.size = md->num_pages << EFI_PAGE_SHIFT;
> > - process_mem_region(&region, minimum, image_size);
> > + if (md->attribute & EFI_MEMORY_MORE_RELIABLE)
> > efi_mirror_found = true;
>
> Here, we should define a local variable of bool type to mark if mirrored
> region is found.

OK. efi_mirror_found need not be a global variable any longer.

> > + }
> > +
> > + for (i = 0; i < nr_desc; i++) {
> > + md = (efi_memory_desc_t *)(pmap + (i * e->efi_memdesc_size));
> > +
> > + /*
> > + * EFI_BOOT_SERVICES_{CODE|DATA} are avoided because boot
> > + * services regions could be accessed after ExitBootServices()
> > + * due to the workaround for buggy firmware.
> > + */
> > + if (!(md->type == EFI_LOADER_CODE ||
> > + md->type == EFI_LOADER_DATA ||
> > + md->type == EFI_CONVENTIONAL_MEMORY))
> > + continue;
> > +
> > + if (efi_mirror_found &&
> > + !(md->attribute & EFI_MEMORY_MORE_RELIABLE))
> > + continue;
> > +
> > + region.start = md->phys_addr;
> > + region.size = md->num_pages << EFI_PAGE_SHIFT;
> > + process_mem_region(&region, minimum, image_size);
>
> And can define a new global variable like efi_processed here.
> efi_processed = true;

I intented that this is represented by the return value of process_efi_entry(),
but if you have more readable/clearer options, I'm fine with that.

>
>
> And yes, I missed this snippet of code to break loop if slot_area has
> been full, this saves time.

No problem, feel free to move this snippet to your 2/2 patch.

Thanks,
Naoya Horiguchi