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

Naoya Horiguchi
2017-07-06T09:23:45Z

On Thu, Jul 06, 2017 at 05:13:32PM +0800, Chao Fan wrote:
> On Thu, Jul 06, 2017 at 08:31:07AM +0000, 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?
> >
> >[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;
>
> Hi Horiguchi-san,
>
> If efi_mirror_found is changed to be true, we won't need to walk other
> entries, so I think:
> if (md->attribute & EFI_MEMORY_MORE_RELIABLE) {
> efi_mirror_found = true;
> break;
> }
> will be enough to show that mirror regions exist. And will walk
> less entries. How do you think about this?

Thank you for the review, Chao.
And you're right, I'll add break here.

# I'll post revised one tomorrow waiting for more comments.

> Another question: what's the benifit of putting this part of
> "efi_mirror_found = true" to a independent cycle.

We can't easily cancel process_mem_region(), so if we process a few normal
regions like EFI_CONVENTIONAL_MEMORY and then find a EFI_MEMORY_MORE_RELIABLE
region, that's a bit troublesome.
So I decided to first check whether EFI_MEMORY_MORE_RELIABLE region exists or not.

Thanks,
Naoya Horiguchi