[PATCH V2 1/2] x86/efi: Unmap EFI boot services code/data regions from efi_pgd

Sai Praneeth Prakhya
2018-10-26T21:40:23Z

Ideally, after kernel assumes control of the platform, firmware shouldn't
access EFI boot services code/data regions. But, it's noticed that this is
not so true in many x86 platforms. Hence, during boot, kernel reserves EFI
boot services code/data regions [1] and maps [2] them to efi_pgd so that
call to set_virtual_address_map() doesn't fail. After returning from
set_virtual_address_map(), kernel frees the reserved regions [3] but they
still remain mapped.

This means that any code that's running in efi_pgd address space (e.g: any
EFI runtime service) would still be able to access EFI boot services
code/data regions but the contents of these regions would have long been
over written by someone else as they are freed by efi_free_boot_services().
So, it's important to unmap these regions. After unmapping EFI boot
services code/data regions, any illegal access by buggy firmware to these
regions would result in page fault which will be handled by efi specific
fault handler.

Unmapping EFI boot services code/data regions will result in clearing
PAGE_PRESENT bit and it shouldn't bother L1TF cases because it's already
handled by protnone_mask() at arch/x86/include/asm/pgtable-invert.h.

[1] Please see efi_reserve_boot_services()
[2] Please see efi_map_region() -> __map_region()
[3] Please see efi_free_boot_services()

Signed-off-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Bhupesh Sharma <bhsharma@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
arch/x86/include/asm/pgtable_types.h | 2 ++
arch/x86/mm/pageattr.c | 26 ++++++++++++++++++++++++++
arch/x86/platform/efi/quirks.c | 25 +++++++++++++++++++++++++
3 files changed, 53 insertions(+)

diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
index b64acb08a62b..cda04ecf5432 100644
--- a/arch/x86/include/asm/pgtable_types.h
+++ b/arch/x86/include/asm/pgtable_types.h
@@ -566,6 +566,8 @@ extern pmd_t *lookup_pmd_address(unsigned long address);
extern phys_addr_t slow_virt_to_phys(void *__address);
extern int kernel_map_pages_in_pgd(pgd_t *pgd, u64 pfn, unsigned long address,
unsigned numpages, unsigned long page_flags);
+extern int kernel_unmap_pages_in_pgd(pgd_t *pgd, unsigned long address,
+ unsigned long numpages);
#endif /* !__ASSEMBLY__ */

#endif /* _ASM_X86_PGTABLE_DEFS_H */
diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index 51a5a69ecac9..248f16181bed 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -2147,6 +2147,32 @@ int kernel_map_pages_in_pgd(pgd_t *pgd, u64 pfn, unsigned long address,
return retval;
}

+int kernel_unmap_pages_in_pgd(pgd_t *pgd, unsigned long address,
+ unsigned long numpages)
+{
+ int retval;
+
+ /*
+ * The typical sequence for unmapping is to find a pte through
+ * lookup_address_in_pgd() (ideally, it should never return NULL because
+ * the address is already mapped) and change it's protections.
+ * As pfn is the *target* of a mapping, it's not useful while unmapping.
+ */
+ struct cpa_data cpa = {
+ .vaddr = &address,
+ .pgd = pgd,
+ .numpages = numpages,
+ .mask_set = __pgprot(0),
+ .mask_clr = __pgprot(_PAGE_PRESENT | _PAGE_RW),
+ .flags = 0,
+ };
+
+ retval = __change_page_attr_set_clr(&cpa, 0);
+ __flush_tlb_all();
+
+ return retval;
+}
+
/*
* The testcases use internal knowledge of the implementation that shouldn't
* be exposed to the rest of the kernel. Include these directly here.
diff --git a/arch/x86/platform/efi/quirks.c b/arch/x86/platform/efi/quirks.c
index 669babcaf245..fb1c44b11235 100644
--- a/arch/x86/platform/efi/quirks.c
+++ b/arch/x86/platform/efi/quirks.c
@@ -370,6 +370,24 @@ void __init efi_reserve_boot_services(void)
}
}

+/*
+ * Apart from having VA mappings for EFI boot services code/data regions,
+ * (duplicate) 1:1 mappings were also created as a quirk for buggy firmware. So,
+ * unmap both 1:1 and VA mappings.
+ */
+static void __init efi_unmap_pages(efi_memory_desc_t *md)
+{
+ pgd_t *pgd = efi_mm.pgd;
+ u64 pa = md->phys_addr;
+ u64 va = md->virt_addr;
+
+ if (kernel_unmap_pages_in_pgd(pgd, pa, md->num_pages))
+ pr_err("Failed to unmap 1:1 mapping for 0x%llx\n", pa);
+
+ if (kernel_unmap_pages_in_pgd(pgd, va, md->num_pages))
+ pr_err("Failed to unmap VA mapping for 0x%llx\n", va);
+}
+
void __init efi_free_boot_services(void)
{
phys_addr_t new_phys, new_size;
@@ -394,6 +412,13 @@ void __init efi_free_boot_services(void)
continue;
}

+ /*
+ * Before calling set_virtual_address_map(), EFI boot services
+ * code/data regions were mapped as a quirk for buggy firmware.
+ * Unmap them from efi_pgd before freeing them up.
+ */
+ efi_unmap_pages(md);
+
/*
* Nasty quirk: if all sub-1MB memory is used for boot
* services, we can get here without having allocated the
--
2.19.1