[PATCH v3] x86/efi: Correct ident mapping of efi old_map when kalsr enabled

Baoquan He
2017-05-13T03:56:51Z

For EFI with 'efi=old_map' kernel option specified, Kernel will panic
when kaslr is enabled.

The back trace is:

BUG: unable to handle kernel paging request at 000000007febd57e
IP: 0x7febd57e
PGD 1025a067
PUD 0

Oops: 0010 [#1] SMP
[ ... ]
Call Trace:
? efi_call+0x58/0x90
? printk+0x58/0x6f
efi_enter_virtual_mode+0x3c5/0x50d
start_kernel+0x40f/0x4b8
? set_init_arg+0x55/0x55
? early_idt_handler_array+0x120/0x120
x86_64_start_reservations+0x24/0x26
x86_64_start_kernel+0x14c/0x16f
start_cpu+0x14/0x14

The root cause is the ident mapping is not built correctly in old_map case.

For nokaslr kernel, PAGE_OFFSET is 0xffff880000000000 which is PGDIR_SIZE
aligned. We can borrow the pud table from direct mapping safely. Given a
physical address X, we have pud_index(X) == pud_index(__va(X)). However,
for kaslr kernel, PAGE_OFFSET is PUD_SIZE aligned. For a given physical
address X, pud_index(X) != pud_index(__va(X)). We can't only copy pgd entry
from direct mapping to build ident mapping, instead need copy pud entry
one by one from direct mapping.

Fix it.

Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Dave Young <dyoung@redhat.com>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Thomas Garnier <thgarnie@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: x86@kernel.org
Cc: linux-efi@vger.kernel.org
---
v2->v3:
1. Rewrite code to copy pud entry one by one so that code can be understood
better. Usually we only have less than 1TB or several TB memory, pud entry
copy one by one won't impact efficiency.

2. Adding p4d page table handling.

v1->v2:
Change code and add description according to Thomas's suggestion as below:

1. Add checking if pud table is allocated successfully. If not just break
the for loop.

2. Add code comment to explain how the 1:1 mapping is built in efi_call_phys_prolog

3. Other minor change

arch/x86/platform/efi/efi_64.c | 69 +++++++++++++++++++++++++++++++++++++-----
1 file changed, 61 insertions(+), 8 deletions(-)

diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
index c488625..c9dffec 100644
--- a/arch/x86/platform/efi/efi_64.c
+++ b/arch/x86/platform/efi/efi_64.c
@@ -71,11 +71,13 @@ static void __init early_code_mapping_set_exec(int executable)

pgd_t * __init efi_call_phys_prolog(void)
{
- unsigned long vaddress;
- pgd_t *save_pgd;
+ unsigned long vaddr, addr_pgd, addr_p4d, addr_pud;
+ pgd_t *save_pgd, *pgd_k, *pgd_efi;
+ p4d_t *p4d, *p4d_k, *p4d_efi;
+ pud_t *pud;

int pgd;
- int n_pgds;
+ int n_pgds, i, j;

if (!efi_enabled(EFI_OLD_MEMMAP)) {
save_pgd = (pgd_t *)read_cr3();
@@ -88,10 +90,44 @@ pgd_t * __init efi_call_phys_prolog(void)
n_pgds = DIV_ROUND_UP((max_pfn << PAGE_SHIFT), PGDIR_SIZE);
save_pgd = kmalloc_array(n_pgds, sizeof(*save_pgd), GFP_KERNEL);

+ /*
+ * Build 1:1 ident mapping for old_map usage. It needs to be noticed
+ * that PAGE_OFFSET is PGDIR_SIZE aligned with KASLR disabled, while
+ * PUD_SIZE ALIGNED with KASLR enabled. So for a given physical
+ * address X, the pud_index(X) != pud_index(__va(X)), we can only copy
+ * pud entry of __va(X) to fill in pud entry of X to build 1:1 mapping
+ * . Means here we can only reuse pmd table of direct mapping.
+ */
for (pgd = 0; pgd < n_pgds; pgd++) {
- save_pgd[pgd] = *pgd_offset_k(pgd * PGDIR_SIZE);
- vaddress = (unsigned long)__va(pgd * PGDIR_SIZE);
- set_pgd(pgd_offset_k(pgd * PGDIR_SIZE), *pgd_offset_k(vaddress));
+ addr_pgd = (unsigned long)(pgd * PGDIR_SIZE);
+ vaddr = (unsigned long)__va(pgd * PGDIR_SIZE);
+ pgd_efi = pgd_offset_k(addr_pgd);
+ save_pgd[pgd] = *pgd_efi;
+ p4d = p4d_alloc(&init_mm, pgd_efi, addr_pgd);
+
+ if (!p4d) {
+ pr_err("Failed to allocate p4d table \n");
+ goto out;
+ }
+ for(i=0; i<PTRS_PER_P4D; i++) {
+ addr_p4d = addr_pgd + i * P4D_SIZE;
+ p4d_efi = p4d + p4d_index(addr_p4d);
+ pud = pud_alloc(&init_mm, p4d_efi, addr_p4d);
+ if (!pud) {
+ pr_err("Failed to allocate pud table \n");
+ goto out;
+ }
+ for(j=0; j<PTRS_PER_PUD; j++) {
+ addr_pud = addr_p4d + j * PUD_SIZE;
+ if (addr_pud > (max_pfn << PAGE_SHIFT))
+ break;
+ vaddr = (unsigned long)__va(addr_pud);
+
+ pgd_k = pgd_offset_k(vaddr);
+ p4d_k = p4d_offset(pgd_k, vaddr);
+ pud[j] = *pud_offset(p4d_k, vaddr);
+ }
+ }
}
out:
__flush_tlb_all();
@@ -104,8 +140,11 @@ void __init efi_call_phys_epilog(pgd_t *save_pgd)
/*
* After the lock is released, the original page table is restored.
*/
- int pgd_idx;
+ int pgd_idx, i;
int nr_pgds;
+ pgd_t *pgd;
+ p4d_t *p4d;
+ pud_t *pud;

if (!efi_enabled(EFI_OLD_MEMMAP)) {
write_cr3((unsigned long)save_pgd);
@@ -115,9 +154,23 @@ void __init efi_call_phys_epilog(pgd_t *save_pgd)

nr_pgds = DIV_ROUND_UP((max_pfn << PAGE_SHIFT) , PGDIR_SIZE);

- for (pgd_idx = 0; pgd_idx < nr_pgds; pgd_idx++)
+ for (pgd_idx = 0; pgd_idx < nr_pgds; pgd_idx++) {
+ pgd = pgd_offset_k(pgd_idx * PGDIR_SIZE);
set_pgd(pgd_offset_k(pgd_idx * PGDIR_SIZE), save_pgd[pgd_idx]);

+ if (!(pgd_val(*pgd) & _PAGE_PRESENT))
+ continue;
+ for(i=0; i<PTRS_PER_P4D; i++) {
+ p4d = p4d_offset(pgd, pgd_idx * PGDIR_SIZE + i * P4D_SIZE);
+ if (!(p4d_val(*p4d) & _PAGE_PRESENT))
+ continue;
+ pud = (pud_t*)p4d_page_vaddr(*p4d);
+ pud_free(&init_mm, pud);
+ }
+ p4d = (p4d_t*)pgd_page_vaddr(*pgd);
+ p4d_free(&init_mm, p4d);
+ }
+
kfree(save_pgd);

__flush_tlb_all();
--
2.5.5