[Crash-utility] [PATCH v5 3/3] kaslr: get offset by walking page tree
Alexey Makhalov
amakhalov at vmware.com
Mon Nov 9 23:14:32 UTC 2020
This method requires only valid CR3. It walks through
page tree starting from __START_KERNEL_map to get real
_stext and its physical address.
_stext_vmlinux has to be initialized. So, requesting it
by "st->_stext_vmlinux = UNINITIALIZED;" for sadump and
vmware backends. Other backends may use "--kaslr=auto"
to get similar effect.
Use this method as a default and IDTR base as a backup.
Signed-off-by: Alexey Makhalov <amakhalov at vmware.com>
Acked-by: Lianbo Jiang <lijiang at redhat.com>
---
kaslr_helper.c | 155 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
symbols.c | 5 +-
2 files changed, 157 insertions(+), 3 deletions(-)
diff --git a/kaslr_helper.c b/kaslr_helper.c
index f9ed56b..5945a3d 100644
--- a/kaslr_helper.c
+++ b/kaslr_helper.c
@@ -379,6 +379,152 @@ verify_kaslr_offset(ulong kaslr_offset)
}
/*
+ * Find virtual (VA) and physical (PA) addresses of kernel start
+ *
+ * va:
+ * Actual address of the kernel start (_stext) placed
+ * randomly by kaslr feature. To be more accurate,
+ * VA = _stext(from vmlinux) + kaslr_offset
+ *
+ * pa:
+ * Physical address where the kerenel is placed.
+ *
+ * In nokaslr case, VA = _stext (from vmlinux)
+ * In kaslr case, virtual address of the kernel placement goes
+ * in this range: ffffffff80000000..ffffffff9fffffff, or
+ * __START_KERNEL_map..+512MB
+ *
+ * https://www.kernel.org/doc/Documentation/x86/x86_64/mm.txt
+ *
+ * Randomized VA will be the first valid page starting from
+ * ffffffff80000000 (__START_KERNEL_map). Page tree entry of
+ * this page will contain the PA of the kernel start.
+ */
+static int
+find_kernel_start(uint64_t pgd, ulong *va, ulong *pa)
+{
+ int pgd_idx, p4d_idx, pud_idx, pmd_idx, pte_idx;
+ uint64_t pgd_pte = 0, pud_pte, pmd_pte, pte;
+
+ pgd_idx = pgd_index(__START_KERNEL_map);
+ if (machdep->flags & VM_5LEVEL)
+ p4d_idx = p4d_index(__START_KERNEL_map);
+ pud_idx = pud_index(__START_KERNEL_map);
+ pmd_idx = pmd_index(__START_KERNEL_map);
+ pte_idx = pte_index(__START_KERNEL_map);
+
+ /* If the VM is in 5-level page table */
+ if (machdep->flags & VM_5LEVEL)
+ *va = ~((1UL << 57) - 1);
+ else
+ *va = ~__VIRTUAL_MASK;
+
+ FILL_PGD(pgd & PHYSICAL_PAGE_MASK, PHYSADDR, PAGESIZE());
+ for (; pgd_idx < PTRS_PER_PGD; pgd_idx++) {
+ pgd_pte = ULONG(machdep->pgd + pgd_idx * sizeof(uint64_t));
+ if (pgd_pte & _PAGE_PRESENT)
+ break;
+ p4d_idx = pud_idx = pmd_idx = pte_idx = 0;
+ }
+ if (pgd_idx == PTRS_PER_PGD)
+ return FALSE;
+ *va |= (ulong)pgd_idx << __PGDIR_SHIFT;
+
+ if (machdep->flags & VM_5LEVEL) {
+ FILL_P4D(pgd_pte & PHYSICAL_PAGE_MASK, PHYSADDR, PAGESIZE());
+ for (; p4d_idx < PTRS_PER_P4D; p4d_idx++) {
+ /* reuse pgd_pte */
+ pgd_pte = ULONG(machdep->machspec->p4d + p4d_idx * sizeof(uint64_t));
+ if (pgd_pte & _PAGE_PRESENT)
+ break;
+ pud_idx = pmd_idx = pte_idx = 0;
+ }
+ if (p4d_idx == PTRS_PER_P4D)
+ return FALSE;
+ *va |= (ulong)p4d_idx << P4D_SHIFT;
+ }
+
+ FILL_PUD(pgd_pte & PHYSICAL_PAGE_MASK, PHYSADDR, PAGESIZE());
+ for (; pud_idx < PTRS_PER_PUD; pud_idx++) {
+ pud_pte = ULONG(machdep->pud + pud_idx * sizeof(uint64_t));
+ if (pud_pte & _PAGE_PRESENT)
+ break;
+ pmd_idx = pte_idx = 0;
+ }
+ if (pud_idx == PTRS_PER_PUD)
+ return FALSE;
+ *va |= (ulong)pud_idx << PUD_SHIFT;
+ if (pud_pte & _PAGE_PSE) {
+ /* 1GB page */
+ *pa = pud_pte & PHYSICAL_PAGE_MASK;
+ return TRUE;
+ }
+
+ FILL_PMD(pud_pte & PHYSICAL_PAGE_MASK, PHYSADDR, PAGESIZE());
+ for (; pmd_idx < PTRS_PER_PMD; pmd_idx++) {
+ pmd_pte = ULONG(machdep->pmd + pmd_idx * sizeof(uint64_t));
+ if (pmd_pte & _PAGE_PRESENT)
+ break;
+ pte_idx = 0;
+ }
+ if (pmd_idx == PTRS_PER_PMD)
+ return FALSE;
+ *va |= pmd_idx << PMD_SHIFT;
+ if (pmd_pte & _PAGE_PSE) {
+ /* 2MB page */
+ *pa = pmd_pte & PHYSICAL_PAGE_MASK;
+ return TRUE;
+ }
+
+ FILL_PTBL(pmd_pte & PHYSICAL_PAGE_MASK, PHYSADDR, PAGESIZE());
+ for (; pte_idx < PTRS_PER_PTE; pte_idx++) {
+ pte = ULONG(machdep->ptbl + pte_idx * sizeof(uint64_t));
+ if (pte & _PAGE_PRESENT)
+ break;
+ }
+ if (pte_idx == PTRS_PER_PTE)
+ return FALSE;
+
+ *va |= pte_idx << PAGE_SHIFT;
+ *pa = pmd_pte & PHYSICAL_PAGE_MASK;
+ return TRUE;
+}
+
+/*
+ * Page Tables based method to calculate kaslr_offset and phys_base.
+ * It uses VA and PA of kernel start.
+ *
+ * kaslr offset and phys_base are calculated as follows:
+ *
+ * kaslr_offset = VA - st->_stext_vmlinux
+ * phys_base = PA - (VA - __START_KERNEL_map)
+ */
+static int
+calc_kaslr_offset_from_page_tables(uint64_t pgd, ulong *kaslr_offset,
+ ulong *phys_base)
+{
+ ulong va, pa;
+
+ if (!st->_stext_vmlinux || st->_stext_vmlinux == UNINITIALIZED) {
+ fprintf(fp, "%s: st->_stext_vmlinux must be initialized\n",
+ __FUNCTION__);
+ return FALSE;
+ }
+ if (!find_kernel_start(pgd, &va, &pa))
+ return FALSE;
+
+ if (CRASHDEBUG(1)) {
+ fprintf(fp, "calc_kaslr_offset: _stext(vmlinux): %lx\n", st->_stext_vmlinux);
+ fprintf(fp, "calc_kaslr_offset: kernel start VA: %lx\n", va);
+ fprintf(fp, "calc_kaslr_offset: kernel start PA: %lx\n", pa);
+ }
+
+ *kaslr_offset = va - st->_stext_vmlinux;
+ *phys_base = pa - (va - __START_KERNEL_map);
+ return TRUE;
+}
+
+/*
* IDT based method to calculate kaslr_offset and phys_base
*
* kaslr offset and phys_base are calculated as follows:
@@ -534,8 +680,13 @@ calc_kaslr_offset(ulong *ko, ulong *pb)
"pgd", RETURN_ON_ERROR))
continue;
- if (!calc_kaslr_offset_from_idt(idtr, pgd, &kaslr_offset, &phys_base))
- continue;
+ if (!calc_kaslr_offset_from_page_tables(pgd, &kaslr_offset,
+ &phys_base)) {
+ if (!calc_kaslr_offset_from_idt(idtr, pgd,
+ &kaslr_offset,
+ &phys_base))
+ continue;
+ }
if (verify_kaslr_offset(kaslr_offset))
goto found;
diff --git a/symbols.c b/symbols.c
index 70b1455..08e9995 100644
--- a/symbols.c
+++ b/symbols.c
@@ -634,8 +634,11 @@ kaslr_init(void)
}
}
- if (SADUMP_DUMPFILE() || VMSS_DUMPFILE())
+ if (SADUMP_DUMPFILE() || QEMU_MEM_DUMP_NO_VMCOREINFO() || VMSS_DUMPFILE()) {
+ /* Need for kaslr_offset and phys_base */
kt->flags2 |= KASLR_CHECK;
+ st->_stext_vmlinux = UNINITIALIZED;
+ }
}
/*
--
2.11.0
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