arm64: mm: Take potential load offset into account when KASLR is off

We enable CONFIG_RELOCATABLE even when CONFIG_RANDOMIZE_BASE is
disabled, and this permits the loader (i.e., EFI) to place the kernel
anywhere in physical memory as long as the base address is 64k aligned.

This means that the 'KASLR' case described in the header that defines
the size of the statically allocated page tables could take effect even
when CONFIG_RANDMIZE_BASE=n. So check for CONFIG_RELOCATABLE instead.

Signed-off-by: Ard Biesheuvel <[email protected]>
Reviewed-by: Anshuman Khandual <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Will Deacon <[email protected]>

Author: ardbiesheuvel

arch/arm64/include/asm/kernel-pgtable.h

@@ -37,27 +37,12 @@
 
 
 /*
- * If KASLR is enabled, then an offset K is added to the kernel address
- * space. The bottom 21 bits of this offset are zero to guarantee 2MB
- * alignment for PA and VA.
- *
- * For each pagetable level of the swapper, we know that the shift will
- * be larger than 21 (for the 4KB granule case we use section maps thus
- * the smallest shift is actually 30) thus there is the possibility that
- * KASLR can increase the number of pagetable entries by 1, so we make
- * room for this extra entry.
- *
- * Note KASLR cannot increase the number of required entries for a level
- * by more than one because it increments both the virtual start and end
- * addresses equally (the extra entry comes from the case where the end
- * address is just pushed over a boundary and the start address isn't).
+ * A relocatable kernel may execute from an address that differs from the one at
+ * which it was linked. In the worst case, its runtime placement may intersect
+ * with two adjacent PGDIR entries, which means that an additional page table
+ * may be needed at each subordinate level.
  */
-
-#ifdef CONFIG_RANDOMIZE_BASE
-#define EARLY_KASLR	(1)
-#else
-#define EARLY_KASLR	(0)
-#endif
+#define EXTRA_PAGE	__is_defined(CONFIG_RELOCATABLE)
 
 #define SPAN_NR_ENTRIES(vstart, vend, shift) \
 	((((vend) - 1) >> (shift)) - ((vstart) >> (shift)) + 1)
@@ -83,7 +68,7 @@
 			+ EARLY_PGDS((vstart), (vend), add) 	/* each PGDIR needs a next level page table */	\
 			+ EARLY_PUDS((vstart), (vend), add)	/* each PUD needs a next level page table */	\
 			+ EARLY_PMDS((vstart), (vend), add))	/* each PMD needs a next level page table */
-#define INIT_DIR_SIZE (PAGE_SIZE * EARLY_PAGES(KIMAGE_VADDR, _end, EARLY_KASLR))
+#define INIT_DIR_SIZE (PAGE_SIZE * EARLY_PAGES(KIMAGE_VADDR, _end, EXTRA_PAGE))
 
 /* the initial ID map may need two extra pages if it needs to be extended */
 #if VA_BITS < 48