powerpc/mm: Enable full randomisation of memory mappings

Do like most other architectures and provide randomisation also to
"legacy" memory mappings, by adding the random factor to
mm->mmap_base in arch_pick_mmap_layout().

See commit 8b8addf891 ("x86/mm/32: Enable full randomization on
i386 and X86_32") for all explanations and benefits of that mmap
randomisation.

At the moment, slice_find_area_bottomup() doesn't use mm->mmap_base
but uses the fixed TASK_UNMAPPED_BASE instead.
slice_find_area_bottomup() being used as a fallback to
slice_find_area_topdown(), it can't use mm->mmap_base
directly.

Instead of always using TASK_UNMAPPED_BASE as base address, leave
it to the caller. When called from slice_find_area_topdown()
TASK_UNMAPPED_BASE is used. Otherwise mm->mmap_base is used.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/417fb10dde828534c73a03138b49621d74f4e5be.1649523076.git.christophe.leroy@csgroup.eu

arch/powerpc/mm/book3s64/slice.c

@@ -276,20 +276,18 @@ static bool slice_scan_available(unsigned long addr,
 }
 
 static unsigned long slice_find_area_bottomup(struct mm_struct *mm,
-					      unsigned long len,
+					      unsigned long addr, unsigned long len,
 					      const struct slice_mask *available,
 					      int psize, unsigned long high_limit)
 {
 	int pshift = max_t(int, mmu_psize_defs[psize].shift, PAGE_SHIFT);
-	unsigned long addr, found, next_end;
+	unsigned long found, next_end;
 	struct vm_unmapped_area_info info;
 
 	info.flags = 0;
 	info.length = len;
 	info.align_mask = PAGE_MASK & ((1ul << pshift) - 1);
 	info.align_offset = 0;
-
-	addr = TASK_UNMAPPED_BASE;
 	/*
 	 * Check till the allow max value for this mmap request
 	 */
@@ -322,12 +320,12 @@ static unsigned long slice_find_area_bottomup(struct mm_struct *mm,
 }
 
 static unsigned long slice_find_area_topdown(struct mm_struct *mm,
-					     unsigned long len,
+					     unsigned long addr, unsigned long len,
 					     const struct slice_mask *available,
 					     int psize, unsigned long high_limit)
 {
 	int pshift = max_t(int, mmu_psize_defs[psize].shift, PAGE_SHIFT);
-	unsigned long addr, found, prev;
+	unsigned long found, prev;
 	struct vm_unmapped_area_info info;
 	unsigned long min_addr = max(PAGE_SIZE, mmap_min_addr);
 
@@ -335,8 +333,6 @@ static unsigned long slice_find_area_topdown(struct mm_struct *mm,
 	info.length = len;
 	info.align_mask = PAGE_MASK & ((1ul << pshift) - 1);
 	info.align_offset = 0;
-
-	addr = mm->mmap_base;
 	/*
 	 * If we are trying to allocate above DEFAULT_MAP_WINDOW
 	 * Add the different to the mmap_base.
@@ -377,7 +373,7 @@ static unsigned long slice_find_area_topdown(struct mm_struct *mm,
 	 * can happen with large stack limits and large mmap()
 	 * allocations.
 	 */
-	return slice_find_area_bottomup(mm, len, available, psize, high_limit);
+	return slice_find_area_bottomup(mm, TASK_UNMAPPED_BASE, len, available, psize, high_limit);
 }
 
 
@@ -386,9 +382,9 @@ static unsigned long slice_find_area(struct mm_struct *mm, unsigned long len,
 				     int topdown, unsigned long high_limit)
 {
 	if (topdown)
-		return slice_find_area_topdown(mm, len, mask, psize, high_limit);
+		return slice_find_area_topdown(mm, mm->mmap_base, len, mask, psize, high_limit);
 	else
-		return slice_find_area_bottomup(mm, len, mask, psize, high_limit);
+		return slice_find_area_bottomup(mm, mm->mmap_base, len, mask, psize, high_limit);
 }
 
 static inline void slice_copy_mask(struct slice_mask *dst,

arch/powerpc/mm/mmap.c

@@ -96,7 +96,7 @@ void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
 	 * bit is set, or if the expected stack growth is unlimited:
 	 */
 	if (mmap_is_legacy(rlim_stack)) {
-		mm->mmap_base = TASK_UNMAPPED_BASE;
+		mm->mmap_base = TASK_UNMAPPED_BASE + random_factor;
 		mm->get_unmapped_area = arch_get_unmapped_area;
 	} else {
 		mm->mmap_base = mmap_base(random_factor, rlim_stack);