[PATCH] mm/usercopy: harden bounds checking for vmalloc allocations

[Dev Jain <dev.jain@arm.com>]

The vmalloc allocator stores the actual allocation size inside the
vm_struct structure. We can use this bound in usercopy instead of the
page-aligned va_end to catch usercopy beyond the actual allocation size.

For vmap, the requested_size field is always page-aligned since it maps a
certain number of pages. Same for vm_map_ram (alongwith, not even having
a vm_struct). So the check is only relevant for vmalloc mappings.

Because there are early vm areas registered even before vmalloc_init,
requested_size may be zero. So also check whether the requested_size
is set.

Signed-off-by: Dev Jain <dev.jain@arm.com>
---
Applies on mm-new (3d18f3499c48).

 mm/usercopy.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/mm/usercopy.c b/mm/usercopy.c
index 5de7a518b1b1c..772681aff8477 100644
--- a/mm/usercopy.c
+++ b/mm/usercopy.c
@@ -176,10 +176,28 @@ static inline void check_heap_object(const void *ptr, unsigned long n,
 
 	if (is_vmalloc_addr(ptr) && !pagefault_disabled()) {
 		struct vmap_area *area = find_vmap_area(addr);
+		struct vm_struct *vm;
 
 		if (!area)
 			usercopy_abort("vmalloc", "no area", to_user, 0, n);
 
+		vm = area->vm;
+		/*
+		 * Mappings with a vm_struct track the originally requested
+		 * size. Check against that rather than the page-rounded
+		 * vmap_area->va_end so copies cannot reach vmalloc tail
+		 * padding. vmap mappings are always page aligned.
+		 */
+		if (vm && (vm->flags & VM_ALLOC) && vm->requested_size) {
+			unsigned long size = vm->requested_size;
+
+			offset = addr - area->va_start;
+			if (offset > size || n > size - offset)
+				usercopy_abort("vmalloc", NULL, to_user,
+					       offset, n);
+			return;
+		}
+
 		if (n > area->va_end - addr) {
 			offset = addr - area->va_start;
 			usercopy_abort("vmalloc", NULL, to_user, offset, n);
-- 
2.43.0