mm: vmalloc: only zero-init on vrealloc shrink

The common case is to grow reallocations, and since init_on_alloc will
have already zeroed the whole allocation, we only need to zero when
shrinking the allocation.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: a0309fa ("mm: vmalloc: support more granular vrealloc() sizing")
Signed-off-by: Kees Cook <[email protected]>
Tested-by: Pawan Gupta <[email protected]>
Cc: Danilo Krummrich <[email protected]>
Cc: Eduard Zingerman <[email protected]>
Cc: "Erhard F." <[email protected]>
Cc: Shung-Hsi Yu <[email protected]>
Cc: "Uladzislau Rezki (Sony)" <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: kees

mm/vmalloc.c

@@ -4093,8 +4093,8 @@ void *vrealloc_noprof(const void *p, size_t size, gfp_t flags)
 	 * would be a good heuristic for when to shrink the vm_area?
 	 */
 	if (size <= old_size) {
-		/* Zero out "freed" memory. */
-		if (want_init_on_free())
+		/* Zero out "freed" memory, potentially for future realloc. */
+		if (want_init_on_free() || want_init_on_alloc(flags))
 			memset((void *)p + size, 0, old_size - size);
 		vm->requested_size = size;
 		kasan_poison_vmalloc(p + size, old_size - size);
@@ -4107,9 +4107,11 @@ void *vrealloc_noprof(const void *p, size_t size, gfp_t flags)
 	if (size <= alloced_size) {
 		kasan_unpoison_vmalloc(p + old_size, size - old_size,
 				       KASAN_VMALLOC_PROT_NORMAL);
-		/* Zero out "alloced" memory. */
-		if (want_init_on_alloc(flags))
-			memset((void *)p + old_size, 0, size - old_size);
+		/*
+		 * No need to zero memory here, as unused memory will have
+		 * already been zeroed at initial allocation time or during
+		 * realloc shrink time.
+		 */
 		vm->requested_size = size;
 		return (void *)p;
 	}