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/20250515214217.619685-2-kees@kernel.org
Fixes: a0309faf1cb0 ("mm: vmalloc: support more granular vrealloc() sizing")
Signed-off-by: Kees Cook <kees@kernel.org>
Tested-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: Eduard Zingerman <eddyz87@gmail.com>
Cc: "Erhard F." <erhard_f@mailbox.org>
Cc: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Cc: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 74bd00fd734dd4418569b8b31085de9dc74dab85..00cf1b575c8962852a5075436769c15304052908 100644 (file)
--- a/mm/vmalloc.c
+++ b/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;
        }