mm: vmalloc: support more granular vrealloc() sizing

Introduce struct vm_struct::requested_size so that the requested
(re)allocation size is retained separately from the allocated area size. 
This means that KASAN will correctly poison the correct spans of requested
bytes.  This also means we can support growing the usable portion of an
allocation that can already be supported by the existing area's existing
allocation.

Link: https://lkml.kernel.org/r/20250426001105.it.679-kees@kernel.org
Fixes: 3ddc2fe ("mm: vmalloc: implement vrealloc()")
Signed-off-by: Kees Cook <kees@kernel.org>
Reported-by: Erhard Furtner <erhard_f@mailbox.org>
Closes: https://lore.kernel.org/all/20250408192503.6149a816@outsider.home/
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/vmalloc.h

@@ -61,6 +61,7 @@ struct vm_struct {
 	unsigned int		nr_pages;
 	phys_addr_t		phys_addr;
 	const void		*caller;
+	unsigned long		requested_size;
 };
 
 struct vmap_area {

mm/vmalloc.c

@@ -1940,7 +1940,7 @@ static inline void setup_vmalloc_vm(struct vm_struct *vm,
 {
 	vm->flags = flags;
 	vm->addr = (void *)va->va_start;
-	vm->size = va_size(va);
+	vm->size = vm->requested_size = va_size(va);
 	vm->caller = caller;
 	va->vm = vm;
 }
@@ -3133,6 +3133,7 @@ struct vm_struct *__get_vm_area_node(unsigned long size,
 
 	area->flags = flags;
 	area->caller = caller;
+	area->requested_size = requested_size;
 
 	va = alloc_vmap_area(size, align, start, end, node, gfp_mask, 0, area);
 	if (IS_ERR(va)) {
@@ -4063,6 +4064,8 @@ EXPORT_SYMBOL(vzalloc_node_noprof);
  */
 void *vrealloc_noprof(const void *p, size_t size, gfp_t flags)
 {
+	struct vm_struct *vm = NULL;
+	size_t alloced_size = 0;
 	size_t old_size = 0;
 	void *n;
 
@@ -4072,30 +4075,44 @@ void *vrealloc_noprof(const void *p, size_t size, gfp_t flags)
 	}
 
 	if (p) {
-		struct vm_struct *vm;
-
 		vm = find_vm_area(p);
 		if (unlikely(!vm)) {
 			WARN(1, "Trying to vrealloc() nonexistent vm area (%p)\n", p);
 			return NULL;
 		}
 
-		old_size = get_vm_area_size(vm);
+		alloced_size = get_vm_area_size(vm);
+		old_size = vm->requested_size;
+		if (WARN(alloced_size < old_size,
+			 "vrealloc() has mismatched area vs requested sizes (%p)\n", p))
+			return NULL;
 	}
 
 	/*
 	 * TODO: Shrink the vm_area, i.e. unmap and free unused pages. What
 	 * would be a good heuristic for when to shrink the vm_area?
 	 */
 	if (size <= old_size) {
-		/* Zero out spare memory. */
-		if (want_init_on_alloc(flags))
+		/* Zero out "freed" memory. */
+		if (want_init_on_free())
 			memset((void *)p + size, 0, old_size - size);
+		vm->requested_size = size;
 		kasan_poison_vmalloc(p + size, old_size - size);
-		kasan_unpoison_vmalloc(p, size, KASAN_VMALLOC_PROT_NORMAL);
 		return (void *)p;
 	}
 
+	/*
+	 * We already have the bytes available in the allocation; use them.
+	 */
+	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);
+		vm->requested_size = size;
+	}
+
 	/* TODO: Grow the vm_area, i.e. allocate and map additional pages. */
 	n = __vmalloc_noprof(size, flags);
 	if (!n)