mm/kasan: fix KASAN poisoning in vrealloc()

A KASAN warning can be triggered when vrealloc() changes the requested
size to a value that is not aligned to KASAN_GRANULE_SIZE.

------------[ cut here ]------------
WARNING: CPU: 2 PID: 1 at mm/kasan/shadow.c:174 kasan_unpoison+0x40/0x48
...
pc : kasan_unpoison+0x40/0x48
lr : __kasan_unpoison_vmalloc+0x40/0x68
Call trace:
kasan_unpoison+0x40/0x48 (P)
vrealloc_node_align_noprof+0x200/0x320
bpf_patch_insn_data+0x90/0x2f0
convert_ctx_accesses+0x8c0/0x1158
bpf_check+0x1488/0x1900
bpf_prog_load+0xd20/0x1258
__sys_bpf+0x96c/0xdf0
__arm64_sys_bpf+0x50/0xa0
invoke_syscall+0x90/0x160

Introduce a dedicated kasan_vrealloc() helper that centralizes KASAN
handling for vmalloc reallocations. The helper accounts for KASAN granule
alignment when growing or shrinking an allocation and ensures that partial
granules are handled correctly.

Use this helper from vrealloc_node_align_noprof() to fix poisoning logic.

[[e-mail]: move kasan_enabled() check, fix build]
Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Fixes: d699440f58ce ("mm: fix vrealloc()'s KASAN poisoning logic")
Signed-off-by: Andrey Ryabinin <[e-mail]>
Reported-by: Maciej Żenczykowski <[e-mail]>
Reported-by: <[e-mail]>
Closes: https://lkml.kernel.org/r/[e-mail]
Reviewed-by: Andrey Konovalov <[e-mail]>
Tested-by: Maciej Wieczor-Retman <[e-mail]>
Cc: Alexander Potapenko <[e-mail]>
Cc: Dmitriy Vyukov <[e-mail]>
Cc: Dmitry Vyukov <[e-mail]>
Cc: Uladzislau Rezki <[e-mail]>
Cc: Vincenzo Frascino <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/kasan.h

···
 		__kasan_unpoison_vmap_areas(vms, nr_vms, flags);
 }
 
+void __kasan_vrealloc(const void *start, unsigned long old_size,
+		unsigned long new_size);
+
+static __always_inline void kasan_vrealloc(const void *start,
+					unsigned long old_size,
+					unsigned long new_size)
+{
+	if (kasan_enabled())
+		__kasan_vrealloc(start, old_size, new_size);
+}
+
 #else /* CONFIG_KASAN_VMALLOC */
 
 static inline void kasan_populate_early_vm_area_shadow(void *start,
···
 kasan_unpoison_vmap_areas(struct vm_struct **vms, int nr_vms,
 			  kasan_vmalloc_flags_t flags)
 { }
+
+static inline void kasan_vrealloc(const void *start, unsigned long old_size,
+				unsigned long new_size) { }
 
 #endif /* CONFIG_KASAN_VMALLOC */
 

mm/kasan/common.c

···
 			__kasan_unpoison_vmalloc(addr, size, flags | KASAN_VMALLOC_KEEP_TAG);
 	}
 }
+
+void __kasan_vrealloc(const void *addr, unsigned long old_size,
+		unsigned long new_size)
+{
+	if (new_size < old_size) {
+		kasan_poison_last_granule(addr, new_size);
+
+		new_size = round_up(new_size, KASAN_GRANULE_SIZE);
+		old_size = round_up(old_size, KASAN_GRANULE_SIZE);
+		if (new_size < old_size)
+			__kasan_poison_vmalloc(addr + new_size,
+					old_size - new_size);
+	} else if (new_size > old_size) {
+		old_size = round_down(old_size, KASAN_GRANULE_SIZE);
+		__kasan_unpoison_vmalloc(addr + old_size,
+					new_size - old_size,
+					KASAN_VMALLOC_PROT_NORMAL |
+					KASAN_VMALLOC_VM_ALLOC |
+					KASAN_VMALLOC_KEEP_TAG);
+	}
+}
 #endif

mm/vmalloc.c

···
 		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);
+		kasan_vrealloc(p, old_size, size);
 		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 |
-				       KASAN_VMALLOC_VM_ALLOC |
-				       KASAN_VMALLOC_KEEP_TAG);
 		/*
 		 * 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;
+		kasan_vrealloc(p, old_size, size);
 		return (void *)p;
 	}