mm: fix vrealloc()'s KASAN poisoning logic

When vrealloc() reuses already allocated vmap_area, we need to re-annotate
poisoned and unpoisoned portions of underlying memory according to the new
size.

This results in a KASAN splat recorded at [1].  A KASAN mis-reporting
issue where there is none.

Note, hard-coding KASAN_VMALLOC_PROT_NORMAL might not be exactly correct,
but KASAN flag logic is pretty involved and spread out throughout
__vmalloc_node_range_noprof(), so I'm using the bare minimum flag here and
leaving the rest to mm people to refactor this logic and reuse it here.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lore.kernel.org/bpf/[email protected]/ [1]
Fixes: 3ddc2fefe6 ("mm: vmalloc: implement vrealloc()")
Signed-off-by: Andrii Nakryiko <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Uladzislau Rezki (Sony) <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/vmalloc.c

@@ -4093,7 +4093,8 @@ void *vrealloc_noprof(const void *p, size_t size, gfp_t flags)
 		/* Zero out spare memory. */
 		if (want_init_on_alloc(flags))
 			memset((void *)p + size, 0, old_size - size);
-
+		kasan_poison_vmalloc(p + size, old_size - size);
+		kasan_unpoison_vmalloc(p, size, KASAN_VMALLOC_PROT_NORMAL);
 		return (void *)p;
 	}