linux-kernel - KASAN vs realloc

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Message-ID: <CANP3RGeuRW53vukDy7WDO3FiVgu34-xVJYkfpm08oLO3odYFrA@mail.gmail.com>
Date: Tue, 6 Jan 2026 13:42:45 +0100
From: Maciej Żenczykowski <maze@...gle.com>
To: Maciej Wieczor-Retman <m.wieczorretman@...me>
Cc: joonki.min@...sung-slsi.corp-partner.google.com, 
	Kees Cook <keescook@...gle.com>, Andrew Morton <akpm@...gle.com>, 
	Andrey Ryabinin <ryabinin.a.a@...il.com>, Alexander Potapenko <glider@...gle.com>, 
	Andrey Konovalov <andreyknvl@...il.com>, Dmitry Vyukov <dvyukov@...gle.com>, 
	Vincenzo Frascino <vincenzo.frascino@....com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Uladzislau Rezki <urezki@...il.com>, Danilo Krummrich <dakr@...nel.org>, Kees Cook <kees@...nel.org>, 
	jiayuan.chen@...ux.dev, syzbot+997752115a851cb0cf36@...kaller.appspotmail.com, 
	Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>, kasan-dev@...glegroups.com, 
	Kernel hackers <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: KASAN vs realloc

We've got internal reports (b/467571011 - from CC'ed Samsung
developer) that kasan realloc is broken for sizes that are not a
multiple of the granule.  This appears to be triggered during Android
bootup by some ebpf program loading operations (a struct is 88 bytes
in size, which is a multiple of 8, but not 16, which is the granule
size).

(this is on 6.18 with
https://lore.kernel.org/all/38dece0a4074c43e48150d1e242f8242c73bf1a5.1764874575.git.m.wieczorretman@pm.me/
already included)

joonki.min@...sung-slsi.corp-partner.google.com summarized it as
"When newly requested size is not bigger than allocated size and old
size was not 16 byte aligned, it failed to unpoison extended area."

and *very* rough comment:

Right. "size - old_size" is not guaranteed 16-byte alignment in this case.

I think we may unpoison 16-byte alignment size, but it allowed more
than requested :(

I'm not sure that's right approach.

if (size <= alloced_size) {
- kasan_unpoison_vmalloc(p + old_size, size - old_size,
+               kasan_unpoison_vmalloc(p + old_size, round_up(size -
old_size, KASAN_GRANULE_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;
+               vm->requested_size = round_up(size, KASAN_GRANULE_SIZE);

my personal guess is that

But just above the code you quoted in mm/vmalloc.c I see:
        if (size <= old_size) {
...
                kasan_poison_vmalloc(p + size, old_size - size);

is also likely wrong?? Considering:

mm/kasan/shadow.c

void __kasan_poison_vmalloc(const void *start, unsigned long size)
{
        if (!is_vmalloc_or_module_addr(start))
                return;

        size = round_up(size, KASAN_GRANULE_SIZE);
        kasan_poison(start, size, KASAN_VMALLOC_INVALID, false);
}

This doesn't look right - if start isn't a multiple of the granule.

--
Maciej Żenczykowski, Kernel Networking Developer @ Google