linux-kernel - Re: [syzbot] KMSAN: uninit-value in mpol_rebind

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Date:   Wed, 5 Jan 2022 15:50:36 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     syzbot <syzbot+217f792c92599518a2ab@...kaller.appspotmail.com>
Cc:     glider@...gle.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] KMSAN: uninit-value in mpol_rebind_task (2)

On Tue, 04 Jan 2022 02:03:17 -0800 syzbot <syzbot+217f792c92599518a2ab@...kaller.appspotmail.com> wrote:

> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    81c325bbf94e kmsan: hooks: do not check memory in kmsan_in..
> git tree:       https://github.com/google/kmsan.git master
> console output: https://syzkaller.appspot.com/x/log.txt?x=112b8f7db00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=2d8b9a11641dc9aa
> dashboard link: https://syzkaller.appspot.com/bug?extid=217f792c92599518a2ab
> compiler:       clang version 14.0.0 (/usr/local/google/src/llvm-git-monorepo 2b554920f11c8b763cd9ed9003f4e19b919b8e1f), GNU ld (GNU Binutils for Debian) 2.35.2
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=13366ea5b00000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14526cb3b00000

Thanks.  I don't get it.

> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+217f792c92599518a2ab@...kaller.appspotmail.com
> 
> =====================================================
> BUG: KMSAN: uninit-value in mpol_rebind_policy mm/mempolicy.c:352 [inline]
> BUG: KMSAN: uninit-value in mpol_rebind_task+0x2ac/0x2c0 mm/mempolicy.c:368
>  mpol_rebind_policy mm/mempolicy.c:352 [inline]

Appears to be

	if (!mpol_store_user_nodemask(pol) &&

But if so, why didn't it also report

	mpol_store_user_nodemask mm/mempolicy.c:184 [inline]

which is where the 

	return pol->flags & MPOL_MODE_FLAGS;

actually happened?

>  mpol_rebind_task+0x2ac/0x2c0 mm/mempolicy.c:368
>  cpuset_change_task_nodemask kernel/cgroup/cpuset.c:1711 [inline]
>  cpuset_attach+0x787/0x15e0 kernel/cgroup/cpuset.c:2278
>  cgroup_migrate_execute+0x1023/0x1d20 kernel/cgroup/cgroup.c:2515
>  cgroup_migrate kernel/cgroup/cgroup.c:2771 [inline]
>  cgroup_attach_task+0x540/0x8b0 kernel/cgroup/cgroup.c:2804
>  __cgroup1_procs_write+0x5cc/0x7a0 kernel/cgroup/cgroup-v1.c:520
>  cgroup1_tasks_write+0x94/0xb0 kernel/cgroup/cgroup-v1.c:539
>  cgroup_file_write+0x4c2/0x9e0 kernel/cgroup/cgroup.c:3852
>  kernfs_fop_write_iter+0x66a/0x9f0 fs/kernfs/file.c:296
>  call_write_iter include/linux/fs.h:2162 [inline]
>  new_sync_write fs/read_write.c:503 [inline]
>  vfs_write+0x1318/0x2030 fs/read_write.c:590
>  ksys_write+0x28b/0x510 fs/read_write.c:643
>  __do_sys_write fs/read_write.c:655 [inline]
>  __se_sys_write fs/read_write.c:652 [inline]
>  __x64_sys_write+0xdb/0x120 fs/read_write.c:652
>  do_syscall_x64 arch/x86/entry/common.c:51 [inline]
>  do_syscall_64+0x54/0xd0 arch/x86/entry/common.c:82
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> Uninit was created at:
>  slab_post_alloc_hook mm/slab.h:524 [inline]
>  slab_alloc_node mm/slub.c:3251 [inline]
>  slab_alloc mm/slub.c:3259 [inline]
>  kmem_cache_alloc+0x902/0x11c0 mm/slub.c:3264
>  mpol_new mm/mempolicy.c:293 [inline]

But mpol_new() does

	policy->flags = flags;


>  do_set_mempolicy+0x421/0xb70 mm/mempolicy.c:853
>  kernel_set_mempolicy mm/mempolicy.c:1504 [inline]
>  __do_sys_set_mempolicy mm/mempolicy.c:1510 [inline]
>  __se_sys_set_mempolicy+0x44c/0xb60 mm/mempolicy.c:1507
>  __x64_sys_set_mempolicy+0xd8/0x110 mm/mempolicy.c:1507
>  do_syscall_x64 arch/x86/entry/common.c:51 [inline]
>  do_syscall_64+0x54/0xd0 arch/x86/entry/common.c:82
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> CPU: 1 PID: 3479 Comm: syz-executor124 Not tainted 5.16.0-rc5-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> =====================================================