Re: [PATCH v1] mm/mempolicy: fix migrate_to_node() assuming there is at least one VMA in a MM

[David Hildenbrand <david@redhat.com>]

On 20.11.24 21:27, Liam R. Howlett wrote:
> * David Hildenbrand <david@redhat.com> [241120 15:12]:
>> We currently assume that there is at least one VMA in a MM, which isn't
>> true.
>>
>> So we might end up having find_vma() return NULL, to then de-reference
>> NULL. So properly handle find_vma() returning NULL.
>>
>> This fixes the report:
>>
>> Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN PTI
>> KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
>> CPU: 1 UID: 0 PID: 6021 Comm: syz-executor284 Not tainted 6.12.0-rc7-syzkaller-00187-gf868cd251776 #0
>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/30/2024
>> RIP: 0010:migrate_to_node mm/mempolicy.c:1090 [inline]
>> RIP: 0010:do_migrate_pages+0x403/0x6f0 mm/mempolicy.c:1194
>> Code: ...
>> RSP: 0018:ffffc9000375fd08 EFLAGS: 00010246
>> RAX: 0000000000000000 RBX: ffffc9000375fd78 RCX: 0000000000000000
>> RDX: ffff88807e171300 RSI: dffffc0000000000 RDI: ffff88803390c044
>> RBP: ffff88807e171428 R08: 0000000000000014 R09: fffffbfff2039ef1
>> R10: ffffffff901cf78f R11: 0000000000000000 R12: 0000000000000003
>> R13: ffffc9000375fe90 R14: ffffc9000375fe98 R15: ffffc9000375fdf8
>> FS:  00005555919e1380(0000) GS:ffff8880b8700000(0000) knlGS:0000000000000000
>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> CR2: 00005555919e1ca8 CR3: 000000007f12a000 CR4: 00000000003526f0
>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>> Call Trace:
>>   <TASK>
>>   kernel_migrate_pages+0x5b2/0x750 mm/mempolicy.c:1709
>>   __do_sys_migrate_pages mm/mempolicy.c:1727 [inline]
>>   __se_sys_migrate_pages mm/mempolicy.c:1723 [inline]
>>   __x64_sys_migrate_pages+0x96/0x100 mm/mempolicy.c:1723
>>   do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>>   do_syscall_64+0xcd/0x250 arch/x86/entry/common.c:83
>>   entry_SYSCALL_64_after_hwframe+0x77/0x7f
>>
>> Fixes: 39743889aaf7 ("[PATCH] Swap Migration V5: sys_migrate_pages interface")
>> Reported-by: syzbot+3511625422f7aa637f0d@syzkaller.appspotmail.com
>> Closes: https://lore.kernel.org/lkml/673d2696.050a0220.3c9d61.012f.GAE@google.com/T/
>> Cc: <stable@vger.kernel.org>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Christoph Lameter <cl@linux.com>
>> Cc: Liam R. Howlett <Liam.Howlett@Oracle.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
> 
> I hate the extra check because syzbot can cause this as this should
> basically never happen in real life, but it seems we have to add it.

I think the reproducer achieves it by doing an MADV_DONTFORK on all VMAs 
and then fork'ing. Likely it doesn't make sense to have a new MM without 
any VMAs, because it cannot do anything reasonable.

But then, I'm not 100% sure if there are other creative ways to 
obtain/achieve the same.

$ git grep "find_vma(mm, 0)"
mm/mempolicy.c: vma = find_vma(mm, 0);

Apart from that there seems to be
kernel/bpf/task_iter.c where we do
	curr_vma = find_vma(curr_mm, 0);

and properly check for NULL later.

So this one sticks out, and is not on anything that I consider a fast 
path ... easy fix. :)

Thanks!

-- 
Cheers,

David / dhildenb