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SubjectRe: kvm: access to invalid memory in mmu_zap_unsync_children
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On 2016/01/18 14:00, Xiao Guangrong wrote:
> On 01/16/2016 01:06 AM, Paolo Bonzini wrote:
>> On 15/01/2016 17:54, Sasha Levin wrote:
>>> Hi all,
>>>
>>> While fuzzing with syzkaller on the latest -next kernel running on a
>>> KVM tools
>>> guest, I've hit the following invalid memory access:
>>>
>>> [ 547.956284] UBSAN: Undefined behaviour in arch/x86/kvm/mmu.c:2011:17
>>>
>>> [ 547.956940] index 3 is out of range for type 'kvm_mmu_page *[3]'
>>>
>>> [ 547.957567] CPU: 0 PID: 21577 Comm: syz-executor Tainted: G
>>> D 4.4.0-next-20160114-sasha-00021-gf1273d1-dirty #2798
>>>
>>> [ 547.958739] 1ffff1001819be5c 000000002fa0e55b ffff8800c0cdf360
>>> ffffffff83433c4e
>>>
>>> [ 547.972448] 0000000041b58ab3 ffffffff8f960c38 ffffffff83433b86
>>> ffff8800c0cdf328
>>>
>>> [ 547.973277] 0000000000000001 000000002fa0e55b ffffffff8feb8440
>>> ffff8800c0cdf3f0
>>>
>>> [ 547.974102] Call Trace:
>>>
>>> [ 547.974424] dump_stack (lib/dump_stack.c:52)
>>> [ 547.975774] ubsan_epilogue (lib/ubsan.c:165)
>>> [ 547.976408] __ubsan_handle_out_of_bounds (lib/ubsan.c:382)
>>> [ 547.980877] mmu_zap_unsync_children (arch/x86/kvm/mmu.c:2011
>>> arch/x86/kvm/mmu.c:2272)
>>
>> Marcelo/Takuya/Xiao,
>>
>> do you know what's the point in the assignment in kvm_mmu_pages_init?
>>
>> It seems to me that it should be
>>
>> parents->parent[0] = NULL;
>>
>> since the only user of the ->parent[] array, mmu_pages_clear_parents,
>> walks the array up from parents->parent[0].

Triggering "index 3 is out of range for type 'kvm_mmu_page *[3]'"
in the first kvm_mmu_pages_init() call in mmu_zap_unsync_children()
means the parent passed in to mmu_zap_unsync_children() has its
->role.level set to PT64_ROOT_LEVEL.

Is this the problem being reported?

Maybe, I'm just confused by the incorrect line-numbers and it's
possible that the problem was triggered in the while loop.

> Yes, it is bugly and it surprised me that it was not triggered in nested
> env.

Yes, the code is not changed much from the following commit:
KVM: MMU: use page array in unsync walk
commit 60c8aec6e2c9923492dabbd6b67e34692bd26c20

What, including my recent cleanups, could change the situation to make
this happen? I'm not sure. It's almost seven years.

>> Any other opinions?
>
> The idea we use the array as [PT64_ROOT_LEVEL-1] is because we never take
> the last level (level = 1) into account.

Yes, this is reasonable.

> I think this diff can fix this, but it has not tested yet.

I'm still reading the code but not sure what changed the situation.

Takuya

> +#define INVALID_INDEX (-1)
> +
> static int mmu_unsync_walk(struct kvm_mmu_page *sp,
> struct kvm_mmu_pages *pvec)
> {
> if (!sp->unsync_children)
> return 0;
>
> - mmu_pages_add(pvec, sp, 0);
> + /*
> + * do not count the index in the parent of the sp we're
> + * walking start from.
> + */
> + mmu_pages_add(pvec, sp, INVALID_INDEX);
> return __mmu_unsync_walk(sp, pvec);
> }
>
> @@ -1980,8 +1986,11 @@ static int mmu_pages_next(struct kvm_mmu_pages
> *pvec,
> return n;
> }
>
> - parents->parent[sp->role.level-2] = sp;
> - parents->idx[sp->role.level-1] = pvec->page[n].idx;
> + parents->parent[sp->role.level - 2] = sp;
> +
> + /* skip setting idex of the sp we start from. */
> + if (pvec->page[n].idx != INVALID_INDEX)
> + parents->idx[sp->role.level - 1] = pvec->page[n].idx;
> }
>
> return n;
> @@ -1999,6 +2008,7 @@ static void mmu_pages_clear_parents(struct
> mmu_page_path *parents)
> if (!sp)
> return;
>
> + WARN_ON(idx != INVALID_INDEX);
> clear_unsync_child_bit(sp, idx);
> level++;
> } while (level < PT64_ROOT_LEVEL-1 && !sp->unsync_children);
> @@ -2008,7 +2018,7 @@ static void kvm_mmu_pages_init(struct kvm_mmu_page
> *parent,
> struct mmu_page_path *parents,
> struct kvm_mmu_pages *pvec)
> {
> - parents->parent[parent->role.level-1] = NULL;
> + parents->parent[parent->role.level - 2] = NULL;
> pvec->nr = 0;
> }




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