Re: [PATCH] mm/hugetlb: avoid get wrong ptep caused by race

[Longpeng (Mike)]

在 2020/2/19 4:37, Sean Christopherson 写道:
> On Tue, Feb 18, 2020 at 08:10:25PM +0800, Longpeng(Mike) wrote:
>> Our machine encountered a panic after run for a long time and
>> the calltrace is:
> 
> What's the actual panic?  Is it a BUG() in hugetlb_fault(), a bad pointer
> dereference, etc...?
> 
A bad pointer dereference.

pgd -> pud -> user 1G hugepage
huge_pte_offset() wants to return NULL or pud (point to the entry), but it maybe
return the a bad pointer of the user 1G hugepage.

>> RIP: 0010:[<ffffffff9dff0587>]  [<ffffffff9dff0587>] hugetlb_fault+0x307/0xbe0
>> RSP: 0018:ffff9567fc27f808  EFLAGS: 00010286
>> RAX: e800c03ff1258d48 RBX: ffffd3bb003b69c0 RCX: e800c03ff1258d48
>> RDX: 17ff3fc00eda72b7 RSI: 00003ffffffff000 RDI: e800c03ff1258d48
>> RBP: ffff9567fc27f8c8 R08: e800c03ff1258d48 R09: 0000000000000080
>> R10: ffffaba0704c22a8 R11: 0000000000000001 R12: ffff95c87b4b60d8
>> R13: 00005fff00000000 R14: 0000000000000000 R15: ffff9567face8074
>> FS:  00007fe2d9ffb700(0000) GS:ffff956900e40000(0000) knlGS:0000000000000000
>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> CR2: ffffd3bb003b69c0 CR3: 000000be67374000 CR4: 00000000003627e0
>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>> Call Trace:
>>  [<ffffffff9df9b71b>] ? unlock_page+0x2b/0x30
>>  [<ffffffff9dff04a2>] ? hugetlb_fault+0x222/0xbe0
>>  [<ffffffff9dff1405>] follow_hugetlb_page+0x175/0x540
>>  [<ffffffff9e15b825>] ? cpumask_next_and+0x35/0x50
>>  [<ffffffff9dfc7230>] __get_user_pages+0x2a0/0x7e0
>>  [<ffffffff9dfc648d>] __get_user_pages_unlocked+0x15d/0x210
>>  [<ffffffffc068cfc5>] __gfn_to_pfn_memslot+0x3c5/0x460 [kvm]
>>  [<ffffffffc06b28be>] try_async_pf+0x6e/0x2a0 [kvm]
>>  [<ffffffffc06b4b41>] tdp_page_fault+0x151/0x2d0 [kvm]
>>  [<ffffffffc075731c>] ? vmx_vcpu_run+0x2ec/0xc80 [kvm_intel]
>>  [<ffffffffc0757328>] ? vmx_vcpu_run+0x2f8/0xc80 [kvm_intel]
>>  [<ffffffffc06abc11>] kvm_mmu_page_fault+0x31/0x140 [kvm]
>>  [<ffffffffc074d1ae>] handle_ept_violation+0x9e/0x170 [kvm_intel]
>>  [<ffffffffc075579c>] vmx_handle_exit+0x2bc/0xc70 [kvm_intel]
>>  [<ffffffffc074f1a0>] ? __vmx_complete_interrupts.part.73+0x80/0xd0 [kvm_intel]
>>  [<ffffffffc07574c0>] ? vmx_vcpu_run+0x490/0xc80 [kvm_intel]
>>  [<ffffffffc069f3be>] vcpu_enter_guest+0x7be/0x13a0 [kvm]
>>  [<ffffffffc06cf53e>] ? kvm_check_async_pf_completion+0x8e/0xb0 [kvm]
>>  [<ffffffffc06a6f90>] kvm_arch_vcpu_ioctl_run+0x330/0x490 [kvm]
>>  [<ffffffffc068d919>] kvm_vcpu_ioctl+0x309/0x6d0 [kvm]
>>  [<ffffffff9deaa8c2>] ? dequeue_signal+0x32/0x180
>>  [<ffffffff9deae34d>] ? do_sigtimedwait+0xcd/0x230
>>  [<ffffffff9e03aed0>] do_vfs_ioctl+0x3f0/0x540
>>  [<ffffffff9e03b0c1>] SyS_ioctl+0xa1/0xc0
>>  [<ffffffff9e53879b>] system_call_fastpath+0x22/0x27
>>
>> ( The kernel we used is older, but we think the latest kernel also has this
>>   bug after dig into this problem. )
>>
>> For 1G hugepages, huge_pte_offset() wants to return NULL or pudp, but it
>> may return a wrong 'pmdp' if there is a race. Please look at the following
>> code snippet:
>>     ...
>>     pud = pud_offset(p4d, addr);
>>     if (sz != PUD_SIZE && pud_none(*pud))
>>         return NULL;
>>     /* hugepage or swap? */
>>     if (pud_huge(*pud) || !pud_present(*pud))
>>         return (pte_t *)pud;
>>
>>     pmd = pmd_offset(pud, addr);
>>     if (sz != PMD_SIZE && pmd_none(*pmd))
>>         return NULL;
>>     /* hugepage or swap? */
>>     if (pmd_huge(*pmd) || !pmd_present(*pmd))
>>         return (pte_t *)pmd;
>>     ...
>>
>> The following sequence would trigger this bug:
>> 1. CPU0: sz = PUD_SIZE and *pud = 0 , continue
>> 1. CPU0: "pud_huge(*pud)" is false
>> 2. CPU1: calling hugetlb_no_page and set *pud to xxxx8e7(PRESENT)
>> 3. CPU0: "!pud_present(*pud)" is false, continue
>> 4. CPU0: pmd = pmd_offset(pud, addr) and maybe return a wrong pmdp
>> However, we want CPU0 to return NULL or pudp.
>>
>> We can avoid this race by read the pud only once.
> 
> Are there any other options for avoiding the panic you hit?  I ask because
> there are a variety of flows that use a very similar code pattern, e.g.
> lookup_address_in_pgd(), and using READ_ONCE() in huge_pte_offset() but not
> other flows could be confusing (or in my case, anxiety inducing[*]).  At
> the least, adding a comment in huge_pte_offset() to explain the need for
> READ_ONCE() would be helpful.
>
I hope the hugetlb and mm maintainers could give some other options if they
approve this bug.
We change the code from
	if (pud_huge(*pud) || !pud_present(*pud))
to
	if (pud_huge(*pud)
		return (pte_t *)pud;
	busy loop for 500ms
	if (!pud_present(*pud))
		return (pte_t *)pud;
and the panic will be hit quickly.

ARM64 has already use READ/WRITE_ONCE to access the pagetable, look at this
commit 20a004e7 (arm64: mm: Use READ_ONCE/WRITE_ONCE when accessing page tables).

The root cause is: 'if (pud_huge(*pud) || !pud_present(*pud))' read entry from
pud twice and the *pud maybe change in a race, so if we only read the pud once.
I use READ_ONCE here is just for safe, to prevents the complier mischief if
possible.

I'll add comments in v2.

> [*] In kernel 5.6, KVM is moving to using lookup_address_in_pgd() (via
>     lookup_address_in_mm()) to identify large page mappings.  The function
>     itself is susceptible to such a race, but KVM only does the lookup
>     after it has done gup() and also ensures any zapping of ptes will cause
>     KVM to restart the faulting (guest) instruction or that the zap will be
>     blocked until after KVM does the lookup, i.e. racing with a transition
>     from !PRESENT -> PRESENT should be impossible (in theory).
> 
This bug is from hugetlb core, we could trigger it in other usages even if the
latest KVM won't.

>> Signed-off-by: Longpeng(Mike) <longpeng2@xxxxxxxxxx>
>> ---
>>  mm/hugetlb.c | 34 ++++++++++++++++++----------------
>>  1 file changed, 18 insertions(+), 16 deletions(-)
>>
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index dd8737a..3bde229 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -4908,31 +4908,33 @@ pte_t *huge_pte_alloc(struct mm_struct *mm,
>>  pte_t *huge_pte_offset(struct mm_struct *mm,
>>  		       unsigned long addr, unsigned long sz)
>>  {
>> -	pgd_t *pgd;
>> -	p4d_t *p4d;
>> -	pud_t *pud;
>> -	pmd_t *pmd;
>> +	pgd_t *pgdp;
>> +	p4d_t *p4dp;
>> +	pud_t *pudp, pud;
>> +	pmd_t *pmdp, pmd;
>>  
>> -	pgd = pgd_offset(mm, addr);
>> -	if (!pgd_present(*pgd))
>> +	pgdp = pgd_offset(mm, addr);
>> +	if (!pgd_present(*pgdp))
>>  		return NULL;
>> -	p4d = p4d_offset(pgd, addr);
>> -	if (!p4d_present(*p4d))
>> +	p4dp = p4d_offset(pgdp, addr);
>> +	if (!p4d_present(*p4dp))
>>  		return NULL;
>>  
>> -	pud = pud_offset(p4d, addr);
>> -	if (sz != PUD_SIZE && pud_none(*pud))
>> +	pudp = pud_offset(p4dp, addr);
>> +	pud = READ_ONCE(*pudp);
>> +	if (sz != PUD_SIZE && pud_none(pud))
>>  		return NULL;
>>  	/* hugepage or swap? */
>> -	if (pud_huge(*pud) || !pud_present(*pud))
>> -		return (pte_t *)pud;
>> +	if (pud_huge(pud) || !pud_present(pud))
>> +		return (pte_t *)pudp;
>>  
>> -	pmd = pmd_offset(pud, addr);
>> -	if (sz != PMD_SIZE && pmd_none(*pmd))
>> +	pmdp = pmd_offset(pudp, addr);
>> +	pmd = READ_ONCE(*pmdp);
>> +	if (sz != PMD_SIZE && pmd_none(pmd))
>>  		return NULL;
>>  	/* hugepage or swap? */
>> -	if (pmd_huge(*pmd) || !pmd_present(*pmd))
>> -		return (pte_t *)pmd;
>> +	if (pmd_huge(pmd) || !pmd_present(pmd))
>> +		return (pte_t *)pmdp;
>>  
>>  	return NULL;
>>  }
>> -- 
>> 1.8.3.1
>>
>>
> 
> .
> 


-- 
Regards,
Longpeng(Mike)