Date:   Tue, 18 Feb 2020 17:58:36 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     "Longpeng (Mike)" <longpeng2@...wei.com>
Cc:     mike.kravetz@...cle.com, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        arei.gonglei@...wei.com, weidong.huang@...wei.com,
        weifuqiang@...wei.com, kvm@...r.kernel.org
Subject: Re: [PATCH] mm/hugetlb: avoid get wrong ptep caused by race

On Wed, Feb 19, 2020 at 09:39:59AM +0800, Longpeng (Mike) wrote:
> 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.

The race and the fix make sense.  I assumed dereferencing garbage from the
huge page was the issue, but I wasn't 100% that was the case, which is why
I asked about alternative fixes.

> 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.

FWIW, I'd be in favor of going the READ/WRITE_ONCE() route for x86, e.g.
convert everything as a follow-up patch (or patches).  I'm fairly confident
that KVM's usage of lookup_address_in_mm() is safe, but I wouldn't exactly
bet my life on it.  I'd much rather the failing scenario be that KVM uses
a sub-optimal page size as opposed to exploding on a bad pointer.

> 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.

I was actually worried about the opposite, introducing a bug by moving to
lookup_address_in_mm().

> >> Signed-off-by: Longpeng(Mike) <longpeng2@...wei.com>
> >> ---
> >>  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)
> 

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