Date:   Sat, 21 Mar 2020 16:38:19 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     "Longpeng (Mike)" <longpeng2@...wei.com>, akpm@...ux-foundation.org
Cc:     kirill.shutemov@...ux.intel.com, linux-kernel@...r.kernel.org,
        arei.gonglei@...wei.com, weidong.huang@...wei.com,
        weifuqiang@...wei.com, kvm@...r.kernel.org, linux-mm@...ck.org,
        Matthew Wilcox <willy@...radead.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH v2] mm/hugetlb: fix a addressing exception caused by
 huge_pte_offset()

On 2/21/20 7:33 PM, Longpeng(Mike) wrote:
> From: Longpeng <longpeng2@...wei.com>
> 
> Our machine encountered a panic(addressing exception) after run
> for a long time and the calltrace is:
> 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. What's more, we also use
> READ_ONCE to access the entries for safe(e.g. avoid the compilier mischief)
> 
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Sean Christopherson <sean.j.christopherson@...el.com>
> Cc: Mike Kravetz <mike.kravetz@...cle.com>
> Cc: stable@...r.kernel.org
> Signed-off-by: Longpeng <longpeng2@...wei.com>

Andrew dropped this patch from his tree which caused me to go back and
look at the status of this patch/issue.

It is pretty obvious that code in the current huge_pte_offset routine
is racy.  I checked out the assembly code produced by my compiler and
verified that the line,

	if (pud_huge(*pud) || !pud_present(*pud))

does actually dereference *pud twice.  So, the value could change between
those two dereferences.   Longpeng (Mike) could easlily recreate the issue
if he put a delay between the two dereferences.  I believe the only
reservations/concerns about the patch below was the use of READ_ONCE().
Is that correct?

Are there any objections to the patch if the READ_ONCE() calls are removed?

Longpeng (Mike), can you recreate the issue by adding the delay and removing
the READ_ONCE() calls?
-- 
Mike Kravetz

> ---
> v1 -> v2:
>   - avoid renaming    [Matthew, Mike]
> 
> ---
>  mm/hugetlb.c | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index dd8737a..90daf37 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4910,28 +4910,30 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
>  {
>  	pgd_t *pgd;
>  	p4d_t *p4d;
> -	pud_t *pud;
> -	pmd_t *pmd;
> +	pud_t *pud, pud_entry;
> +	pmd_t *pmd, pmd_entry;
>  
>  	pgd = pgd_offset(mm, addr);
> -	if (!pgd_present(*pgd))
> +	if (!pgd_present(READ_ONCE(*pgd)))
>  		return NULL;
>  	p4d = p4d_offset(pgd, addr);
> -	if (!p4d_present(*p4d))
> +	if (!p4d_present(READ_ONCE(*p4d)))
>  		return NULL;
>  
>  	pud = pud_offset(p4d, addr);
> -	if (sz != PUD_SIZE && pud_none(*pud))
> +	pud_entry = READ_ONCE(*pud);
> +	if (sz != PUD_SIZE && pud_none(pud_entry))
>  		return NULL;
>  	/* hugepage or swap? */
> -	if (pud_huge(*pud) || !pud_present(*pud))
> +	if (pud_huge(pud_entry) || !pud_present(pud_entry))
>  		return (pte_t *)pud;
>  
>  	pmd = pmd_offset(pud, addr);
> -	if (sz != PMD_SIZE && pmd_none(*pmd))
> +	pmd_entry = READ_ONCE(*pmd);
> +	if (sz != PMD_SIZE && pmd_none(pmd_entry))
>  		return NULL;
>  	/* hugepage or swap? */
> -	if (pmd_huge(*pmd) || !pmd_present(*pmd))
> +	if (pmd_huge(pmd_entry) || !pmd_present(pmd_entry))
>  		return (pte_t *)pmd;
>  
>  	return NULL;
> 

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