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From:   "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
To:     Christophe Leroy <christophe.leroy@csgroup.eu>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>
Cc:     linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] powerpc: Fix random segfault when freeing hugetlb range
In-Reply-To: <f0cb2a5477cd87d1eaadb128042e20aeb2bc2859.1598860677.git.christophe.leroy@csgroup.eu>
References: <f0cb2a5477cd87d1eaadb128042e20aeb2bc2859.1598860677.git.christophe.leroy@csgroup.eu>
Date:   Wed, 02 Sep 2020 08:53:28 +0530
Message-ID: <875z8weua7.fsf@linux.ibm.com>
MIME-Version: 1.0
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Sender: linux-kernel-owner@vger.kernel.org
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Christophe Leroy <christophe.leroy@csgroup.eu> writes:

> The following random segfault is observed from time to time with
> map_hugetlb selftest:
>
> root@localhost:~# ./map_hugetlb 1 19
> 524288 kB hugepages
> Mapping 1 Mbytes
> Segmentation fault
>
> [   31.219972] map_hugetlb[365]: segfault (11) at 117 nip 77974f8c lr 779a6834 code 1 in ld-2.23.so[77966000+21000]
> [   31.220192] map_hugetlb[365]: code: 9421ffc0 480318d1 93410028 90010044 9361002c 93810030 93a10034 93c10038
> [   31.220307] map_hugetlb[365]: code: 93e1003c 93210024 8123007c 81430038 <80e90004> 814a0004 7f443a14 813a0004
> [   31.221911] BUG: Bad rss-counter state mm:(ptrval) type:MM_FILEPAGES val:33
> [   31.229362] BUG: Bad rss-counter state mm:(ptrval) type:MM_ANONPAGES val:5
>
> This fault is due to hugetlb_free_pgd_range() freeing page tables
> that are also used by regular pages.
>
> As explain in the comment at the beginning of
> hugetlb_free_pgd_range(), the verification done in free_pgd_range()
> on floor and ceiling is not done here, which means
> hugetlb_free_pte_range() can free outside the expected range.
>
> As the verification cannot be done in hugetlb_free_pgd_range(), it
> must be done in hugetlb_free_pte_range().
>

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>

> Fixes: b250c8c08c79 ("powerpc/8xx: Manage 512k huge pages as standard pages.")
> Cc: stable@vger.kernel.org
> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> ---
>  arch/powerpc/mm/hugetlbpage.c | 18 ++++++++++++++++--
>  1 file changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/arch/powerpc/mm/hugetlbpage.c b/arch/powerpc/mm/hugetlbpage.c
> index 26292544630f..e7ae2a2c4545 100644
> --- a/arch/powerpc/mm/hugetlbpage.c
> +++ b/arch/powerpc/mm/hugetlbpage.c
> @@ -330,10 +330,24 @@ static void free_hugepd_range(struct mmu_gather *tlb, hugepd_t *hpdp, int pdshif
>  				 get_hugepd_cache_index(pdshift - shift));
>  }
>  
> -static void hugetlb_free_pte_range(struct mmu_gather *tlb, pmd_t *pmd, unsigned long addr)
> +static void hugetlb_free_pte_range(struct mmu_gather *tlb, pmd_t *pmd,
> +				   unsigned long addr, unsigned long end,
> +				   unsigned long floor, unsigned long ceiling)
>  {
> +	unsigned long start = addr;
>  	pgtable_t token = pmd_pgtable(*pmd);
>  
> +	start &= PMD_MASK;
> +	if (start < floor)
> +		return;
> +	if (ceiling) {
> +		ceiling &= PMD_MASK;
> +		if (!ceiling)
> +			return;
> +	}
> +	if (end - 1 > ceiling - 1)
> +		return;
> +

We do repeat that for pud/pmd/pte hugetlb_free_range. Can we consolidate
that with comment explaining we are checking if the pgtable entry is
mapping outside the range?

>  	pmd_clear(pmd);
>  	pte_free_tlb(tlb, token, addr);
>  	mm_dec_nr_ptes(tlb->mm);
> @@ -363,7 +377,7 @@ static void hugetlb_free_pmd_range(struct mmu_gather *tlb, pud_t *pud,
>  			 */
>  			WARN_ON(!IS_ENABLED(CONFIG_PPC_8xx));
>  
> -			hugetlb_free_pte_range(tlb, pmd, addr);
> +			hugetlb_free_pte_range(tlb, pmd, addr, end, floor, ceiling);
>  
>  			continue;
>  		}
> -- 
> 2.25.0