linux-kernel - Re: [PATCH] powerpc/hugetlb: fix page rights verification in gup

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Message-Id: <87r2xdrfny.fsf@skywalker.in.ibm.com>
Date:   Tue, 18 Jul 2017 19:49:29 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:     Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Scott Wood <oss@...error.net>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/hugetlb: fix page rights verification in gup_hugepte()

Christophe Leroy <christophe.leroy@....fr> writes:

> gup_hugepte() checks if pages are present and readable, and
> when  'write' is set, also checks if the pages are writable.
>
> Initially this was done by checking if _PAGE_PRESENT and
> _PAGE_READ were set. In addition, _PAGE_WRITE was verified for write
> accesses.
>
> The problem is that we have to handle the three following cases:
> 1/ The target defines __PAGE_READ and __PAGE_WRITE
> 2/ The target defines __PAGE_RW
> 3/ The target defines __PAGE_RO
>
> In case 1/, this is obvious
> In case 2/, __PAGE_READ is defined as 0 and __PAGE_WRITE as __PAGE_RW
> so it works as well.
> But in case 3, __PAGE_RW is defined as 0, which means __PAGE_WRITE is 0
> and then the test returns true (page writable) in all cases.
>
> A first correction was attempted in commit 6b8cb66a6a7cc ("powerpc: Fix
> usage of _PAGE_RO in hugepage"), but that fix is wrong:
> instead of checking that the page is writable when write is requested,
> it checks that the page is NOT writable when write is NOT requested.
>
> This patch adds a new pte_read() helper to check whether a page is
> readable or not. This avoids handling all possible cases in
> gup_hugepte().
>
> Then gup_hugepte() is modified to use pte_present(), pte_read()
> and pte_write() instead of the raw flags.
>

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

> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
> ---
>  arch/powerpc/include/asm/book3s/32/pgtable.h |  1 +
>  arch/powerpc/include/asm/book3s/64/pgtable.h |  5 +++++
>  arch/powerpc/include/asm/nohash/pgtable.h    |  1 +
>  arch/powerpc/mm/hugetlbpage.c                | 15 +++------------
>  4 files changed, 10 insertions(+), 12 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/book3s/32/pgtable.h b/arch/powerpc/include/asm/book3s/32/pgtable.h
> index 7fb755880409..2a67b861e6e1 100644
> --- a/arch/powerpc/include/asm/book3s/32/pgtable.h
> +++ b/arch/powerpc/include/asm/book3s/32/pgtable.h
> @@ -301,6 +301,7 @@ int map_kernel_page(unsigned long va, phys_addr_t pa, int flags);
>  
>  /* Generic accessors to PTE bits */
>  static inline int pte_write(pte_t pte)		{ return !!(pte_val(pte) & _PAGE_RW);}
> +static inline int pte_read(pte_t pte)		{ return 1; }
>  static inline int pte_dirty(pte_t pte)		{ return !!(pte_val(pte) & _PAGE_DIRTY); }
>  static inline int pte_young(pte_t pte)		{ return !!(pte_val(pte) & _PAGE_ACCESSED); }
>  static inline int pte_special(pte_t pte)	{ return !!(pte_val(pte) & _PAGE_SPECIAL); }
> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h
> index c0737c86a362..83cb73c8c3bb 100644
> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h
> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
> @@ -409,6 +409,11 @@ static inline int pte_write(pte_t pte)
>  	return __pte_write(pte) || pte_savedwrite(pte);
>  }
>  
> +static inline int pte_read(pte_t pte)
> +{
> +	return !!(pte_raw(pte) & cpu_to_be64(_PAGE_READ));
> +}
> +
>  #define __HAVE_ARCH_PTEP_SET_WRPROTECT
>  static inline void ptep_set_wrprotect(struct mm_struct *mm, unsigned long addr,
>  				      pte_t *ptep)
> diff --git a/arch/powerpc/include/asm/nohash/pgtable.h b/arch/powerpc/include/asm/nohash/pgtable.h
> index e5805ad78e12..17989c3d9a24 100644
> --- a/arch/powerpc/include/asm/nohash/pgtable.h
> +++ b/arch/powerpc/include/asm/nohash/pgtable.h
> @@ -14,6 +14,7 @@ static inline int pte_write(pte_t pte)
>  {
>  	return (pte_val(pte) & (_PAGE_RW | _PAGE_RO)) != _PAGE_RO;
>  }
> +static inline int pte_read(pte_t pte)		{ return 1; }
>  static inline int pte_dirty(pte_t pte)		{ return pte_val(pte) & _PAGE_DIRTY; }
>  static inline int pte_young(pte_t pte)		{ return pte_val(pte) & _PAGE_ACCESSED; }
>  static inline int pte_special(pte_t pte)	{ return pte_val(pte) & _PAGE_SPECIAL; }
> diff --git a/arch/powerpc/mm/hugetlbpage.c b/arch/powerpc/mm/hugetlbpage.c
> index e1bf5ca397fe..1226932579a0 100644
> --- a/arch/powerpc/mm/hugetlbpage.c
> +++ b/arch/powerpc/mm/hugetlbpage.c
> @@ -977,7 +977,6 @@ EXPORT_SYMBOL_GPL(__find_linux_pte_or_hugepte);
>  int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
>  		unsigned long end, int write, struct page **pages, int *nr)
>  {
> -	unsigned long mask;
>  	unsigned long pte_end;
>  	struct page *head, *page;
>  	pte_t pte;
> @@ -988,18 +987,10 @@ int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
>  		end = pte_end;
>  
>  	pte = READ_ONCE(*ptep);
> -	mask = _PAGE_PRESENT | _PAGE_READ;
> -
> -	/*
> -	 * On some CPUs like the 8xx, _PAGE_RW hence _PAGE_WRITE is defined
> -	 * as 0 and _PAGE_RO has to be set when a page is not writable
> -	 */
> -	if (write)
> -		mask |= _PAGE_WRITE;
> -	else
> -		mask |= _PAGE_RO;
>  
> -	if ((pte_val(pte) & mask) != mask)
> +	if (!pte_present(pte) || !pte_read(pte))
> +		return 0;
> +	if (write && !pte_write(pte))
>  		return 0;
>  
>  	/* hugepages are never "special" */
> -- 
> 2.12.0