linux-kernel - Re: [PATCH 4.9 105/125] mm, gup: add missing refcount overflow checks on x86 and s390

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Date:   Wed, 4 Dec 2019 19:27:44 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Cc:     stable@...r.kernel.org, Sasha Levin <sashal@...nel.org>,
        Ben Hutchings <ben.hutchings@...ethink.co.uk>
Subject: Re: [PATCH 4.9 105/125] mm, gup: add missing refcount overflow checks
 on x86 and s390

On 12/4/19 6:56 PM, Greg Kroah-Hartman wrote:
> From: Vlastimil Babka <vbabka@...e.cz>
> 
> The mainline commit 8fde12ca79af ("mm: prevent get_user_pages() from
> overflowing page refcount") was backported to 4.9.y stable as commit
> 2ed768cfd895. The backport however missed that in 4.9, there are several
> arch-specific gup.c versions with fast gup implementations, so these do not
> prevent refcount overflow.
> 
> This is partially fixed for x86 in stable-only commit d73af79742e7 ("x86, mm,
> gup: prevent get_page() race with munmap in paravirt guest"). This stable-only
> commit adds missing parts to x86 version, as well as s390 version, both taken
> from the SUSE SLES/openSUSE 4.12-based kernels.
> 
> The remaining architectures with own gup.c are sparc, mips, sh. It's unlikely
> the known overflow scenario based on FUSE, which needs 140GB of RAM, is a
> problem for those architectures, and I don't feel confident enough to patch
> them.
> 
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> Signed-off-by: Sasha Levin <sashal@...nel.org>

No, this one had a leak bug and I've sent updated version:
https://lore.kernel.org/linux-mm/e274291b-054f-2fad-28e8-59fabf312e61@suse.cz/

> ---
>  arch/s390/mm/gup.c |  9 ++++++---
>  arch/x86/mm/gup.c  | 10 ++++++++--
>  2 files changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/s390/mm/gup.c b/arch/s390/mm/gup.c
> index 97fc449a74707..33a940389a6d1 100644
> --- a/arch/s390/mm/gup.c
> +++ b/arch/s390/mm/gup.c
> @@ -38,7 +38,8 @@ static inline int gup_pte_range(pmd_t *pmdp, pmd_t pmd, unsigned long addr,
>  		VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
>  		page = pte_page(pte);
>  		head = compound_head(page);
> -		if (!page_cache_get_speculative(head))
> +		if (unlikely(WARN_ON_ONCE(page_ref_count(head) < 0)
> +		    || !page_cache_get_speculative(head)))
>  			return 0;
>  		if (unlikely(pte_val(pte) != pte_val(*ptep))) {
>  			put_page(head);
> @@ -76,7 +77,8 @@ static inline int gup_huge_pmd(pmd_t *pmdp, pmd_t pmd, unsigned long addr,
>  		refs++;
>  	} while (addr += PAGE_SIZE, addr != end);
>  
> -	if (!page_cache_add_speculative(head, refs)) {
> +	if (unlikely(WARN_ON_ONCE(page_ref_count(head) < 0)
> +	    || !page_cache_add_speculative(head, refs))) {
>  		*nr -= refs;
>  		return 0;
>  	}
> @@ -150,7 +152,8 @@ static int gup_huge_pud(pud_t *pudp, pud_t pud, unsigned long addr,
>  		refs++;
>  	} while (addr += PAGE_SIZE, addr != end);
>  
> -	if (!page_cache_add_speculative(head, refs)) {
> +	if (unlikely(WARN_ON_ONCE(page_ref_count(head) < 0)
> +	    || !page_cache_add_speculative(head, refs))) {
>  		*nr -= refs;
>  		return 0;
>  	}
> diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c
> index d7db45bdfb3bd..551fc7fea046d 100644
> --- a/arch/x86/mm/gup.c
> +++ b/arch/x86/mm/gup.c
> @@ -202,10 +202,12 @@ static int __gup_device_huge_pmd(pmd_t pmd, unsigned long addr,
>  			undo_dev_pagemap(nr, nr_start, pages);
>  			return 0;
>  		}
> +		if (unlikely(!try_get_page(page))) {
> +			put_dev_pagemap(pgmap);
> +			return 0;
> +		}
>  		SetPageReferenced(page);
>  		pages[*nr] = page;
> -		get_page(page);
> -		put_dev_pagemap(pgmap);
>  		(*nr)++;
>  		pfn++;
>  	} while (addr += PAGE_SIZE, addr != end);
> @@ -230,6 +232,8 @@ static noinline int gup_huge_pmd(pmd_t pmd, unsigned long addr,
>  
>  	refs = 0;
>  	head = pmd_page(pmd);
> +	if (WARN_ON_ONCE(page_ref_count(head) <= 0))
> +		return 0;
>  	page = head + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
>  	do {
>  		VM_BUG_ON_PAGE(compound_head(page) != head, page);
> @@ -289,6 +293,8 @@ static noinline int gup_huge_pud(pud_t pud, unsigned long addr,
>  
>  	refs = 0;
>  	head = pud_page(pud);
> +	if (WARN_ON_ONCE(page_ref_count(head) <= 0))
> +		return 0;
>  	page = head + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
>  	do {
>  		VM_BUG_ON_PAGE(compound_head(page) != head, page);
>