linux-kernel - Re: [PATCH 1/1] MIPS: Don't clear _PAGE_SPECIAL in _PAGE_CHG

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Message-ID: <20230424080124.GA4889@alpha.franken.de>
Date:   Mon, 24 Apr 2023 10:01:24 +0200
From:   Thomas Bogendoerfer <tsbogend@...ha.franken.de>
To:     Henry Willard <henry.willard@...cle.com>
Cc:     linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] MIPS: Don't clear _PAGE_SPECIAL in _PAGE_CHG_MASK

On Wed, Apr 12, 2023 at 03:29:53PM -0600, Henry Willard wrote:
> In the special case where
> 
> 	p = mmap(NULL, ALLOC_SIZE, PROT_READ,
>                 MAP_PRIVATE | MAP_ANONYMOUS | MAP_POPULATE, -1, 0);
> 
> is followed by
> 
> 	rc = mprotect(p, ALLOC_SIZE, PROT_NONE);
> 
> the _PAGE_SPECIAL bit in the page tables will be cleared by
> mistake and the later unmapped operations will incorrectly
> modify the struct page for the the zero page. This sequence
> occurs in the madvise05 test of the Linux Test Project
> suite of tests.
> 
> This was discovered while testing an older version of the kernel
> (5.4.17) on a MIPS device. Unfortunately, support for this device
> is not available in newer kernels, so I can't test this with the
> latest Linux kernel code. It looks like the problem exists in
> newer kernels, but I can't verify it. Except for the LTP test,
> this sequence of calls is probably not common.
> 
> Passing it along in the hope it will be useful to someone.
> 
> Signed-off-by: Henry Willard <henry.willard@...cle.com>
> ---
>  arch/mips/include/asm/pgtable-bits.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/mips/include/asm/pgtable-bits.h b/arch/mips/include/asm/pgtable-bits.h
> index 2362842ee2b5..1c576679aa87 100644
> --- a/arch/mips/include/asm/pgtable-bits.h
> +++ b/arch/mips/include/asm/pgtable-bits.h
> @@ -280,6 +280,7 @@ static inline uint64_t pte_to_entrylo(unsigned long pte_val)
>  #define __WRITEABLE	(_PAGE_SILENT_WRITE | _PAGE_WRITE | _PAGE_MODIFIED)
>  
>  #define _PAGE_CHG_MASK	(_PAGE_ACCESSED | _PAGE_MODIFIED |	\
> -			 _PAGE_SOFT_DIRTY | _PFN_MASK | _CACHE_MASK)
> +			 _PAGE_SOFT_DIRTY | _PFN_MASK |   \
> +			 _CACHE_MASK | _PAGE_SPECIAL)
>  
>  #endif /* _ASM_PGTABLE_BITS_H */
> -- 
> 2.31.1

applied to mips-next.

Thomas.

-- 
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea.                                                [ RFC1925, 2.3 ]