Re: [PATCH 1/1] MIPS: Don't clear _PAGE_SPECIAL in _PAGE_CHG_MASK

Thomas Bogendoerfer
2023-04-24T08:01:52Z

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@oracle.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.

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