linux-kernel - Re: [PATCH -next v2] powerpc/64s/pgtable: fix an undefined behaviour

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Date:   Thu, 5 Mar 2020 20:22:06 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Qian Cai <cai@....pw>, mpe@...erman.id.au,
        akpm@...ux-foundation.org
Cc:     rashmicy@...il.com, linux-mm@...ck.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next v2] powerpc/64s/pgtable: fix an undefined behaviour



Le 05/03/2020 à 15:32, Qian Cai a écrit :
> Booting a power9 server with hash MMU could trigger an undefined
> behaviour because pud_offset(p4d, 0) will do,
> 
> 0 >> (PAGE_SHIFT:16 + PTE_INDEX_SIZE:8 + H_PMD_INDEX_SIZE:10)
> 
> Fix it by converting pud_offset() and friends to static inline
> functions.

I was suggesting to convert pud_index() to static inline, because that's 
where the shift sits. Is it not possible ?

Here you seems to fix the problem for now, but if someone reuses 
pud_index() in another macro one day, the same problem may happen again.

Christophe

> 
>   UBSAN: shift-out-of-bounds in arch/powerpc/mm/ptdump/ptdump.c:282:15
>   shift exponent 34 is too large for 32-bit type 'int'
>   CPU: 6 PID: 1 Comm: swapper/0 Not tainted 5.6.0-rc4-next-20200303+ #13
>   Call Trace:
>   dump_stack+0xf4/0x164 (unreliable)
>   ubsan_epilogue+0x18/0x78
>   __ubsan_handle_shift_out_of_bounds+0x160/0x21c
>   walk_pagetables+0x2cc/0x700
>   walk_pud at arch/powerpc/mm/ptdump/ptdump.c:282
>   (inlined by) walk_pagetables at arch/powerpc/mm/ptdump/ptdump.c:311
>   ptdump_check_wx+0x8c/0xf0
>   mark_rodata_ro+0x48/0x80
>   kernel_init+0x74/0x194
>   ret_from_kernel_thread+0x5c/0x74
> 
> Suggested-by: Christophe Leroy <christophe.leroy@....fr>
> Signed-off-by: Qian Cai <cai@....pw>
> ---
>   arch/powerpc/include/asm/book3s/64/pgtable.h | 20 ++++++++++++++------
>   1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h
> index fa60e8594b9f..4967bc9e25e2 100644
> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h
> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
> @@ -1016,12 +1016,20 @@ static inline bool p4d_access_permitted(p4d_t p4d, bool write)
>   
>   #define pgd_offset(mm, address)	 ((mm)->pgd + pgd_index(address))
>   
> -#define pud_offset(p4dp, addr)	\
> -	(((pud_t *) p4d_page_vaddr(*(p4dp))) + pud_index(addr))
> -#define pmd_offset(pudp,addr) \
> -	(((pmd_t *) pud_page_vaddr(*(pudp))) + pmd_index(addr))
> -#define pte_offset_kernel(dir,addr) \
> -	(((pte_t *) pmd_page_vaddr(*(dir))) + pte_index(addr))
> +static inline pud_t *pud_offset(p4d_t *p4d, unsigned long address)
> +{
> +	return (pud_t *)p4d_page_vaddr(*p4d) + pud_index(address);
> +}
> +
> +static inline pmd_t *pmd_offset(pud_t *pud, unsigned long address)
> +{
> +	return (pmd_t *)pud_page_vaddr(*pud) + pmd_index(address);
> +}
> +
> +static inline pte_t *pte_offset_kernel(pmd_t *pmd, unsigned long address)
> +{
> +	return (pte_t *)pmd_page_vaddr(*pmd) + pte_index(address);
> +}
>   
>   #define pte_offset_map(dir,addr)	pte_offset_kernel((dir), (addr))
>   
>