Re: [PATCH -next] powerpc/mm/ptdump: fix an undefined behaviour

[Christophe Leroy <christophe.leroy@c-s.fr>]

Le 05/03/2020 à 05:47, 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)
> 
>   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
> 
> Fixes: 8eb07b187000 ("powerpc/mm: Dump linux pagetables")
> Signed-off-by: Qian Cai <cai@lca.pw>
> ---
> 
> Notes for maintainers:
> 
> This is on the top of the linux-next commit "powerpc: add support for
> folded p4d page tables" which is in the Andrew's tree.
> 
>   arch/powerpc/mm/ptdump/ptdump.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/mm/ptdump/ptdump.c b/arch/powerpc/mm/ptdump/ptdump.c
> index 9d6256b61df3..b530f81398a7 100644
> --- a/arch/powerpc/mm/ptdump/ptdump.c
> +++ b/arch/powerpc/mm/ptdump/ptdump.c
> @@ -279,7 +279,7 @@ static void walk_pmd(struct pg_state *st, pud_t *pud, unsigned long start)
>   
>   static void walk_pud(struct pg_state *st, p4d_t *p4d, unsigned long start)
>   {
> -	pud_t *pud = pud_offset(p4d, 0);
> +	pud_t *pud = pud_offset(p4d, 0UL);

Is that the only place we have to do this ?

(In 5.6-rc) I see the same in:
/arch/powerpc/mm/ptdump/hashpagetable.c
/arch/powerpc/kvm/book3s_64_mmu_radix.c

Wouldn't it be better to:
- Either cast addr to unsigned long in pud_index() macro
- Or change pud_index() macro to a static inline function as x86 ?

Christophe