Re: [PATCH v2 2/2] arm64/mm: fix incorrect file_map_count for invalid pmd/pud

[Liu Shixin]

On 2022/11/18 22:34, Will Deacon wrote:
> On Thu, Nov 17, 2022 at 03:56:02PM +0800, Liu Shixin wrote:
>> The page table check trigger BUG_ON() unexpectedly when split hugepage:
>>
>>  ------------[ cut here ]------------
>>  kernel BUG at mm/page_table_check.c:119!
>>  Internal error: Oops - BUG: 00000000f2000800 [#1] SMP
>>  Dumping ftrace buffer:
>>     (ftrace buffer empty)
>>  Modules linked in:
>>  CPU: 7 PID: 210 Comm: transhuge-stres Not tainted 6.1.0-rc3+ #748
>>  Hardware name: linux,dummy-virt (DT)
>>  pstate: 20000005 (nzCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>>  pc : page_table_check_set.isra.0+0x398/0x468
>>  lr : page_table_check_set.isra.0+0x1c0/0x468
>> [...]
>>  Call trace:
>>   page_table_check_set.isra.0+0x398/0x468
>>   __page_table_check_pte_set+0x160/0x1c0
>>   __split_huge_pmd_locked+0x900/0x1648
>>   __split_huge_pmd+0x28c/0x3b8
>>   unmap_page_range+0x428/0x858
>>   unmap_single_vma+0xf4/0x1c8
>>   zap_page_range+0x2b0/0x410
>>   madvise_vma_behavior+0xc44/0xe78
>>   do_madvise+0x280/0x698
>>   __arm64_sys_madvise+0x90/0xe8
>>   invoke_syscall.constprop.0+0xdc/0x1d8
>>   do_el0_svc+0xf4/0x3f8
>>   el0_svc+0x58/0x120
>>   el0t_64_sync_handler+0xb8/0xc0
>>   el0t_64_sync+0x19c/0x1a0
>> [...]
>>
>> On arm64, pmd_leaf() will return true even if the pmd is invalid due to
>> pmd_present_invalid() check. So in pmdp_invalidate() the file_map_count
>> will not only decrease once but also increase once. Then in set_pte_at(),
>> the file_map_count increase again, and so trigger BUG_ON() unexpectedly.
>>
>> Fix this problem by adding pmd_valid() in pmd_user_accessible_page().
>> Moreover, add pud_valid() for pud_user_accessible_page() too.
>>
>> Fixes: 42b2547137f5 ("arm64/mm: enable ARCH_SUPPORTS_PAGE_TABLE_CHECK")
>> Reported-by: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
>> Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx>
>> Acked-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
>> ---
>>  arch/arm64/include/asm/pgtable.h | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
>> index edf6625ce965..3bc64199aa2e 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -863,12 +863,12 @@ static inline bool pte_user_accessible_page(pte_t pte)
>>  
>>  static inline bool pmd_user_accessible_page(pmd_t pmd)
>>  {
>> -	return pmd_leaf(pmd) && (pmd_user(pmd) || pmd_user_exec(pmd));
>> +	return pmd_valid(pmd) && pmd_leaf(pmd) && (pmd_user(pmd) || pmd_user_exec(pmd));
> Hmm, doesn't this have a funny interaction with PROT_NONE where the pmd is
> invalid but present? If you don't care about PROT_NONE, then you could just
> do:
>
>   pmd_valid(pmd) && !pmd_table(pmd) && (pmd_user(pmd) || pmd_user_exec(pmd))
>
> but if you do care then you could do:
>
>   pmd_leaf(pmd) && !pmd_present_invalid(pmd) && (pmd_user(pmd) || pmd_user_exec(pmd))
I prefer the latter. I will fix and resend later.
>>  static inline bool pud_user_accessible_page(pud_t pud)
>>  {
>> -	return pud_leaf(pud) && pud_user(pud);
>> +	return pud_valid(pud) && pud_leaf(pud) && pud_user(pud);
> Not caused by this patch, but why don't we have something like a
> pud_user_exec() check here like we do for the pte and pmd levels?
As far as I know, there is no user use the user executable pud on arm64, so didn't define pud_user_exec().

Thanks,

>
> Will
> .
>