Re: [PATCH] arm64/hugetlb: Fix page table walk in huge_pte_alloc()

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 4/15/24 12:08, Ard Biesheuvel wrote:
> Hello Anshuman,
> 
> On Mon, 15 Apr 2024 at 08:31, Anshuman Khandual
> <anshuman.khandual@arm.com> wrote:
>>
>> Currently normal HugeTLB fault ends up crashing the kernel, as p4dp derived
>> from p4d_offset() is an invalid address when PGTABLE_LEVEL = 5. A p4d level
>> entry needs to be allocated when not available while walking the page table
>> during HugeTLB faults. Let's call p4d_alloc() to allocate such entries when
>> required instead of current p4d_offset().
>>
>>  Unable to handle kernel paging request at virtual address ffffffff80000000
>>  Mem abort info:
>>    ESR = 0x0000000096000005
>>    EC = 0x25: DABT (current EL), IL = 32 bits
>>    SET = 0, FnV = 0
>>    EA = 0, S1PTW = 0
>>    FSC = 0x05: level 1 translation fault
>>  Data abort info:
>>    ISV = 0, ISS = 0x00000005, ISS2 = 0x00000000
>>    CM = 0, WnR = 0, TnD = 0, TagAccess = 0
>>    GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
>>  swapper pgtable: 4k pages, 52-bit VAs, pgdp=0000000081da9000
>>  [ffffffff80000000] pgd=1000000082cec003, p4d=0000000082c32003, pud=0000000000000000
>>  Internal error: Oops: 0000000096000005 [#1] PREEMPT SMP
>>  Modules linked in:
>>  CPU: 1 PID: 108 Comm: high_addr_hugep Not tainted 6.9.0-rc4 #48
>>  Hardware name: Foundation-v8A (DT)
>>  pstate: 01402005 (nzcv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--)
>>  pc : huge_pte_alloc+0xd4/0x334
>>  lr : hugetlb_fault+0x1b8/0xc68
>>  sp : ffff8000833bbc20
>>  x29: ffff8000833bbc20 x28: fff000080080cb58 x27: ffff800082a7cc58
>>  x26: 0000000000000000 x25: fff0000800378e40 x24: fff00008008d6c60
>>  x23: 00000000de9dbf07 x22: fff0000800378e40 x21: 0004000000000000
>>  x20: 0004000000000000 x19: ffffffff80000000 x18: 1ffe00010011d7a1
>>  x17: 0000000000000001 x16: ffffffffffffffff x15: 0000000000000001
>>  x14: 0000000000000000 x13: ffff8000816120d0 x12: ffffffffffffffff
>>  x11: 0000000000000000 x10: fff00008008ebd0c x9 : 0004000000000000
>>  x8 : 0000000000001255 x7 : fff00008003e2000 x6 : 00000000061d54b0
>>  x5 : 0000000000001000 x4 : ffffffff80000000 x3 : 0000000000200000
>>  x2 : 0000000000000004 x1 : 0000000080000000 x0 : 0000000000000000
>>  Call trace:
>>  huge_pte_alloc+0xd4/0x334
>>  hugetlb_fault+0x1b8/0xc68
>>  handle_mm_fault+0x260/0x29c
>>  do_page_fault+0xfc/0x47c
>>  do_translation_fault+0x68/0x74
>>  do_mem_abort+0x44/0x94
>>  el0_da+0x2c/0x9c
>>  el0t_64_sync_handler+0x70/0xc4
>>  el0t_64_sync+0x190/0x194
>>  Code: aa000084 cb010084 b24c2c84 8b130c93 (f9400260)
>>  ---[ end trace 0000000000000000 ]---
>>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Ard Biesheuvel <ardb@kernel.org>
>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>> Cc: Mark Rutland <mark.rutland@arm.com>
>> Cc: linux-arm-kernel@lists.infradead.org
>> Cc: linux-kernel@vger.kernel.org
>> Fixes: a6bbf5d4d9d1 ("arm64: mm: Add definitions to support 5 levels of paging")
>> Reported-by: Dev Jain <dev.jain@arm.com>
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>> ---
>> This patch applies on v6.9-rc4
>>
>>  arch/arm64/mm/hugetlbpage.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
> 
> Acked-by: Ard Biesheuvel <ardb@kernel.org>
> 
> Thanks for fixing this. One question below.
> 
> 
>> diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
>> index 0f0e10bb0a95..5c819459555a 100644
>> --- a/arch/arm64/mm/hugetlbpage.c
>> +++ b/arch/arm64/mm/hugetlbpage.c
>> @@ -276,7 +276,7 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma,
>>         pte_t *ptep = NULL;
>>
>>         pgdp = pgd_offset(mm, addr);
>> -       p4dp = p4d_offset(pgdp, addr);
>> +       p4dp = p4d_alloc(mm, pgdp, addr);
> 
> Shouldn't we check p4dp for NULL here?

Sure, makes sense. I will add a check for p4dp, just like pudp below.

> 
>>         pudp = pud_alloc(mm, p4dp, addr);
>>         if (!pudp)
>>                 return NULL;
>> --
>> 2.25.1
>>

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