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

[Ard Biesheuvel]

Hello Anshuman,

On Mon, 15 Apr 2024 at 08:31, Anshuman Khandual
<anshuman.khandual at 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 at arm.com>
> Cc: Will Deacon <will at kernel.org>
> Cc: Ard Biesheuvel <ardb at kernel.org>
> Cc: Ryan Roberts <ryan.roberts at arm.com>
> Cc: Mark Rutland <mark.rutland at arm.com>
> Cc: linux-arm-kernel at lists.infradead.org
> Cc: linux-kernel at vger.kernel.org
> Fixes: a6bbf5d4d9d1 ("arm64: mm: Add definitions to support 5 levels of paging")
> Reported-by: Dev Jain <dev.jain at arm.com>
> Signed-off-by: Anshuman Khandual <anshuman.khandual at 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 at 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?

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