Re: [PATCH] x86/mm: don't free p4d table when it is folded at runtime.

[Kirill A. Shutemov]

On Mon, Jun 25, 2018 at 01:24:27PM +0300, Andrey Ryabinin wrote:
> When the p4d page table layer is folded at runtime, the p4d_free()
> should do nothing, the same as in <asm-generic/pgtable-nop4d.h>.
> 
> It seems this bug should cause double-free in efi_call_phys_epilog(),
> but I don't know how to trigger that code path, so I can't confirm that
> by testing.

+ Baoquan, Matt.

There's other bug in the efi_call_phys_epilog() that prevents the bug from
being triggered.

With the patch below. You can trigger the bug with efi=old_map in kernel
command line + KALSR and CONFIG_X86_5LEVEL=y:

	page:fffff6bec0000000 count:0 mapcount:1 mapping:0000000000000000 index:0x0
	flags: 0x800(reserved)
	raw: 0000000000000800 fffff6bec0000008 fffff6bec0000008 0000000000000000
	raw: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
	page dumped because: VM_BUG_ON_PAGE(page_ref_count(page) == 0)
	------------[ cut here ]------------
	kernel BUG at /home/kas/linux/la57/include/linux/mm.h:499!
	invalid opcode: 0000 [#1] PREEMPT SMP
	CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.18.0-rc2-00037-g6f0d349d922b-dirty #58
	Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
	RIP: 0010:__free_pages+0x28/0x30
	Code: 00 00 8b 47 34 85 c0 74 15 f0 ff 4f 34 75 09 85 f6 74 06 e9 ca d8 ff ff c3 e9 64 ff ff ff 48
	RSP: 0000:ffffffff9a403e90 EFLAGS: 00000246
	RAX: 000000000000003e RBX: ffffffff9a41d000 RCX: 0000000000000002
	RDX: 0000000000000000 RSI: 0000000000000000 RDI: 00000000ffffffff
	RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
	R10: 0000000000000000 R11: 0000000000000000 R12: ffff9092af089000
	R13: ffffffff9a598a80 R14: 0000000000000001 R15: 0000000000000001
	FS:  0000000000000000(0000) GS:ffff9092bfc00000(0000) knlGS:0000000000000000
	CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
	CR2: ffff9092bffff000 CR3: 0000000198e1d000 CR4: 00000000000006b0
	Call Trace:
	 efi_call_phys_epilog+0x17d/0x1bb
	 efi_enter_virtual_mode+0x457/0x4ca
	 start_kernel+0x443/0x4dc
	 secondary_startup_64+0xb7/0xc0
	Modules linked in:
	---[ end trace 61e271260b11acdd ]---

I'll send patch for efi_call_phys_epilog().

> 
> Fixes: 98219dda2ab5 ("x86/mm: Fold p4d page table layer at runtime")
> Signed-off-by: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx>

Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # 4.17

> ---
>  arch/x86/include/asm/pgalloc.h | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/include/asm/pgalloc.h b/arch/x86/include/asm/pgalloc.h
> index ada6410fd2ec..fbd578daa66e 100644
> --- a/arch/x86/include/asm/pgalloc.h
> +++ b/arch/x86/include/asm/pgalloc.h
> @@ -184,6 +184,9 @@ static inline p4d_t *p4d_alloc_one(struct mm_struct *mm, unsigned long addr)
>  
>  static inline void p4d_free(struct mm_struct *mm, p4d_t *p4d)
>  {
> +	if (!pgtable_l5_enabled())
> +		return;
> +
>  	BUG_ON((unsigned long)p4d & (PAGE_SIZE-1));
>  	free_page((unsigned long)p4d);
>  }

diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
index e01f7ceb9e7a..77873ce700ae 100644
--- a/arch/x86/platform/efi/efi_64.c
+++ b/arch/x86/platform/efi/efi_64.c
@@ -166,14 +166,14 @@ void __init efi_call_phys_epilog(pgd_t *save_pgd)
                pgd = pgd_offset_k(pgd_idx * PGDIR_SIZE);
                set_pgd(pgd_offset_k(pgd_idx * PGDIR_SIZE), save_pgd[pgd_idx]);

-               if (!(pgd_val(*pgd) & _PAGE_PRESENT))
+               if (!pgd_present(*pgd))
                        continue;

                for (i = 0; i < PTRS_PER_P4D; i++) {
                        p4d = p4d_offset(pgd,
                                         pgd_idx * PGDIR_SIZE + i * P4D_SIZE);

-                       if (!(p4d_val(*p4d) & _PAGE_PRESENT))
+                       if (!p4d_present(*p4d))
                                continue;

                        pud = (pud_t *)p4d_page_vaddr(*p4d);
-- 
 Kirill A. Shutemov