Re: [PATCH v4] x86/efi: Correct ident mapping of efi old_map when kalsr enabled

[Dave Young]

Hi Baoquan,
On 05/18/17 at 02:39pm, Baoquan He wrote:
> For EFI with 'efi=old_map' kernel option specified, Kernel will panic
> when kaslr is enabled.
> 
> The back trace is:
> 
> BUG: unable to handle kernel paging request at 000000007febd57e
> IP: 0x7febd57e
> PGD 1025a067
> PUD 0
> 
> Oops: 0010 [#1] SMP
> [ ... ]
> Call Trace:
>  ? efi_call+0x58/0x90
>  ? printk+0x58/0x6f
>  efi_enter_virtual_mode+0x3c5/0x50d
>  start_kernel+0x40f/0x4b8
>  ? set_init_arg+0x55/0x55
>  ? early_idt_handler_array+0x120/0x120
>  x86_64_start_reservations+0x24/0x26
>  x86_64_start_kernel+0x14c/0x16f
>  start_cpu+0x14/0x14
> 
> The root cause is the ident mapping is not built correctly in old_map case.
> 
> For nokaslr kernel, PAGE_OFFSET is 0xffff880000000000 which is PGDIR_SIZE
> aligned. We can borrow the pud table from direct mapping safely. Given a
> physical address X, we have pud_index(X) == pud_index(__va(X)). However,
> for kaslr kernel, PAGE_OFFSET is PUD_SIZE aligned. For a given physical
> address X, pud_index(X) != pud_index(__va(X)). We can't only copy pgd entry
> from direct mapping to build ident mapping, instead need copy pud entry
> one by one from direct mapping.
> 
> Fix it.
> 
> Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
> Signed-off-by: Dave Young <dyoung@xxxxxxxxxx>

Although I put some effort on debugging the problem, the patch should
be your credit, also I'm not familiar with the pgtable walking code
especially p4d usage, it should be reviewed by x86/efi/mm maintainer.

It would be better to drop my signed-off-by 

> Cc: Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx>
> Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Thomas Garnier <thgarnie@xxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Russ Anderson <rja@xxxxxxx>
> Cc: Frank Ramsay <frank.ramsay@xxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Bhupesh Sharma <bhsharma@xxxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: linux-efi@xxxxxxxxxxxxxxx
> ---
> v3->v4:
>     1. Forget running scripts/checkpatch.pl to check patch, there are several
>     code stype issue. Correct them in this version.
> 
> v2->v3:
>     1. Rewrite code to copy pud entry one by one so that code can be understood
>     better. Usually we only have less than 1TB or several TB memory, pud entry
>     copy one by one won't impact efficiency.
> 
>     2. Adding p4d page table handling.
> 
> v1->v2:
>     Change code and add description according to Thomas's suggestion as below:
> 
>     1. Add checking if pud table is allocated successfully. If not just break
>     the for loop.
> 
>     2. Add code comment to explain how the 1:1 mapping is built in efi_call_phys_prolog
> 
>     3. Other minor change
> 
>  arch/x86/platform/efi/efi_64.c | 70 +++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 62 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
> index c488625..087aafc 100644
> --- a/arch/x86/platform/efi/efi_64.c
> +++ b/arch/x86/platform/efi/efi_64.c
> @@ -71,11 +71,13 @@ static void __init early_code_mapping_set_exec(int executable)
>  
>  pgd_t * __init efi_call_phys_prolog(void)
>  {
> -	unsigned long vaddress;
> -	pgd_t *save_pgd;
> +	unsigned long vaddr, addr_pgd, addr_p4d, addr_pud;
> +	pgd_t *save_pgd, *pgd_k, *pgd_efi;
> +	p4d_t *p4d, *p4d_k, *p4d_efi;
> +	pud_t *pud;
>  
>  	int pgd;
> -	int n_pgds;
> +	int n_pgds, i, j;
>  
>  	if (!efi_enabled(EFI_OLD_MEMMAP)) {
>  		save_pgd = (pgd_t *)read_cr3();
> @@ -88,10 +90,44 @@ pgd_t * __init efi_call_phys_prolog(void)
>  	n_pgds = DIV_ROUND_UP((max_pfn << PAGE_SHIFT), PGDIR_SIZE);
>  	save_pgd = kmalloc_array(n_pgds, sizeof(*save_pgd), GFP_KERNEL);
>  
> +	/*
> +	 * Build 1:1 ident mapping for old_map usage. It needs to be noticed
> +	 * that PAGE_OFFSET is PGDIR_SIZE aligned with KASLR disabled, while
> +	 * PUD_SIZE ALIGNED with KASLR enabled. So for a given physical
> +	 * address X, the pud_index(X) != pud_index(__va(X)), we can only copy
> +	 * pud entry of __va(X) to fill in pud entry of X to build 1:1 mapping
> +	 * . Means here we can only reuse pmd table of direct mapping.
> +	 */
>  	for (pgd = 0; pgd < n_pgds; pgd++) {
> -		save_pgd[pgd] = *pgd_offset_k(pgd * PGDIR_SIZE);
> -		vaddress = (unsigned long)__va(pgd * PGDIR_SIZE);
> -		set_pgd(pgd_offset_k(pgd * PGDIR_SIZE), *pgd_offset_k(vaddress));
> +		addr_pgd = (unsigned long)(pgd * PGDIR_SIZE);
> +		vaddr = (unsigned long)__va(pgd * PGDIR_SIZE);
> +		pgd_efi = pgd_offset_k(addr_pgd);
> +		save_pgd[pgd] = *pgd_efi;
> +		p4d =  p4d_alloc(&init_mm, pgd_efi, addr_pgd);
> +
> +		if (!p4d) {
> +			pr_err("Failed to allocate p4d table!\n");
> +			goto out;
> +		}
> +		for (i = 0; i < PTRS_PER_P4D; i++) {
> +			addr_p4d = addr_pgd + i * P4D_SIZE;
> +			p4d_efi = p4d + p4d_index(addr_p4d);
> +			pud = pud_alloc(&init_mm, p4d_efi, addr_p4d);
> +			if (!pud) {
> +				pr_err("Failed to allocate pud table!\n");
> +				goto out;
> +			}
> +			for (j = 0; j < PTRS_PER_PUD; j++) {
> +				addr_pud = addr_p4d + j * PUD_SIZE;
> +				if (addr_pud > (max_pfn << PAGE_SHIFT))
> +					break;
> +				vaddr = (unsigned long)__va(addr_pud);
> +
> +				pgd_k = pgd_offset_k(vaddr);
> +				p4d_k = p4d_offset(pgd_k, vaddr);
> +				pud[j] = *pud_offset(p4d_k, vaddr);
> +			}
> +		}

Is it better to add or use some common function for this instead of
doing these only in efi code?

>  	}
>  out:
>  	__flush_tlb_all();
> @@ -104,8 +140,11 @@ void __init efi_call_phys_epilog(pgd_t *save_pgd)
>  	/*
>  	 * After the lock is released, the original page table is restored.
>  	 */
> -	int pgd_idx;
> +	int pgd_idx, i;
>  	int nr_pgds;
> +	pgd_t *pgd;
> +	p4d_t *p4d;
> +	pud_t *pud;
>  
>  	if (!efi_enabled(EFI_OLD_MEMMAP)) {
>  		write_cr3((unsigned long)save_pgd);
> @@ -115,9 +154,24 @@ void __init efi_call_phys_epilog(pgd_t *save_pgd)
>  
>  	nr_pgds = DIV_ROUND_UP((max_pfn << PAGE_SHIFT) , PGDIR_SIZE);
>  
> -	for (pgd_idx = 0; pgd_idx < nr_pgds; pgd_idx++)
> +	for (pgd_idx = 0; pgd_idx < nr_pgds; pgd_idx++) {
> +		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))
> +			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))
> +				continue;
> +			pud = (pud_t *)p4d_page_vaddr(*p4d);
> +			pud_free(&init_mm, pud);
> +		}
> +		p4d = (p4d_t *)pgd_page_vaddr(*pgd);
> +		p4d_free(&init_mm, p4d);
> +	}
> +
>  	kfree(save_pgd);
>  
>  	__flush_tlb_all();
> -- 
> 2.5.5
> 

Thanks
Dave