Re: [PATCH 4.9 23/35] x86/mm: Expand static page table for fixmap space

[Ben Hutchings]

On Thu, 2018-10-11 at 17:35 +0200, Greg Kroah-Hartman wrote:
> 4.9-stable review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Feng Tang <feng.tang@xxxxxxxxx>
> 
> commit 05ab1d8a4b36ee912b7087c6da127439ed0a903e upstream.

This backport is incorrect.  The part that updated __startup_64() in
arch/x86/kernel/head64.c was dropped, presumably because that function
doesn't exist in 4.9.  However that seems to be an essential of the
fix.  In 4.9 the startup_64 routine in arch/x86/kernel/head_64.S would
need to be changed instead.

I also found that this introduces new boot-time warnings on some
systems if CONFIG_DEBUG_WX is enabled.

So, unless someone provides fixes for those issues, I think this should
be reverted for the 4.9 branch.

Ben.

> We met a kernel panic when enabling earlycon, which is due to the fixmap
> address of earlycon is not statically setup.
> 
> Currently the static fixmap setup in head_64.S only covers 2M virtual
> address space, while it actually could be in 4M space with different
> kernel configurations, e.g. when VSYSCALL emulation is disabled.
> 
> So increase the static space to 4M for now by defining FIXMAP_PMD_NUM to 2,
> and add a build time check to ensure that the fixmap is covered by the
> initial static page tables.
> 
> Fixes: 1ad83c858c7d ("x86_64,vsyscall: Make vsyscall emulation configurable")
> Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Tested-by: kernel test robot <rong.a.chen@xxxxxxxxx>
> Reviewed-by: Juergen Gross <jgross@xxxxxxxx> (Xen parts)
> Cc: H Peter Anvin <hpa@xxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Yinghai Lu <yinghai@xxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Cc: Andy Lutomirsky <luto@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Link: https://lkml.kernel.org/r/20180920025828.23699-1-feng.tang@xxxxxxxxx
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/fixmap.h     |   10 ++++++++++
>  arch/x86/include/asm/pgtable_64.h |    3 ++-
>  arch/x86/kernel/head_64.S         |   16 ++++++++++++----
>  arch/x86/mm/pgtable.c             |    9 +++++++++
>  arch/x86/xen/mmu.c                |    8 ++++++--
>  5 files changed, 39 insertions(+), 7 deletions(-)
> 
> --- a/arch/x86/include/asm/fixmap.h
> +++ b/arch/x86/include/asm/fixmap.h
> @@ -14,6 +14,16 @@
>  #ifndef _ASM_X86_FIXMAP_H
>  #define _ASM_X86_FIXMAP_H
>  
> +/*
> + * Exposed to assembly code for setting up initial page tables. Cannot be
> + * calculated in assembly code (fixmap entries are an enum), but is sanity
> + * checked in the actual fixmap C code to make sure that the fixmap is
> + * covered fully.
> + */
> +#define FIXMAP_PMD_NUM	2
> +/* fixmap starts downwards from the 507th entry in level2_fixmap_pgt */
> +#define FIXMAP_PMD_TOP	507
> +
>  #ifndef __ASSEMBLY__
>  #include <linux/kernel.h>
>  #include <asm/acpi.h>
> --- a/arch/x86/include/asm/pgtable_64.h
> +++ b/arch/x86/include/asm/pgtable_64.h
> @@ -13,13 +13,14 @@
>  #include <asm/processor.h>
>  #include <linux/bitops.h>
>  #include <linux/threads.h>
> +#include <asm/fixmap.h>
>  
>  extern pud_t level3_kernel_pgt[512];
>  extern pud_t level3_ident_pgt[512];
>  extern pmd_t level2_kernel_pgt[512];
>  extern pmd_t level2_fixmap_pgt[512];
>  extern pmd_t level2_ident_pgt[512];
> -extern pte_t level1_fixmap_pgt[512];
> +extern pte_t level1_fixmap_pgt[512 * FIXMAP_PMD_NUM];
>  extern pgd_t init_level4_pgt[];
>  
>  #define swapper_pg_dir init_level4_pgt
> --- a/arch/x86/kernel/head_64.S
> +++ b/arch/x86/kernel/head_64.S
> @@ -23,6 +23,7 @@
>  #include "../entry/calling.h"
>  #include <asm/export.h>
>  #include <asm/nospec-branch.h>
> +#include <asm/fixmap.h>
>  
>  #ifdef CONFIG_PARAVIRT
>  #include <asm/asm-offsets.h>
> @@ -493,13 +494,20 @@ NEXT_PAGE(level2_kernel_pgt)
>  		KERNEL_IMAGE_SIZE/PMD_SIZE)
>  
>  NEXT_PAGE(level2_fixmap_pgt)
> -	.fill	506,8,0
> -	.quad	level1_fixmap_pgt - __START_KERNEL_map + _PAGE_TABLE
> -	/* 8MB reserved for vsyscalls + a 2MB hole = 4 + 1 entries */
> -	.fill	5,8,0
> +	.fill	(512 - 4 - FIXMAP_PMD_NUM),8,0
> +	pgtno = 0
> +	.rept (FIXMAP_PMD_NUM)
> +	.quad level1_fixmap_pgt + (pgtno << PAGE_SHIFT) - __START_KERNEL_map \
> +		+ _PAGE_TABLE;
> +	pgtno = pgtno + 1
> +	.endr
> +	/* 6 MB reserved space + a 2MB hole */
> +	.fill	4,8,0
>  
>  NEXT_PAGE(level1_fixmap_pgt)
> +	.rept (FIXMAP_PMD_NUM)
>  	.fill	512,8,0
> +	.endr
>  
>  #undef PMDS
>  
> --- a/arch/x86/mm/pgtable.c
> +++ b/arch/x86/mm/pgtable.c
> @@ -536,6 +536,15 @@ void __native_set_fixmap(enum fixed_addr
>  {
>  	unsigned long address = __fix_to_virt(idx);
>  
> +#ifdef CONFIG_X86_64
> +       /*
> +	* Ensure that the static initial page tables are covering the
> +	* fixmap completely.
> +	*/
> +	BUILD_BUG_ON(__end_of_permanent_fixed_addresses >
> +		     (FIXMAP_PMD_NUM * PTRS_PER_PTE));
> +#endif
> +
>  	if (idx >= __end_of_fixed_addresses) {
>  		BUG();
>  		return;
> --- a/arch/x86/xen/mmu.c
> +++ b/arch/x86/xen/mmu.c
> @@ -1936,7 +1936,7 @@ void __init xen_setup_kernel_pagetable(p
>  		 * L3_k[511] -> level2_fixmap_pgt */
>  		convert_pfn_mfn(level3_kernel_pgt);
>  
> -		/* L3_k[511][506] -> level1_fixmap_pgt */
> +		/* L3_k[511][508-FIXMAP_PMD_NUM ... 507] -> level1_fixmap_pgt */
>  		convert_pfn_mfn(level2_fixmap_pgt);
>  	}
>  	/* We get [511][511] and have Xen's version of level2_kernel_pgt */
> @@ -1970,7 +1970,11 @@ void __init xen_setup_kernel_pagetable(p
>  		set_page_prot(level2_ident_pgt, PAGE_KERNEL_RO);
>  		set_page_prot(level2_kernel_pgt, PAGE_KERNEL_RO);
>  		set_page_prot(level2_fixmap_pgt, PAGE_KERNEL_RO);
> -		set_page_prot(level1_fixmap_pgt, PAGE_KERNEL_RO);
> +
> +		for (i = 0; i < FIXMAP_PMD_NUM; i++) {
> +			set_page_prot(level1_fixmap_pgt + i * PTRS_PER_PTE,
> +				      PAGE_KERNEL_RO);
> +		}
>  
>  		/* Pin down new L4 */
>  		pin_pagetable_pfn(MMUEXT_PIN_L4_TABLE,
> 
> 
-- 
Ben Hutchings
Absolutum obsoletum. (If it works, it's out of date.) - Stafford Beer

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