Re: another pagetable initialization crash on xen

[Konrad Rzeszutek Wilk]

On Tue, Mar 29, 2011 at 06:25:56PM +0100, Stefano Stabellini wrote:
> On Mon, 28 Mar 2011, Stefano Stabellini wrote:
> > On Mon, 28 Mar 2011, Stefano Stabellini wrote:
> > > Hi Yinghai,
> > > unfortunately I found another pagetable initialization bug on xen
> > > affecting linux 2.6.39-rc0.
> > > The problem is that on xen we need to make sure that all the pagetable pages
> > > are mapped read-only, in fact in xen_set_pte we have this check:
> > > 
> > > if (pfn >= pgt_buf_start && pfn < pgt_buf_end)
> > >     /* make the pte read-only */
> > > 
> > > however pgt_buf_end is where the kernel pagetable *currently* ends, so
> > > some kernel pagetable pages allocated after pgt_buf_end might be marked
> > > read-write by mistake.  A simple way to fix the issue would be to use
> > > pgt_buf_top instead:
> > > 
> > > if (pfn >= pgt_buf_start && pfn < pgt_buf_top)
> > >     /* make the pte read-only */
> > > 
> > > however after building the kernel pagetable in init_memory_mapping we
> > > only reserve memory between pgt_buf_start and pgt_buf_end:
> > > 
> > > if (!after_bootmem && pgt_buf_end > pgt_buf_start)
> > >         memblock_x86_reserve_range(pgt_buf_start << PAGE_SHIFT,
> > >                  pgt_buf_end << PAGE_SHIFT, "PGTABLE");
> > > 
> > > so feature allocations might use memory between the final value of
> > > pgt_buf_end and pgt_buf_top that has been marked read-only in the xen
> > > specific code, causing a crash.
> > > The only way I could find to fix the crash is to reserve also the memory
> > > region between pgt_buf_start and pgt_buf_top on xen, but that would
> > > require an ugly if(xen_domain()) at the of init_memory_mapping or
> > > the introduction of a new pvop function to reserve the pagetable memory.
> > > I don't like the idea, but I couldn't find anything better.
> > > Yinghai, do you have any better suggestions?
> > 
> > Just to make it clearer, this is the not-so-pretty patch I am talking
> > about:
> 
> 
> I kept working on refactoring this code, and I think I was able to come
> up with a slightly better patch.
> Do you think this is acceptable?
> 
> 
> 
> commit c576ff205060e92b36224e80a989492b245e806d
> Author: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> Date:   Tue Mar 29 10:59:02 2011 +0000
> 
>     x86: introduce kernel_pagetable hooks
>     
>     Introduce two new pvop hooks:
>     
>     - kernel_pagetable_alloc is used to allocate the initial kernel
>     pagetable pages;
>     
>     - kernel_pagetable_reserve is used to reserved the kernel pagetable
>     memory range;
>     
>     Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> 
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index 7db7723..a3ab967 100644
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -299,6 +299,12 @@ int phys_mem_access_prot_allowed(struct file *file, unsigned long pfn,
>  /* Install a pte for a particular vaddr in kernel space. */
>  void set_pte_vaddr(unsigned long vaddr, pte_t pte);
>  
> +struct map_range;
> +void native_kernel_pagetable_alloc(struct map_range *mr,
> +		unsigned long *pgt_start,
> +		unsigned long *pgt_top);
> +void native_kernel_pagetable_reserve(unsigned long pgt_start,
> +		unsigned long pgt_end);
>  #ifdef CONFIG_X86_32
>  extern void native_pagetable_setup_start(pgd_t *base);
>  extern void native_pagetable_setup_done(pgd_t *base);
> diff --git a/arch/x86/include/asm/x86_init.h b/arch/x86/include/asm/x86_init.h
> index 643ebf2..984ce6f 100644
> --- a/arch/x86/include/asm/x86_init.h
> +++ b/arch/x86/include/asm/x86_init.h
> @@ -69,10 +69,26 @@ struct x86_init_oem {
>  
>  /**
>   * struct x86_init_paging - platform specific paging functions
> + * @kernel_pagetable_alloc:	platform specific kernel pagetable
> + *                                      allocation function
> + * @kernel_pagetable_reserve:	platform specific kernel pagetable
> + *                                      reserve memory function
>   * @pagetable_setup_start:	platform specific pre paging_init() call
>   * @pagetable_setup_done:	platform specific post paging_init() call
>   */
>  struct x86_init_paging {
> +	/* @mr:     (in) memory range to map
> +	 * @pgt_start - @pgt_top:  (out) memory allocated for the kernel
> +	 *								pagetable pages
> +	 */
> +	void (*kernel_pagetable_alloc)(struct map_range *mr,
> +		unsigned long *pgt_start,
> +		unsigned long *pgt_top);
> +	/* @pgt_start - @pgt_end: memory region used for the kernel
> +	 *							pagetable pages
> +	 */
> +	void (*kernel_pagetable_reserve)(unsigned long pgt_start,
> +			unsigned long pgt_end);
>  	void (*pagetable_setup_start)(pgd_t *base);
>  	void (*pagetable_setup_done)(pgd_t *base);
>  };
> diff --git a/arch/x86/kernel/x86_init.c b/arch/x86/kernel/x86_init.c
> index c11514e..1048a3d 100644
> --- a/arch/x86/kernel/x86_init.c
> +++ b/arch/x86/kernel/x86_init.c
> @@ -62,6 +62,8 @@ struct x86_init_ops x86_init __initdata = {
>  	},
>  
>  	.paging = {
> +		.kernel_pagetable_alloc = native_kernel_pagetable_alloc,
> +		.kernel_pagetable_reserve = native_kernel_pagetable_reserve,
>  		.pagetable_setup_start	= native_pagetable_setup_start,
>  		.pagetable_setup_done	= native_pagetable_setup_done,
>  	},
> diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
> index 286d289..67d0792 100644
> --- a/arch/x86/mm/init.c
> +++ b/arch/x86/mm/init.c
> @@ -30,11 +30,21 @@ int direct_gbpages
>  #endif
>  ;
>  
> -static void __init find_early_table_space(unsigned long end, int use_pse,
> -					  int use_gbpages)
> +struct map_range {
> +	unsigned long start;
> +	unsigned long end;
> +	unsigned page_size_mask;
> +};
> +
> +void __init native_kernel_pagetable_alloc(struct map_range *mr,
> +		unsigned long *pgt_start,
> +		unsigned long *pgt_top)
>  {
> -	unsigned long puds, pmds, ptes, tables, start = 0, good_end = end;
> +	unsigned long puds, pmds, ptes, tables, start = 0,
> +	end = mr->end, good_end = end;

end = good_end = mr->end;

>  	phys_addr_t base;
> +	int use_pse = mr->page_size_mask & PG_LEVEL_2M;
> +	int use_gbpages = mr->page_size_mask & PG_LEVEL_1G;

bool.
>  
>  	puds = (end + PUD_SIZE - 1) >> PUD_SHIFT;
>  	tables = roundup(puds * sizeof(pud_t), PAGE_SIZE);
> @@ -73,19 +83,20 @@ static void __init find_early_table_space(unsigned long end, int use_pse,
>  	if (base == MEMBLOCK_ERROR)
>  		panic("Cannot find space for the kernel page tables");
>  
> -	pgt_buf_start = base >> PAGE_SHIFT;
> -	pgt_buf_end = pgt_buf_start;
> -	pgt_buf_top = pgt_buf_start + (tables >> PAGE_SHIFT);
> +	*pgt_start = base >> PAGE_SHIFT;
> +	*pgt_top = *pgt_start + (tables >> PAGE_SHIFT);
>  
>  	printk(KERN_DEBUG "kernel direct mapping tables up to %lx @ %lx-%lx\n",
> -		end, pgt_buf_start << PAGE_SHIFT, pgt_buf_top << PAGE_SHIFT);
> +		end, *pgt_start << PAGE_SHIFT, *pgt_top << PAGE_SHIFT);
>  }
>  
> -struct map_range {
> -	unsigned long start;
> -	unsigned long end;
> -	unsigned page_size_mask;
> -};
> +void __init native_kernel_pagetable_reserve(unsigned long pgt_start,
> +		unsigned long pgt_end)
> +{
> +	if (pgt_start > pgt_end)
> +		memblock_x86_reserve_range(pgt_start << PAGE_SHIFT,
> +				pgt_end << PAGE_SHIFT, "PGTABLE");
> +}
>  
>  #ifdef CONFIG_X86_32
>  #define NR_RANGE_MR 3
> @@ -257,9 +268,15 @@ unsigned long __init_refok init_memory_mapping(unsigned long start,
>  	 * memory mapped. Unfortunately this is done currently before the
>  	 * nodes are discovered.
>  	 */
> -	if (!after_bootmem)
> -		find_early_table_space(end, use_pse, use_gbpages);
> -
> +	if (!after_bootmem) {
> +		struct map_range map;
> +		map.start = start;
> +		map.end = end;
> +		map.page_size_mask = page_size_mask;
> +		x86_init.paging.kernel_pagetable_alloc(&map, &pgt_buf_start, &pgt_buf_top);
> +		pgt_buf_end = pgt_buf_start;
> +	}
> +		
>  	for (i = 0; i < nr_range; i++)
>  		ret = kernel_physical_mapping_init(mr[i].start, mr[i].end,
>  						   mr[i].page_size_mask);
> @@ -272,9 +289,8 @@ unsigned long __init_refok init_memory_mapping(unsigned long start,
>  
>  	__flush_tlb_all();
>  
> -	if (!after_bootmem && pgt_buf_end > pgt_buf_start)
> -		memblock_x86_reserve_range(pgt_buf_start << PAGE_SHIFT,
> -				 pgt_buf_end << PAGE_SHIFT, "PGTABLE");
> +	if (!after_bootmem)
> +		x86_init.paging.kernel_pagetable_reserve(pgt_buf_start, pgt_buf_end);
>  
>  	if (!after_bootmem)
>  		early_memtest(start, end);
> --
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