Re: [Xen-devel] [PATCH] x86: make show_page_walk() more robust

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

As I said, I found this during debugging - I had added explicit calls to
show_page_walk(), and am also considering adding it to the early page
fault handler (which provides pretty little information at present - the
stub handler for all other exceptions is doing better - and does dubious
[to me] things with a dubious [to me] comment).

And regardless of that, the mfn_valid() check is absolutely required.

Jan

>>> Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> 24.01.08 16:35 >>>
How can show_page_walk() be called before paging_init() sets up the M2P
table? It is invoked only from exception and interrupt handlers, which are
not installed until trap_init() has run. And trap_init() is invoked later
than paging_init() during boot.

 -- Keir

On 24/1/08 15:03, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

> While adding 1Gb page support for the 1:1 mapping I noticed that
> show_page_walk() crashes when used before the M2P table gets created,
> and from looking at the code I realized that it would also crash if a
> corrupt page table with an out of range MFN would be encountered.
> While it would have been possible to make get_gpfn_from_mfn() more
> robust, it seemed like keeping the overhead there low (as in the
> general case proper values can be expected and would likely have been
> checked for already), and hence I made the checks privates to
> show_page_walk().
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>
> 
> Index: 2008-01-18/xen/arch/x86/x86_32/mm.c
> ===================================================================
> --- 2008-01-18.orig/xen/arch/x86/x86_32/mm.c 2008-01-23 11:54:09.000000000
> +0100
> +++ 2008-01-18/xen/arch/x86/x86_32/mm.c 2008-01-23 11:48:16.000000000 +0100
> @@ -41,6 +41,7 @@ l2_pgentry_t __attribute__ ((__section__
>  unsigned int PAGE_HYPERVISOR         = __PAGE_HYPERVISOR;
>  unsigned int PAGE_HYPERVISOR_NOCACHE = __PAGE_HYPERVISOR_NOCACHE;
>  
> +int mpt_valid;
>  static unsigned long mpt_size;
>  
>  void *alloc_xen_pagetable(void)
> @@ -110,6 +111,8 @@ void __init paging_init(void)
>                        pg, (__PAGE_HYPERVISOR | _PAGE_PSE) & ~_PAGE_RW));
>      }
>  
> +    mpt_valid = 1;
> +
>      /* Fill with an obvious debug pattern. */
>      for ( i = 0; i < (mpt_size / BYTES_PER_LONG); i++)
>          set_gpfn_from_mfn(i, 0x55555555);
> Index: 2008-01-18/xen/arch/x86/x86_32/traps.c
> ===================================================================
> --- 2008-01-18.orig/xen/arch/x86/x86_32/traps.c 2008-01-23 11:54:09.000000000
> +0100
> +++ 2008-01-18/xen/arch/x86/x86_32/traps.c 2008-01-23 11:53:58.000000000 +0100
> @@ -132,7 +132,8 @@ void show_page_walk(unsigned long addr)
>      l3t += (cr3 & 0xFE0UL) >> 3;
>      l3e = l3t[l3_table_offset(addr)];
>      mfn = l3e_get_pfn(l3e);
> -    pfn = get_gpfn_from_mfn(mfn);
> +    pfn = mfn_valid(mfn) && mpt_valid ?
> +          get_gpfn_from_mfn(mfn) : INVALID_M2P_ENTRY;
>      printk(" L3[0x%03lx] = %"PRIpte" %08lx\n",
>             l3_table_offset(addr), l3e_get_intpte(l3e), pfn);
>      unmap_domain_page(l3t);
> @@ -143,7 +144,8 @@ void show_page_walk(unsigned long addr)
>      l2t = map_domain_page(mfn);
>      l2e = l2t[l2_table_offset(addr)];
>      mfn = l2e_get_pfn(l2e);
> -    pfn = get_gpfn_from_mfn(mfn);
> +    pfn = mfn_valid(mfn) && mpt_valid ?
> +          get_gpfn_from_mfn(mfn) : INVALID_M2P_ENTRY;
>      printk(" L2[0x%03lx] = %"PRIpte" %08lx %s\n",
>             l2_table_offset(addr), l2e_get_intpte(l2e), pfn,
>             (l2e_get_flags(l2e) & _PAGE_PSE) ? "(PSE)" : "");
> @@ -155,7 +157,8 @@ void show_page_walk(unsigned long addr)
>      l1t = map_domain_page(mfn);
>      l1e = l1t[l1_table_offset(addr)];
>      mfn = l1e_get_pfn(l1e);
> -    pfn = get_gpfn_from_mfn(mfn);
> +    pfn = mfn_valid(mfn) && mpt_valid ?
> +          get_gpfn_from_mfn(mfn) : INVALID_M2P_ENTRY;
>      printk(" L1[0x%03lx] = %"PRIpte" %08lx\n",
>             l1_table_offset(addr), l1e_get_intpte(l1e), pfn);
>      unmap_domain_page(l1t);
> Index: 2008-01-18/xen/arch/x86/x86_64/mm.c
> ===================================================================
> --- 2008-01-18.orig/xen/arch/x86/x86_64/mm.c 2008-01-23 11:54:09.000000000
> +0100
> +++ 2008-01-18/xen/arch/x86/x86_64/mm.c 2008-01-23 11:50:56.000000000 +0100
> @@ -32,6 +32,7 @@
>  #include <asm/msr.h>
>  #include <public/memory.h>
>  
> +int mpt_valid;
>  #ifdef CONFIG_COMPAT
>  unsigned int m2p_compat_vstart = __HYPERVISOR_COMPAT_VIRT_START;
>  #endif
> @@ -144,6 +145,8 @@ void __init paging_init(void)
>          l2_ro_mpt++;
>      }
>  
> +    mpt_valid = 1;
> +
>      /* Create user-accessible L2 directory to map the MPT for compat guests.
> */
>      BUILD_BUG_ON(l4_table_offset(RDWR_MPT_VIRT_START) !=
>                   l4_table_offset(HIRO_COMPAT_MPT_VIRT_START));
> Index: 2008-01-18/xen/arch/x86/x86_64/traps.c
> ===================================================================
> --- 2008-01-18.orig/xen/arch/x86/x86_64/traps.c 2008-01-23 11:54:09.000000000
> +0100
> +++ 2008-01-18/xen/arch/x86/x86_64/traps.c 2008-01-23 11:53:27.000000000 +0100
> @@ -136,7 +136,8 @@ void show_page_walk(unsigned long addr)
>      l4t = mfn_to_virt(mfn);
>      l4e = l4t[l4_table_offset(addr)];
>      mfn = l4e_get_pfn(l4e);
> -    pfn = get_gpfn_from_mfn(mfn);
> +    pfn = mfn_valid(mfn) && mpt_valid ?
> +          get_gpfn_from_mfn(mfn) : INVALID_M2P_ENTRY;
>      printk(" L4[0x%03lx] = %"PRIpte" %016lx\n",
>             l4_table_offset(addr), l4e_get_intpte(l4e), pfn);
>      if ( !(l4e_get_flags(l4e) & _PAGE_PRESENT) )
> @@ -145,7 +146,8 @@ void show_page_walk(unsigned long addr)
>      l3t = mfn_to_virt(mfn);
>      l3e = l3t[l3_table_offset(addr)];
>      mfn = l3e_get_pfn(l3e);
> -    pfn = get_gpfn_from_mfn(mfn);
> +    pfn = mfn_valid(mfn) && mpt_valid ?
> +          get_gpfn_from_mfn(mfn) : INVALID_M2P_ENTRY;
>      printk(" L3[0x%03lx] = %"PRIpte" %016lx\n",
>             l3_table_offset(addr), l3e_get_intpte(l3e), pfn);
>      if ( !(l3e_get_flags(l3e) & _PAGE_PRESENT) )
> @@ -154,7 +156,8 @@ void show_page_walk(unsigned long addr)
>      l2t = mfn_to_virt(mfn);
>      l2e = l2t[l2_table_offset(addr)];
>      mfn = l2e_get_pfn(l2e);
> -    pfn = get_gpfn_from_mfn(mfn);
> +    pfn = mfn_valid(mfn) && mpt_valid ?
> +          get_gpfn_from_mfn(mfn) : INVALID_M2P_ENTRY;
>      printk(" L2[0x%03lx] = %"PRIpte" %016lx %s\n",
>             l2_table_offset(addr), l2e_get_intpte(l2e), pfn,
>             (l2e_get_flags(l2e) & _PAGE_PSE) ? "(PSE)" : "");
> @@ -165,7 +168,8 @@ void show_page_walk(unsigned long addr)
>      l1t = mfn_to_virt(mfn);
>      l1e = l1t[l1_table_offset(addr)];
>      mfn = l1e_get_pfn(l1e);
> -    pfn = get_gpfn_from_mfn(mfn);
> +    pfn = mfn_valid(mfn) && mpt_valid ?
> +          get_gpfn_from_mfn(mfn) : INVALID_M2P_ENTRY;
>      printk(" L1[0x%03lx] = %"PRIpte" %016lx\n",
>             l1_table_offset(addr), l1e_get_intpte(l1e), pfn);
>  }
> Index: 2008-01-18/xen/include/asm-x86/mm.h
> ===================================================================
> --- 2008-01-18.orig/xen/include/asm-x86/mm.h 2008-01-23 11:54:09.000000000
> +0100
> +++ 2008-01-18/xen/include/asm-x86/mm.h 2008-01-23 11:55:37.000000000 +0100
> @@ -267,6 +267,7 @@ TYPE_SAFE(unsigned long,mfn);
>  #define machine_to_phys_mapping  ((unsigned long *)RDWR_MPT_VIRT_START)
>  #define INVALID_M2P_ENTRY        (~0UL)
>  #define VALID_M2P(_e)            (!((_e) & (1UL<<(BITS_PER_LONG-1))))
> +extern int mpt_valid;
>  
>  #ifdef CONFIG_COMPAT
>  #define compat_machine_to_phys_mapping ((unsigned int
> *)RDWR_COMPAT_MPT_VIRT_START)
> 
> 
> 
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