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

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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