[Xen-devel] [PATCH v2] xen/x86: Remap text/data/bss with appropriate permissions

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

c/s cf39362 "x86: use 2M superpages for text/data/bss mappings" served two
purposes; to map the primary code and data with appropriate pagetable
permissions (rather than unilaterally RWX), and to reduce the TLB pressure.

The extra alignment exposed a SYSLinux issue, and was partly reverted by c/s
0b8a172 "x86: partially revert use of 2M mappings for hypervisor image".

This change reinstates the pagetable permission improvements while avoiding
the 2M alignment issue.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>

v2:

Move the remapping earlier in the boot sequence.  This is where
memguart_init() used to reside, which itself used map_pages_to_xen() on the
full virtual region.
---
 xen/arch/x86/setup.c   | 31 +++++++++++++++++++++++++++++++
 xen/arch/x86/xen.lds.S | 16 ++++++++++++++++
 2 files changed, 47 insertions(+)

diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index 1876a28..ee65f55 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -1205,6 +1205,37 @@ void __init noreturn __start_xen(unsigned long mbi_p)
                    ~((1UL << L2_PAGETABLE_SHIFT) - 1);
     destroy_xen_mappings(xen_virt_end, XEN_VIRT_START + BOOTSTRAP_MAP_BASE);
 
+    /*
+     * If not using 2M mappings to gain suitable pagetable permissions
+     * directly from the relocation above, remap the code/data
+     * sections with decreased permissions.
+     */
+    if ( !using_2M_mapping() )
+    {
+        /* Mark .text as RX (avoiding the first 2M superpage). */
+        map_pages_to_xen(XEN_VIRT_START + MB(2),
+                         PFN_DOWN(__pa(XEN_VIRT_START + MB(2))),
+                         PFN_DOWN(__2M_text_end -
+                                  (const char *)(XEN_VIRT_START + MB(2))),
+                         PAGE_HYPERVISOR_RX);
+
+        /* Mark .rodata as RO. */
+        map_pages_to_xen((unsigned long)&__2M_rodata_start,
+                         PFN_DOWN(__pa(__2M_rodata_start)),
+                         PFN_DOWN(__2M_rodata_end - __2M_rodata_start),
+                         PAGE_HYPERVISOR_RO);
+
+        /* Mark .data and .bss as RW. */
+        map_pages_to_xen((unsigned long)&__2M_rwdata_start,
+                         PFN_DOWN(__pa(__2M_rwdata_start)),
+                         PFN_DOWN(__2M_rwdata_end - __2M_rwdata_start),
+                         PAGE_HYPERVISOR_RW);
+
+        /* Drop the remaining mappings in the shattered superpage. */
+        destroy_xen_mappings((unsigned long)&__2M_rwdata_end,
+                             ROUNDUP((unsigned long)&__2M_rwdata_end, MB(2)));
+    }
+
     nr_pages = 0;
     for ( i = 0; i < e820.nr_map; i++ )
         if ( e820.map[i].type == E820_RAM )
diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
index e47771c..5eb825e 100644
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -56,6 +56,8 @@ SECTIONS
 
 #ifdef EFI
   . = ALIGN(MB(2));
+#else
+  . = ALIGN(PAGE_SIZE);
 #endif
   __2M_text_end = .;
 
@@ -99,6 +101,8 @@ SECTIONS
 
 #ifdef EFI
   . = ALIGN(MB(2));
+#else
+  . = ALIGN(PAGE_SIZE);
 #endif
   __2M_rodata_end = .;
 
@@ -168,6 +172,8 @@ SECTIONS
 
 #ifdef EFI
   . = ALIGN(MB(2));
+#else
+  . = ALIGN(PAGE_SIZE);
 #endif
   __2M_init_end = .;
 
@@ -212,6 +218,8 @@ SECTIONS
 
 #ifdef EFI
   . = ALIGN(MB(2));
+#else
+  . = ALIGN(PAGE_SIZE);
 #endif
   __2M_rwdata_end = .;
 
@@ -270,6 +278,14 @@ ASSERT(IS_ALIGNED(__2M_init_start,   MB(2)), 
"__2M_init_start misaligned")
 ASSERT(IS_ALIGNED(__2M_init_end,     MB(2)), "__2M_init_end misaligned")
 ASSERT(IS_ALIGNED(__2M_rwdata_start, MB(2)), "__2M_rwdata_start misaligned")
 ASSERT(IS_ALIGNED(__2M_rwdata_end,   MB(2)), "__2M_rwdata_end misaligned")
+#else
+ASSERT(IS_ALIGNED(__2M_text_end,     PAGE_SIZE), "__2M_text_end misaligned")
+ASSERT(IS_ALIGNED(__2M_rodata_start, PAGE_SIZE), "__2M_rodata_start 
misaligned")
+ASSERT(IS_ALIGNED(__2M_rodata_end,   PAGE_SIZE), "__2M_rodata_end misaligned")
+ASSERT(IS_ALIGNED(__2M_init_start,   PAGE_SIZE), "__2M_init_start misaligned")
+ASSERT(IS_ALIGNED(__2M_init_end,     PAGE_SIZE), "__2M_init_end misaligned")
+ASSERT(IS_ALIGNED(__2M_rwdata_start, PAGE_SIZE), "__2M_rwdata_start 
misaligned")
+ASSERT(IS_ALIGNED(__2M_rwdata_end,   PAGE_SIZE), "__2M_rwdata_end misaligned")
 #endif
 
 ASSERT(IS_ALIGNED(cpu0_stack, STACK_SIZE), "cpu0_stack misaligned")
-- 
2.1.4


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel