[Xen-changelog] [xen staging] xen/arm: Simplify alternative patching of non-writable region

[patchbot@xxxxxxx]

commit 7c98c24e9ba76df3b9f531353d99e5c1bfa8b9a9
Author:     Julien Grall <julien.grall@xxxxxxx>
AuthorDate: Tue Jun 12 12:36:37 2018 +0100
Commit:     Julien Grall <julien.grall@xxxxxxx>
CommitDate: Fri Jun 22 02:59:42 2018 +0100

    xen/arm: Simplify alternative patching of non-writable region
    
    During the MMU setup process, Xen will set SCTLR_EL2.WNX
    (Write-Non-eXecutable) bit. Because of that, the alternative code need
    to re-mapped the region in a difference place in order to modify the
    text section.
    
    At the moment, the function patching the code is only aware of the
    re-mapped region. This requires the caller to mess with Xen internal in
    order to have function such as is_active_kernel_text() working.
    
    All the interactions with Xen internal can be removed by specifying the
    offset between the region patch and the writable region for updating the
    instruction
    
    This simplification will also make it easier to integrate dynamic patching
    in a follow-up patch. Indeed, the callback address should be in
    an original region and not re-mapped only which is writeable non-executable.
    
    This is part of XSA-263.
    
    Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
    Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
---
 xen/arch/arm/alternative.c | 42 +++++++++++++-----------------------------
 1 file changed, 13 insertions(+), 29 deletions(-)

diff --git a/xen/arch/arm/alternative.c b/xen/arch/arm/alternative.c
index 9ffdc475d6..936cf04956 100644
--- a/xen/arch/arm/alternative.c
+++ b/xen/arch/arm/alternative.c
@@ -97,12 +97,16 @@ static u32 get_alt_insn(const struct alt_instr *alt,
 /*
  * The region patched should be read-write to allow __apply_alternatives
  * to replacing the instructions when necessary.
+ *
+ * @update_offset: Offset between the region patched and the writable
+ * region for the update. 0 if the patched region is writable.
  */
-static int __apply_alternatives(const struct alt_region *region)
+static int __apply_alternatives(const struct alt_region *region,
+                                paddr_t update_offset)
 {
     const struct alt_instr *alt;
-    const u32 *replptr;
-    u32 *origptr;
+    const u32 *replptr, *origptr;
+    u32 *updptr;
 
     printk(XENLOG_INFO "alternatives: Patching with alt table %p -> %p\n",
            region->begin, region->end);
@@ -118,6 +122,7 @@ static int __apply_alternatives(const struct alt_region 
*region)
         BUG_ON(alt->alt_len != alt->orig_len);
 
         origptr = ALT_ORIG_PTR(alt);
+        updptr = (void *)origptr + update_offset;
         replptr = ALT_REPL_PTR(alt);
 
         nr_inst = alt->alt_len / sizeof(insn);
@@ -125,7 +130,7 @@ static int __apply_alternatives(const struct alt_region 
*region)
         for ( i = 0; i < nr_inst; i++ )
         {
             insn = get_alt_insn(alt, origptr + i, replptr + i);
-            *(origptr + i) = cpu_to_le32(insn);
+            *(updptr + i) = cpu_to_le32(insn);
         }
 
         /* Ensure the new instructions reached the memory and nuke */
@@ -162,9 +167,6 @@ static int __apply_alternatives_multi_stop(void *unused)
         paddr_t xen_size = _end - _start;
         unsigned int xen_order = get_order_from_bytes(xen_size);
         void *xenmap;
-        struct virtual_region patch_region = {
-            .list = LIST_HEAD_INIT(patch_region.list),
-        };
 
         BUG_ON(patched);
 
@@ -177,31 +179,13 @@ static int __apply_alternatives_multi_stop(void *unused)
         /* Re-mapping Xen is not expected to fail during boot. */
         BUG_ON(!xenmap);
 
-        /*
-         * If we generate a new branch instruction, the target will be
-         * calculated in this re-mapped Xen region. So we have to register
-         * this re-mapped Xen region as a virtual region temporarily.
-         */
-        patch_region.start = xenmap;
-        patch_region.end = xenmap + xen_size;
-        register_virtual_region(&patch_region);
+        region.begin = __alt_instructions;
+        region.end = __alt_instructions_end;
 
-        /*
-         * Find the virtual address of the alternative region in the new
-         * mapping.
-         * alt_instr contains relative offset, so the function
-         * __apply_alternatives will patch in the re-mapped version of
-         * Xen.
-         */
-        region.begin = (void *)__alt_instructions - (void *)_start + xenmap;
-        region.end = (void *)__alt_instructions_end - (void *)_start + xenmap;
-
-        ret = __apply_alternatives(&region);
+        ret = __apply_alternatives(&region, xenmap - (void *)_start);
         /* The patching is not expected to fail during boot. */
         BUG_ON(ret != 0);
 
-        unregister_virtual_region(&patch_region);
-
         vunmap(xenmap);
 
         /* Barriers provided by the cache flushing */
@@ -235,7 +219,7 @@ int apply_alternatives(const struct alt_instr *start, const 
struct alt_instr *en
         .end = end,
     };
 
-    return __apply_alternatives(&region);
+    return __apply_alternatives(&region, 0);
 }
 
 /*
--
generated by git-patchbot for /home/xen/git/xen.git#staging

_______________________________________________
Xen-changelog mailing list
Xen-changelog@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/xen-changelog