References: bnc#823011 XSA-55

From: Ian Jackson <ian.jackson@eu.citrix.com>
Date: Thu, 13 Jun 2013 19:10:14 +0100
Subject: [PATCH 11/21] libelf: Make all callers call elf_check_broken

This arranges that if the new pointer reference error checking
tripped, we actually get a message about it.  In this patch these
messages do not change the actual return values from the various
functions: so pointer reference errors do not prevent loading.  This
is for fear that some existing kernels might cause the code to make
these wild references, which would then break, which is not a good
thing in a security patch.

In xen/arch/x86/domain_build.c we have to introduce an "out" label and
change all of the "return rc" beyond the relevant point into "goto
out".

This is part of the fix to a security issue, XSA-55.

Differences in 4.1 backport:
 * No xen/arch/arm.
 * There was less error handling in xen/arch/x86/domain_build.c
   so less need to change it.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>

--- a/tools/libxc/xc_dom_elfloader.c
+++ b/tools/libxc/xc_dom_elfloader.c
@@ -242,6 +242,13 @@ static int xc_dom_load_elf_symtab(struct
             elf_store_field(elf, shdr, e32.sh_name, 0);
     }
 
+    if ( elf_check_broken(&syms) )
+        xc_dom_printf("%s: symbols ELF broken: %s\n", __FUNCTION__,
+                      elf_check_broken(&syms));
+    if ( elf_check_broken(elf) )
+        xc_dom_printf("%s: ELF broken: %s\n", __FUNCTION__,
+                      elf_check_broken(elf));
+
     if ( tables == 0 )
     {
         xc_dom_printf("%s: no symbol table present\n", __FUNCTION__);
@@ -279,13 +286,16 @@ static int xc_dom_parse_elf_kernel(struc
     {
         xc_dom_panic(XC_INVALID_KERNEL, "%s: ELF image has no shstrtab\n",
                      __FUNCTION__);
-        return -EINVAL;
+        rc = -EINVAL;
+        goto out;
     }
 
     /* parse binary and get xen meta info */
     elf_parse_binary(elf);
     if ( (rc = elf_xen_parse(elf, &dom->parms)) != 0 )
-        return rc;
+    {
+        goto out;
+    }
 
     /* find kernel segment */
     dom->kernel_seg.vstart = dom->parms.virt_kstart;
@@ -298,7 +308,13 @@ static int xc_dom_parse_elf_kernel(struc
     xc_dom_printf("%s: %s: 0x%" PRIx64 " -> 0x%" PRIx64 "\n",
                   __FUNCTION__, dom->guest_type,
                   dom->kernel_seg.vstart, dom->kernel_seg.vend);
-    return 0;
+    rc = 0;
+out:
+    if ( elf_check_broken(elf) )
+        xc_dom_printf("%s: ELF broken: %s\n", __FUNCTION__,
+                      elf_check_broken(elf));
+
+    return rc;
 }
 
 static int xc_dom_load_elf_kernel(struct xc_dom_image *dom)
--- a/tools/libxc/xc_hvm_build.c
+++ b/tools/libxc/xc_hvm_build.c
@@ -316,11 +316,16 @@ static int setup_guest(int xc_handle,
         munmap(page0, PAGE_SIZE);
     }
 
+    if ( elf_check_broken(&elf) )
+        ERROR("HVM ELF broken: %s", elf_check_broken(&elf));
+
     free(page_array);
     return 0;
 
  error_out:
     free(page_array);
+    if ( elf_check_broken(&elf) )
+        ERROR("HVM ELF broken, failing: %s", elf_check_broken(&elf));
     return -1;
 }
 
--- a/tools/xcutils/readnotes.c
+++ b/tools/xcutils/readnotes.c
@@ -219,6 +219,9 @@ int main(int argc, char **argv)
 		printf("__xen_guest: %s\n",
                        elf_strfmt(&elf, elf_section_start(&elf, shdr)));
 
+	if (elf_check_broken(&elf))
+		printf("warning: broken ELF: %s\n", elf_check_broken(&elf));
+
 	return 0;
 }
 
--- a/xen/arch/x86/domain_build.c
+++ b/xen/arch/x86/domain_build.c
@@ -368,7 +368,7 @@ int __init construct_dom0(
 #endif
     elf_parse_binary(&elf);
     if ( (rc = elf_xen_parse(&elf, &parms)) != 0 )
-        return rc;
+        goto out;
 
     /* compatibility check */
     compatible = 0;
@@ -407,7 +407,8 @@ int __init construct_dom0(
     if ( !compatible )
     {
         printk("Mismatch between Xen and DOM0 kernel\n");
-        return -EINVAL;
+        rc = -EINVAL;
+        goto out;
     }
 
 #if defined(__x86_64__)
@@ -646,7 +647,8 @@ int __init construct_dom0(
          (v_end > HYPERVISOR_COMPAT_VIRT_START(d)) )
     {
         printk("DOM0 image overlaps with Xen private area.\n");
-        return -EINVAL;
+        rc = -EINVAL;
+        goto out;
     }
 
     if ( is_pv_32on64_domain(d) )
@@ -819,7 +821,8 @@ int __init construct_dom0(
         {
             write_ptbase(current);
             printk("Invalid HYPERCALL_PAGE field in ELF notes.\n");
-            return -1;
+            rc = -1;
+            goto out;
         }
         hypercall_page_initialise(
             d, (void *)(unsigned long)parms.virt_hypercall);
@@ -1144,9 +1147,19 @@ int __init construct_dom0(
 
     BUG_ON(rc != 0);
 
-    iommu_dom0_init(dom0);
+    if ( elf_check_broken(&elf) )
+        printk(" Xen warning: dom0 kernel broken ELF: %s\n",
+               elf_check_broken(&elf));
 
+    iommu_dom0_init(dom0);
     return 0;
+
+out:
+    if ( elf_check_broken(&elf) )
+        printk(" Xen dom0 kernel broken ELF: %s\n",
+               elf_check_broken(&elf));
+
+    return rc;
 }
 
 /*
