[PATCH v4 4/4] x86/livepatch: perform sanity checks on the payload exception table contents

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

Ensure the entries of a payload exception table only apply to text regions in
the payload itself.  Since the payload exception table needs to be loaded and
active even before a patch is applied (because hooks might already rely on it),
make sure the exception table (if any) only contains fixups for the payload
text section.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Changes since v3:
 - Rename to extable_in_bounds().
 - Use _p() instead of casting to void *.

Changes since v2:
 - New in this version.
---
 xen/arch/x86/extable.c             | 18 ++++++++++++++++++
 xen/arch/x86/include/asm/uaccess.h |  3 +++
 xen/common/livepatch.c             |  9 +++++++++
 3 files changed, 30 insertions(+)

diff --git a/xen/arch/x86/extable.c b/xen/arch/x86/extable.c
index 8415cd1fa249..2be090b92df8 100644
--- a/xen/arch/x86/extable.c
+++ b/xen/arch/x86/extable.c
@@ -228,3 +228,21 @@ unsigned long asmlinkage search_pre_exception_table(struct 
cpu_user_regs *regs)
     }
     return fixup;
 }
+
+#ifdef CONFIG_LIVEPATCH
+bool extable_in_bounds(const struct exception_table_entry *ex_start,
+                       const struct exception_table_entry *ex_end,
+                       const void *start, const void *end)
+{
+    for ( ; ex_start < ex_end; ex_start++ )
+    {
+        const void *addr = _p(ex_addr(ex_start));
+        const void *cont = _p(ex_cont(ex_start));
+
+        if ( addr < start || addr >= end || cont < start || cont >= end )
+            return false;
+    }
+
+    return true;
+}
+#endif
diff --git a/xen/arch/x86/include/asm/uaccess.h 
b/xen/arch/x86/include/asm/uaccess.h
index 48b684c19d44..4995edfdd8db 100644
--- a/xen/arch/x86/include/asm/uaccess.h
+++ b/xen/arch/x86/include/asm/uaccess.h
@@ -426,5 +426,8 @@ extern unsigned long search_exception_table(const struct 
cpu_user_regs *regs,
 extern void sort_exception_tables(void);
 extern void sort_exception_table(struct exception_table_entry *start,
                                  const struct exception_table_entry *stop);
+extern bool extable_in_bounds(const struct exception_table_entry *ex_start,
+                              const struct exception_table_entry *ex_end,
+                              const void *start, const void *end);
 
 #endif /* __X86_UACCESS_H__ */
diff --git a/xen/common/livepatch.c b/xen/common/livepatch.c
index 1afde0281402..4702c06902a9 100644
--- a/xen/common/livepatch.c
+++ b/xen/common/livepatch.c
@@ -912,6 +912,15 @@ static int prepare_payload(struct payload *payload,
         s = sec->load_addr;
         e = sec->load_addr + sec->sec->sh_size;
 
+        if ( !extable_in_bounds(s, e, payload->text_addr,
+                                payload->text_addr + payload->text_size) )
+        {
+            printk(XENLOG_ERR LIVEPATCH
+                   "%s: Invalid exception table with out of bounds entries\n",
+                   elf->name);
+            return -EINVAL;
+        }
+
         sort_exception_table(s ,e);
 
         region->ex = s;
-- 
2.44.0