[PATCH v2] libdw: check memory access in get_(u|s)leb128

[vvvvvv@google.com]

From: Aleksei Vetrov <vvvvvv@google.com>

__libdw_get_uleb128 and __libdw_get_sleb128 should check if addrp has
already reached the end before unrolling the first step. It is done by
moving __libdw_max_len to the beginning of the function, which can
notice, that addrp is beyond the end. Then we just check the result of
this function.

Signed-off-by: Aleksei Vetrov <vvvvvv@google.com>
---
 libdw/memory-access.h | 10 ++++++++--
 tests/leb128.c        | 29 ++++++++++++++++++++++++++++-
 2 files changed, 36 insertions(+), 3 deletions(-)

diff --git a/libdw/memory-access.h b/libdw/memory-access.h
index fca4129a..6d79343c 100644
--- a/libdw/memory-access.h
+++ b/libdw/memory-access.h
@@ -72,13 +72,16 @@ __libdw_max_len_sleb128 (const unsigned char *addr, const unsigned char *end)
 static inline uint64_t
 __libdw_get_uleb128 (const unsigned char **addrp, const unsigned char *end)
 {
+  const size_t max = __libdw_max_len_uleb128 (*addrp, end);
+  if (unlikely (max == 0))
+    return UINT64_MAX;
+
   uint64_t acc = 0;
 
   /* Unroll the first step to help the compiler optimize
      for the common single-byte case.  */
   get_uleb128_step (acc, *addrp, 0);
 
-  const size_t max = __libdw_max_len_uleb128 (*addrp - 1, end);
   for (size_t i = 1; i < max; ++i)
     get_uleb128_step (acc, *addrp, i);
   /* Other implementations set VALUE to UINT_MAX in this
@@ -124,6 +127,10 @@ __libdw_get_uleb128_unchecked (const unsigned char **addrp)
 static inline int64_t
 __libdw_get_sleb128 (const unsigned char **addrp, const unsigned char *end)
 {
+  const size_t max = __libdw_max_len_sleb128 (*addrp, end);
+  if (unlikely (max == 0))
+    return INT64_MAX;
+
   /* Do the work in an unsigned type, but use implementation-defined
      behavior to cast to signed on return.  This avoids some undefined
      behavior when shifting.  */
@@ -133,7 +140,6 @@ __libdw_get_sleb128 (const unsigned char **addrp, const unsigned char *end)
      for the common single-byte case.  */
   get_sleb128_step (acc, *addrp, 0);
 
-  const size_t max = __libdw_max_len_sleb128 (*addrp - 1, end);
   for (size_t i = 1; i < max; ++i)
     get_sleb128_step (acc, *addrp, i);
   if (*addrp == end)
diff --git a/tests/leb128.c b/tests/leb128.c
index 47b57c0d..03090d80 100644
--- a/tests/leb128.c
+++ b/tests/leb128.c
@@ -117,6 +117,19 @@ test_sleb (void)
   return OK;
 }
 
+static int
+test_sleb_safety (void)
+{
+  const int64_t expected_error = INT64_MAX;
+  int64_t value;
+  const unsigned char *test = NULL;
+  get_sleb128 (value, test, test);
+  if (value != expected_error)
+    return FAIL;
+
+  return OK;
+}
+
 static int
 test_one_uleb (const unsigned char *data, size_t len, uint64_t expect)
 {
@@ -166,8 +179,22 @@ test_uleb (void)
   return OK;
 }
 
+static int
+test_uleb_safety (void)
+{
+  const uint64_t expected_error = UINT64_MAX;
+  uint64_t value;
+  const unsigned char *test = NULL;
+  get_uleb128 (value, test, test);
+  if (value != expected_error)
+    return FAIL;
+
+  return OK;
+}
+
 int
 main (void)
 {
-  return test_sleb () || test_uleb ();
+  return test_sleb () || test_sleb_safety () || test_uleb ()
+	 || test_uleb_safety ();
 }
-- 
2.39.1.581.gbfd45094c4-goog