[PATCH] libdwfl: Rewrite GElf_Nhdr reading in dwfl_segment_report_module

[Mark Wielaard <mark@klomp.org>]

Make sure that the notes filesz is not too big. Rewrite reading of the
notes to check for overflow at every step. Also limit the size of the
buildid bytes.

Signed-off-by: Mark Wielaard <mark@klomp.org>
---
 libdwfl/ChangeLog                    |  5 ++
 libdwfl/dwfl_segment_report_module.c | 79 ++++++++++++++++------------
 2 files changed, 49 insertions(+), 35 deletions(-)

diff --git a/libdwfl/ChangeLog b/libdwfl/ChangeLog
index 1f83576d..18ffc347 100644
--- a/libdwfl/ChangeLog
+++ b/libdwfl/ChangeLog
@@ -1,3 +1,8 @@
+2021-12-19  Mark Wielaard  <mark@klomp.org>
+
+	* dwfl_segment_report_module.c (dwfl_segment_report_module): Check
+	notes filesz. Rewrite reading of GElf_Nhdr.
+
 2021-12-08  Mark Wielaard  <mark@klomp.org>
 
 	* dwfl_segment_report_module.c (dwfl_segment_report_module): Make sure
diff --git a/libdwfl/dwfl_segment_report_module.c b/libdwfl/dwfl_segment_report_module.c
index 78c70795..a6d6be85 100644
--- a/libdwfl/dwfl_segment_report_module.c
+++ b/libdwfl/dwfl_segment_report_module.c
@@ -520,6 +520,9 @@ dwfl_segment_report_module (Dwfl *dwfl, int ndx, const char *name,
               if (data_size != 0)
                 filesz = data_size;
 
+	      if (filesz > SIZE_MAX / sizeof (Elf32_Nhdr))
+		continue;
+
               assert (sizeof (Elf32_Nhdr) == sizeof (Elf64_Nhdr));
 
               void *notes;
@@ -532,6 +535,8 @@ dwfl_segment_report_module (Dwfl *dwfl, int ndx, const char *name,
                 {
                   const unsigned int xencoding = ehdr.e32.e_ident[EI_DATA];
 
+		  if (filesz > SIZE_MAX / sizeof (Elf32_Nhdr))
+		    continue;
                   notes = malloc (filesz);
                   if (unlikely (notes == NULL))
                     continue; /* Next header */
@@ -552,44 +557,48 @@ dwfl_segment_report_module (Dwfl *dwfl, int ndx, const char *name,
 
               const GElf_Nhdr *nh = notes;
               size_t len = 0;
-              size_t last_len;
-              while (filesz > len + sizeof (*nh))
+              while (filesz - len > sizeof (*nh))
                 {
-                  const void *note_name;
-                  const void *note_desc;
-                  last_len = len;
-
-                  len += sizeof (*nh);
-                  note_name = notes + len;
-
-                  len += nh->n_namesz;
-                  len = align == 8 ? NOTE_ALIGN8 (len) : NOTE_ALIGN4 (len);
-                  note_desc = notes + len;
-
-                  if (unlikely (filesz < len + nh->n_descsz
-                                || len <= last_len
-                                || len + nh->n_descsz < last_len))
-                    break;
-
-                  if (nh->n_type == NT_GNU_BUILD_ID
-                      && nh->n_descsz > 0
-                      && nh->n_namesz == sizeof "GNU"
-                      && !memcmp (note_name, "GNU", sizeof "GNU"))
-                    {
-                      build_id.vaddr = (note_desc
+		  len += sizeof (*nh);
+
+		  size_t namesz = nh->n_namesz;
+		  namesz = align == 8 ? NOTE_ALIGN8 (namesz) : NOTE_ALIGN4 (namesz);
+		  if (namesz > filesz - len || len + namesz < namesz)
+		    break;
+
+		  void *note_name = notes + len;
+		  len += namesz;
+
+		  size_t descsz = nh->n_descsz;
+		  descsz = align == 8 ? NOTE_ALIGN8 (descsz) : NOTE_ALIGN4 (descsz);
+		  if (descsz > filesz - len || len + descsz < descsz)
+		    break;
+
+		  void *note_desc = notes + len;
+		  len += descsz;
+
+		  /* We don't handle very short or really large build-ids.  We need at
+		     at least 3 and allow for up to 64 (normally ids are 20 long).  */
+#define MIN_BUILD_ID_BYTES 3
+#define MAX_BUILD_ID_BYTES 64
+		  if (nh->n_type == NT_GNU_BUILD_ID
+		      && nh->n_descsz >= MIN_BUILD_ID_BYTES
+		      && nh->n_descsz <= MAX_BUILD_ID_BYTES
+		      && nh->n_namesz == sizeof "GNU"
+		      && !memcmp (note_name, "GNU", sizeof "GNU"))
+		    {
+		      build_id.vaddr = (note_desc
 					- (const void *) notes
 					+ note_vaddr);
-                      build_id.len = nh->n_descsz;
-                      build_id.memory = malloc (build_id.len);
-                      if (likely (build_id.memory != NULL))
-                        memcpy (build_id.memory, note_desc, build_id.len);
-                      break;
-                    }
-
-                  len += nh->n_descsz;
-                  len = align == 8 ? NOTE_ALIGN8 (len) : NOTE_ALIGN4 (len);
-                  nh = (void *) notes + len;
-                }
+		      build_id.len = nh->n_descsz;
+		      build_id.memory = malloc (build_id.len);
+		      if (likely (build_id.memory != NULL))
+			memcpy (build_id.memory, note_desc, build_id.len);
+		      break;
+		    }
+
+		  nh = (void *) notes + len;
+		}
 
               if (notes != data)
                 free (notes);
-- 
2.30.2