libelf: elf_getphdrnum sanity check the returned phnum result.

The internal __elf_getphdrnum_rdlock might return an inconsistent phnum.
Return a sanitized value, or return an error to users that rely on phnum
to be consistent. That way iterating over all phdrs using elf_getphdr
will return consistent results.

Signed-off-by: Mark Wielaard <mjw@redhat.com>

libelf/ChangeLog

@@ -1,3 +1,8 @@
+2014-11-22  Mark Wielaard  <mjw@redhat.com>
+
+	* elf_getphdrnum.c (elf_getphdrnum): Sanity check the
+	__elf_getphdrnum_rdlock result.
+
 2014-11-18  Mark Wielaard  <mjw@redhat.com>
 
 	* version_xlate.h (elf_cvt_Verdef): Check for overflow.

libelf/elf_getphdrnum.c

@@ -97,6 +97,39 @@ elf_getphdrnum (elf, dst)
 
   rwlock_rdlock (elf->lock);
   result = __elf_getphdrnum_rdlock (elf, dst);
+
+  /* Do some sanity checking to make sure phnum and phoff are consistent.  */
+  Elf64_Off off = (elf->class == ELFCLASS32
+		   ? elf->state.elf32.ehdr->e_phoff
+		   : elf->state.elf64.ehdr->e_phoff);
+  if (unlikely (off == 0))
+    {
+      *dst = 0;
+      goto out;
+    }
+
+  if (unlikely (off >= elf->maximum_size))
+    {
+      __libelf_seterrno (ELF_E_INVALID_DATA);
+      result = -1;
+      goto out;
+    }
+
+  /* Check for too many sections.  */
+  size_t phdr_size = (elf->class == ELFCLASS32
+		      ? sizeof (Elf32_Phdr) : sizeof (Elf64_Phdr));
+  if (unlikely (*dst > SIZE_MAX / phdr_size))
+    {
+      __libelf_seterrno (ELF_E_INVALID_DATA);
+      result = -1;
+      goto out;
+    }
+
+  /* Truncated file?  Don't return more than can be indexed.  */
+  if (unlikely (elf->maximum_size - off < *dst * phdr_size))
+    *dst = (elf->maximum_size - off) / phdr_size;
+
+out:
   rwlock_unlock (elf->lock);
 
   return result;