libelf: Make sure string returned by elf_strptr is NUL terminated.

The result of elf_strptr is often used directly to print or strcmp
the string. If the section data was truncated or corrupted that could
lead to invalid memory reads possibly crashing the application.
https://bugzilla.redhat.com/show_bug.cgi?id=1170810#c24

Reported-by: Alexander Cherepanov <cherepan@mccme.ru>
Signed-off-by: Mark Wielaard <mjw@redhat.com>

1 files changed, 21 insertions, 2 deletions

diff --git a/libelf/elf_strptr.c b/libelf/elf_strptr.c
index 62936a0f..e73bf360 100644
--- a/libelf/elf_strptr.c
+++ b/libelf/elf_strptr.c
@@ -86,6 +86,7 @@ elf_strptr (elf, idx, offset)
 	}
     }
 
+  size_t sh_size = 0;
   if (elf->class == ELFCLASS32)
     {
       Elf32_Shdr *shdr = strscn->shdr.e32 ?: __elf32_getshdr_rdlock (strscn);
@@ -96,6 +97,7 @@ elf_strptr (elf, idx, offset)
 	  goto out;
 	}
 
+      sh_size = shdr->sh_size;
       if (unlikely (offset >= shdr->sh_size))
 	{
 	  /* The given offset is too big, it is beyond this section.  */
@@ -113,6 +115,7 @@ elf_strptr (elf, idx, offset)
 	  goto out;
 	}
 
+      sh_size = shdr->sh_size;
       if (unlikely (offset >= shdr->sh_size))
 	{
 	  /* The given offset is too big, it is beyond this section.  */
@@ -141,7 +144,14 @@ elf_strptr (elf, idx, offset)
       // rawdata_base can be set while rawdata.d hasn't been
       // initialized yet (when data_read is zero). So we cannot just
       // look at the rawdata.d.d_size.
-      result = &strscn->rawdata_base[offset];
+
+      /* Make sure the string is NUL terminated.  Start from the end,
+	 which very likely is a NUL char.  */
+      if (likely (memrchr (&strscn->rawdata_base[offset],
+			  '\0', sh_size - offset) != NULL))
+	result = &strscn->rawdata_base[offset];
+      else
+	__libelf_seterrno (ELF_E_INVALID_INDEX);
     }
   else
     {
@@ -153,7 +163,16 @@ elf_strptr (elf, idx, offset)
 	  if (offset >= (size_t) dl->data.d.d_off
 	      && offset < dl->data.d.d_off + dl->data.d.d_size)
 	    {
-	      result = (char *) dl->data.d.d_buf + (offset - dl->data.d.d_off);
+	      /* Make sure the string is NUL terminated.  Start from
+		 the end, which very likely is a NUL char.  */
+	      if (likely (memrchr ((char *) dl->data.d.d_buf
+				   + (offset - dl->data.d.d_off), '\0',
+				   (dl->data.d.d_size
+				    - (offset - dl->data.d.d_off))) != NULL))
+		result = ((char *) dl->data.d.d_buf
+			  + (offset - dl->data.d.d_off));
+	      else
+		__libelf_seterrno (ELF_E_INVALID_INDEX);
 	      break;
 	    }