[PATCH] libelf: ar_size cannot be negative. Fix max ar size.

[PATCH] libelf: ar_size cannot be negative. Fix max ar size.

[Mark Wielaard]

[-- Attachment #1: Type: text/plain, Size: 1503 bytes --]

Elf_Arhdr ar_size is loff_t, which is signed. Make sure it isn't negative.
When the parent start_offset is non-zero maxsize should include it to
compensate for ar offset.

Found with afl-fuzz.

Signed-off-by: Mark Wielaard <mjw@redhat.com>
---
 libelf/ChangeLog   | 5 +++++
 libelf/elf_begin.c | 9 ++++++++-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/libelf/ChangeLog b/libelf/ChangeLog
index 3b88d03..6a1c925 100644
--- a/libelf/ChangeLog
+++ b/libelf/ChangeLog
@@ -1,3 +1,8 @@
+2014-12-25  Mark Wielaard  <mjw@redhat.com>
+
+	* elf_begin.c (__libelf_next_arhdr_wrlock): ar_size cannot be
+	negative. Include start_offset in maxsize.
+
 2014-12-18  Ulrich Drepper  <drepper@gmail.com>
 
 	* Makefile.am: Suppress output of textrel_check command.
diff --git a/libelf/elf_begin.c b/libelf/elf_begin.c
index 30abe0b..947b0ed 100644
--- a/libelf/elf_begin.c
+++ b/libelf/elf_begin.c
@@ -924,9 +924,16 @@ __libelf_next_arhdr_wrlock (elf)
   INT_FIELD (ar_mode);
   INT_FIELD (ar_size);
 
+  if (elf_ar_hdr->ar_size < 0)
+    {
+      __libelf_seterrno (ELF_E_INVALID_ARCHIVE);
+      return -1;
+    }
+
   /* Truncated file?  */
   size_t maxsize;
-  maxsize = elf->maximum_size - elf->state.ar.offset - sizeof (struct ar_hdr);
+  maxsize = (elf->start_offset + elf->maximum_size
+	     - elf->state.ar.offset - sizeof (struct ar_hdr));
   if ((size_t) elf_ar_hdr->ar_size > maxsize)
     elf_ar_hdr->ar_size = maxsize;
 
-- 
1.8.3.1

Re: [PATCH] libelf: ar_size cannot be negative. Fix max ar size.

[Mark Wielaard]

[-- Attachment #1: Type: text/plain, Size: 265 bytes --]

On Thu, 2014-12-25 at 23:28 +0100, Mark Wielaard wrote:
> Elf_Arhdr ar_size is loff_t, which is signed. Make sure it isn't negative.
> When the parent start_offset is non-zero maxsize should include it to
> compensate for ar offset.

I pushed this to master.