buglwIP - A Lightweight TCP/IP stack - Bugs: bug #24517, IP reassembly crashes on unaligned...


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bug #24517: IP reassembly crashes on unaligned IP headers

Submitted by:  Simon Goldschmidt <goldsimon>
Submitted on:  Fri 10 Oct 2008 02:21:56 PM UTC  
Category:  IPv4 Severity:  4 - Important
Item Group:  Crash Error Status:  Fixed
Privacy:  Public Assigned to:  Simon Goldschmidt <goldsimon>
Open/Closed:  Closed Planned Release:  None
lwIP version:  1.3.0

Mon 02 Feb 2009 07:26:51 PM UTC, comment #2: 

Seems to work, does it?

Simon Goldschmidt <goldsimon>
Project AdministratorIn charge of this item.
Wed 15 Oct 2008 06:21:04 PM UTC, comment #1: 

Checked in the fix. Ed, would you please check that it works?

Simon Goldschmidt <goldsimon>
Project AdministratorIn charge of this item.
Fri 10 Oct 2008 02:21:56 PM UTC, original submission:  

I don't know if this is a duplicate for #23309. Since we have a detailed error description here (thanks to Ed), this one is easy to solve.

Ed Sutter described it on lwip-users, here's the mail:

Ok, I've spoken to a few folks off-line about this, and have narrowed it
down a bit further.  There's either a bug or I'm not configuring something

I'm running LWIP 1.3.0 on a Blackfin, which is picky about accessing longs
on long-word boundaries.  I have MEM_ALIGNMENT set to 4.  My network generates
occasional fragmented IP packets, and it causes the function
ip_reass_chain_frag_into_datagram_and_validate() to crash.

Here's the flow...
The packet is received and buffered up using pbuf_alloc() (with each payload
aligned on a 4-byte boundary) then that set of pbufs is passed to ethernet_input().
The ethernet_input() function then sees that it is IP, increments the payload
pointer by 14 (size of ethernet header) and passes the pbufs to ip_input().
Note that at this point, the payload pointer is no longer aligned on a 4-byte
boundary because the original 4-byte-aligned payload pointer is incremented
by 14.

Next, ip_input() sees that the packet is a fragment, so it calls ip_reass()
which then calls ip_reass_chain_frag_into_datagram_and_validate().  About 15
lines from the top, this function overlays a "helper" structure pointer on
the payload area (which is no longer 4-byte aligned).  The helper structure's
first member is a pointer which is initialized to NULL, this causes the crash
because loading the pointer is a 4-byte-wide access, but the pointer is not on
a 4-byte aligned address.

This is only seen on an alignment-sensitive processor when an fragment is
received.  I made a simple change to the helper structure to fix this (see
below); however, I would like to make sure that there isn't some other
configuration parameter that I should be setting to avoid this.

I changed this...

    struct ip_reass_helper {
      struct pbuf *next_pbuf;    <<<< this member access is not aligned
      u16_t start;
      u16_t end;

to this...

    struct ip_reass_helper {
        PACK_STRUCT_FIELD(u16_t start);
        PACK_STRUCT_FIELD(struct pbuf *next_pbuf);    <<<< now it is
        PACK_STRUCT_FIELD(u16_t end);

and my crash has disappeared.  Putting the 'start' member above the next_pbuf
pointer will naturally align the pointer, which as far as I can tell will
always be mis-aligned by 2 bytes when MEM_ALIGNMENT is set to 4.

Is this a valid fix or am I just configuring something incorrectly to cause
this crash?

Sorry for the verbosity!

Simon Goldschmidt <goldsimon>
Project AdministratorIn charge of this item.


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    Follow 3 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2009-02-02 goldsimon StatusReady For Test => Fixed
        Open/ClosedOpen => Closed
    2008-10-15 goldsimon StatusIn Progress => Ready For Test

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