Last Comment Bug 328509 - GIF2.cpp: do_lzw() tries to access memory out of bounds [@ do_lzw]
: GIF2.cpp: do_lzw() tries to access memory out of bounds [@ do_lzw]
[sg:high?] Doesn't affect ff1.0/moz1....
: fixed1.8.0.2, fixed1.8.1, regression, topcrash
Product: Core
Classification: Components
Component: ImageLib (show other bugs)
: Trunk
: PowerPC Mac OS X
-- normal (vote)
: ---
Assigned To: Daniel Veditz [:dveditz]
: Milan Sreckovic [:milan]
: 317536 (view as bug list)
Depends on:
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Reported: 2006-02-24 12:38 PST by jhp (no longer active)
Modified: 2007-10-01 19:14 PDT (History)
8 users (show)
dveditz: blocking1.9a1+
dveditz: blocking1.8.1+
dveditz: blocking1.8.0.2+
See Also:
Crash Signature:
QA Whiteboard:
Iteration: ---
Points: ---
Has Regression Range: ---
Has STR: ---

One line patch to fix clearing of struct (786 bytes, patch)
2006-02-26 02:24 PST, Alfred Kayser
jhpedemonte: review+
pavlov: superreview+
Details | Diff | Splinter Review

Description User image jhp (no longer active) 2006-02-24 12:38:50 PST
Offshoot of bug 328258.

Debugging using the gif attached at bug 328258 (, I noticed that do_lzw() is accessing memory out of bounds of the array.  Specifically, in this code:

      while (code >= clear_code)
        if (code == prefix[code])
          return -1;

        *stackp++ = suffix[code];
        code = prefix[code];

        if (stackp == stack + MAX_BITS)
          return -1;

At one point, when count == 190, the line "code = prefix[code]" sets code to 43690.  The next time through the loop, it tries to access prefix[43690], but prefix is of size 4097.
Comment 1 User image jhp (no longer active) 2006-02-24 14:59:09 PST
In GIFInit, I changed this line:
  memset(gs, 0, offsetof(gif_struct, prefix));
  memset(gs, 0, sizeof(gif_struct));

With this, I don't see it trying to access invalid memory.  Seems the while loop tries to access an offset of 'prefix' that has never been set up.
Comment 2 User image Alfred Kayser 2006-02-26 02:24:37 PST
Created attachment 213232 [details] [diff] [review]
One line patch to fix clearing of struct

The offsetof thing was to clear only part of the struct, but is clearly wrong. 
It is saver anyway to clear the whole struct.
Comment 3 User image jhp (no longer active) 2006-02-26 06:53:20 PST
Comment on attachment 213232 [details] [diff] [review]
One line patch to fix clearing of struct

This is fine.  However, as part of LZW, should the algorithm access an offset of prefix that hasn't previously been set?  With this fix, the result will now be zero.  Is that correct according to the LZW spec?
Comment 4 User image Christian :Biesinger (don't email me, ping me on IRC) 2006-02-26 15:49:59 PST
isn't this a duplicate of bug 243653?
Comment 5 User image Alfred Kayser 2006-02-27 00:39:50 PST
Comment on attachment 213232 [details] [diff] [review]
One line patch to fix clearing of struct

Pavlov, can you take quickly a look at this. This is related to a 'Access Denied' bug...
Comment 6 User image Alfred Kayser 2006-02-28 01:14:44 PST
Thanks, who can check this for me?
Comment 7 User image Alfred Kayser 2006-02-28 10:28:09 PST
Previous comment should be:

Thanks, who can check this in the tree(s) for me?

Also requesting to check it into the 1.8 branch for FF 2.0, because this a crasher (and related to bug 328258).
Comment 8 User image Daniel Veditz [:dveditz] 2006-03-01 14:26:31 PST
The memset line was added as part of bug 274391, well after the 1.7/aviary101 branches.
Comment 9 User image Daniel Veditz [:dveditz] 2006-03-02 12:50:32 PST
I guess I'll take this to land it.
Comment 10 User image Daniel Veditz [:dveditz] 2006-03-03 15:53:36 PST
Fix landed on trunk, 1.8 branch, and 1.8.1 branch

What is the security impact here? Looks like it's only reading random memory, right?
Comment 11 User image Dave Liebreich [:davel] 2006-03-04 13:00:55 PST
Please provide testcase and guidance on how to verify this fix.
Comment 12 User image David Baron :dbaron: ⌚️UTC-8 2006-03-10 12:39:24 PST
FWIW, this patch fixed the uninitialized memory reads and free memory reads reported by valgrind on the GIF in the testcase on bug 317536.  That bug should be marked duplicate or depends+fixed when this bug is opened.

Crashes in do_lzw are a topcrash for Fx; I suspect this fix fixes them.
Comment 13 User image Daniel Veditz [:dveditz] 2006-03-28 17:06:57 PST
fixed per comment 10 on March 3
Comment 14 User image Daniel Veditz [:dveditz] 2006-04-13 16:55:01 PDT
*** Bug 317536 has been marked as a duplicate of this bug. ***

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