GNU bug report logs - #26545
shred: buffer overlap, some data not wiped

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Pádraig Brady <P <at> draigBrady.com>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#26545: closed (shred: buffer overlap, some data not wiped)
Date: Tue, 18 Apr 2017 02:51:02 +0000

[Message part 1 (text/plain, inline)]

Your message dated Mon, 17 Apr 2017 19:50:14 -0700
with message-id <55f0917c-3966-410d-7135-e6598f6830f6 <at> draigBrady.com>
and subject line Re: bug#26545: shred: buffer overlap, some data not wiped
has caused the debbugs.gnu.org bug report #26545,
regarding shred: buffer overlap, some data not wiped
to be marked as done.

(If you believe you have received this mail in error, please contact
help-debbugs <at> gnu.org.)


-- 
26545: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=26545
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Bogdan <bogdandr <at> op.pl>
To: bug-coreutils <at> gnu.org
Subject: shred: buffer overlap, some data not wiped
Date: Mon, 17 Apr 2017 20:53:47 +0200

[Message part 3 (text/plain, inline)]

 Hi.

 I believe I've found a bug in shred.c, function fillpattern(): there
is code that says:

  for (i = 3; i < size / 2; i *= 2)
    memcpy (r + i, r, i);
  if (i < size)
    memcpy (r + i, r, size - i);

The problem occurs for specific values of the "size" variable.
Example: size = 7:
1) size / 2 = 3,
2) the "for" loop sets "i" to 3 and never runs (the condition is "3 <
3"), even though it could (there are 4 bytes left to be wiped now)
3) the "if" condition is true (3 < 7)
4) the memcpy call evaluates to
	memcpy (r + 3, r, 7 - 3)
   in other words:
	memcpy (r + 3, r, 4)

Now, this poses a few problems:

1) copying 4 bytes between areas separated by 3 bytes means that the
areas overlap, which is forbidden for memcpy (results are not guaranteed),

2) copying 4 bytes with only 3 of them wiped means potentially copying
1 byte of the original data (from byte 4 to byte 7) and leaving it there.

 I don't know if it's possible to get "size = 7" with the current code
shape, but there may be other "problematic" values. May look like a
small bug now, may become bigger later.

 Anyway, I'm attaching a simple patch to fix this. The key change is
to write "i * 2 < size" instead of "i < size / 2". Although
mathematically equivalent, with C's integer arithmetic the latter one
will truncate the results.
 You may change left shifts to multiplications, if you wish, but if
overflow happens, it will happen in both versions.

 Things to keep in mind for later:
- size_t may be 32 bits wide, so watch out for buffers of 4GB or more
(may happen one day? :) ),
- if one day "size" could be any value (including 1 or 2), buffer
overflow will happen.

-- 
Pozdrawiam/Regards - Bogdan                     (GNU/Linux & FreeDOS)
Kurs asemblera x86 (DOS, GNU/Linux):            http://bogdro.evai.pl
Grupy dyskusyjne o asm:  pl.comp.lang.asm alt.pl.asm alt.pl.asm.win32
www.Xiph.org www.TorProject.org  Soft(EN): http://bogdro.evai.pl/soft

[shred-buffer-fix.diff (text/x-patch, attachment)]

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From: Pádraig Brady <P <at> draigBrady.com>
To: Bogdan <bogdandr <at> op.pl>, 26545-done <at> debbugs.gnu.org
Subject: Re: bug#26545: shred: buffer overlap, some data not wiped
Date: Mon, 17 Apr 2017 19:50:14 -0700

[Message part 6 (text/plain, inline)]

On 17/04/17 11:53, Bogdan wrote:
>  Hi.
> 
>  I believe I've found a bug in shred.c, function fillpattern(): there
> is code that says:
> 
>   for (i = 3; i < size / 2; i *= 2)
>     memcpy (r + i, r, i);
>   if (i < size)
>     memcpy (r + i, r, size - i);
> 
> The problem occurs for specific values of the "size" variable.
> Example: size = 7:
> 1) size / 2 = 3,
> 2) the "for" loop sets "i" to 3 and never runs (the condition is "3 <
> 3"), even though it could (there are 4 bytes left to be wiped now)
> 3) the "if" condition is true (3 < 7)
> 4) the memcpy call evaluates to
> 	memcpy (r + 3, r, 7 - 3)
>    in other words:
> 	memcpy (r + 3, r, 4)
> 
> Now, this poses a few problems:
> 
> 1) copying 4 bytes between areas separated by 3 bytes means that the
> areas overlap, which is forbidden for memcpy (results are not guaranteed),
> 
> 2) copying 4 bytes with only 3 of them wiped means potentially copying
> 1 byte of the original data (from byte 4 to byte 7) and leaving it there.
> 
>  I don't know if it's possible to get "size = 7" with the current code
> shape, but there may be other "problematic" values. May look like a
> small bug now, may become bigger later.

Very well spotted!
It's easy enough to trigger:

  touch blah
  shred -n4 -s7 blah

valgrind or ASAN will trigger failures due to this issue.

>  Anyway, I'm attaching a simple patch to fix this. The key change is
> to write "i * 2 < size" instead of "i < size / 2". Although
> mathematically equivalent, with C's integer arithmetic the latter one
> will truncate the results.
>  You may change left shifts to multiplications, if you wish, but if
> overflow happens, it will happen in both versions.
> 
>  Things to keep in mind for later:
> - size_t may be 32 bits wide, so watch out for buffers of 4GB or more
> (may happen one day? :) ),
> - if one day "size" could be any value (including 1 or 2), buffer
> overflow will happen.

These overflow issues can be avoided by just changing the < to <=.
I've not noted the issue in NEWS since it really is edge case stuff
not practically relevant to users, especially considering there
is always a random pass written after the patterns.

I've updated your patch in the attached and will push later.

thanks,
Pádraig

[shred-pattern-umr.patch (text/x-patch, attachment)]

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