Re: Bogus buffer length check in linux-2.6.11 read()

Robert Hancock
2005-03-16T14:42:13Z

linux-os wrote:
> I don't know how much more precise I could have been. I show the
> code that will cause the observed condition. I explain that this
> condition is new, that it doesn't correspond to the previous
> behavior.
>
> Never before was some buffer checked for length before some data
> was written to it. The EFAULT is supposed to occur IFF a write
> attempt occurs outside the caller's accessible address space.
> This used to be done by hardware during the write to user-space.
> This had zero impact upon performance. Now there is some
> software added that adds CPU cycles, subtracts performance,
> and cannot possibly do anything useful.
>
> Also, the code was written to show the problem. The code
> is not designed to be an example of good coding practice.
>
> The actual problem observed with the new kernel was
> when some legacy code used gets() instead of fgets().
> The call returned immediately with an EFAULT because
> the 'C' runtime library put some value that the kernel
> didn't 'like' (4096 bytes) in the subsequent read.
>
> This is code for which there are no sources available
> and it is required to be used, cannot be replaced,
> cannot be thrown away and costs about US$ 10,000
> from a company that is no longer in business.
>
> Somebody's arbitrary and capricious addition of spook
> code destroyed an application's functionality.

It appears this was added by the patch shown here:

http://lwn.net/Articles/122581/

The reason given was that if the read or write doesn't use all of the
available space due to end-of-file, etc. the remaining part of the
buffer given by the user is not checked for accessibility, thereby
hiding bugs. It makes little sense for an app to do a read or write with
a buffer larger than the space that they've actually allocated.

I can see how this might be a problem when using gets, since there is no
way to know how big the buffer that has been allocated by the
application is.

Note that access_ok only does a rudimentary check to determine if the
address might be a valid user-space address, it does not check every
page to determine if it is accessible or not like verify_area did (and
copy_to/from_user does).

--
Robert Hancock Saskatoon, SK, Canada
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