Re: Taking strlen of buffers copied from userspace

[Robert Hancock <hancockr@shaw.ca>]

Artem Frolov wrote:
> Hello,
> 
> I am in the process of testing static defect analyzer on a Linux
> kernel source code (see disclosure below).
> 
> I found some potential array bounds violations. The pattern is as
> follows: bytes are copied from the user space and then buffer is
> accessed on index strlen(buf)-1. This is a defect if user data start
> from 0. So the question is: can we make any assumptions what data may
> be received from the user or it could be arbitrary?

In general I don't think any such assumptions should be made. In the 
case of the two below I'm assuming that root access is required to write 
those files, preventing any serious security hole, but it shouldn't 
really be permitted to corrupt kernel memory like this, as would likely 
happen if somebody wrote some data that contained a null as the first 
character.

> 
> For example, in ./drivers/block/cciss.c, function cciss_proc_write
> (line numbers are taken form 2.6.11.3):
>    ....
>    293          if (count > sizeof(cmd)-1) return -EINVAL;
>    294          if (copy_from_user(cmd, buffer, count)) return -EFAULT;
>    295          cmd[count] = '\0';
>    296          len = strlen(cmd);      // above 3 lines ensure safety
>    297          if (cmd[len-1] == '\n')
>    298                  cmd[--len] = '\0';
>    .....
> 
> Another example is arch/i386/kernel/cpu/mtrr/if.c, function mtrr_write:
>    ....
>    107          if (copy_from_user(line, buf, len - 1))
>    108                  return -EFAULT;
>    109          ptr = line + strlen(line) - 1;
>    110          if (*ptr == '\n')
>    111                  *ptr = '\0';
>     ....
> 

This one is also unsafe if somebody writes some data which is not 
null-terminated (assuming that that's possible), since strlen will run 
off the end of the buffer. The first example doesn't have that problem.

-- 
Robert Hancock      Saskatoon, SK, Canada
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