Taking strlen of buffers copied from userspace

[Artem Frolov <artemfrolov@gmail.com>]

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?

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';
    ....


Full disclosure: I am working for Klocwork (http://www.klocwork.com/),
which is a vendor of commercial closed-source proprietary products,
static analyzer for C/C++ is part of its products

Best regards
--
Artem Frolov
Senior software engineer
Klocwork inc