Re: Taking strlen of buffers copied from userspace

From: Robert Hancock
Date: Tue Mar 15 2005 - 19:02:52 EST

Artem Frolov wrote:

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
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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