Re: 2.6.13-mm3 [OOPS] vfs, page_owner, full reproductively, badness in vsnprintf

Alexander Nyberg
2005-09-12T17:54:45Z

On Mon, Sep 12, 2005 at 01:48:54PM +0200 Michal Piotrowski wrote:

> Hi,
>
> [1.] One line summary of the problem:
> System oops when I do F3 (file view) in mc on /proc/page_owner
>
> [2.] Full description of the problem/report:
>
> [3.] Keywords (i.e., modules, networking, kernel):
> vfs, page_owner, full reproductively
>
> [4.] Kernel version (from /proc/version):
> Linux version 2.6.13-mm3 (root@ng02.pl) (gcc version 3.3.5 (Debian
> 1:3.3.5-13)) #4 SMP PREEMPT Mon Sep 12 12:14:24 CEST 2005
>

Gah, I'm such a fantastic programmer.

I don't know what mc is up to but the error checking in read_page_owner
is flawed wrt snprintf which could cause the 'size' argument to snprintf
to become negative and if so overwrite beyond 'buf'.

Again, I fail to see how mc causes this to happen, but this fixes it
by proper error checking.

Signed-off-by: Alexander Nyberg <alexn@telia.com>

Index: mm/fs/proc/proc_misc.c
===================================================================
--- mm.orig/fs/proc/proc_misc.c 2005-09-12 16:52:22.000000000 +0200
+++ mm/fs/proc/proc_misc.c 2005-09-12 17:20:13.000000000 +0200
@@ -567,6 +567,7 @@
char namebuf[128];
unsigned long offset = 0, symsize;
int i;
+ ssize_t num_written = 0;

pfn = min_low_pfn + *ppos;
page = pfn_to_page(pfn);
@@ -588,22 +589,40 @@
if (!kbuf)
return -ENOMEM;

- ret = snprintf(kbuf, 1024, "Page allocated via order %d, mask 0x%x\n",
+ ret = snprintf(kbuf, count, "Page allocated via order %d, mask 0x%x\n",
page->order, page->gfp_mask);
+ if (ret >= count) {
+ ret = -ENOMEM;
+ goto out;
+ }
+
+ num_written = ret;

for (i = 0; i < 8; i++) {
if (!page->trace[i])
break;
symname = kallsyms_lookup(page->trace[i], &symsize, &offset, &modname, namebuf);
- ret += snprintf(kbuf + ret, count - ret, "[0x%lx] %s+%lu\n",
+ ret = snprintf(kbuf + num_written, count - num_written, "[0x%lx] %s+%lu\n",
page->trace[i], namebuf, offset);
+ if (ret >= count - num_written) {
+ ret = -ENOMEM;
+ goto out;
+ }
+ num_written += ret;
}

- ret += snprintf(kbuf + ret, count -ret, "\n");
+ ret = snprintf(kbuf + num_written, count - num_written, "\n");
+ if (ret >= count - num_written) {
+ ret = -ENOMEM;
+ goto out;
+ }
+
+ num_written += ret;
+ ret = num_written;

if (copy_to_user(buf, kbuf, ret))
ret = -EFAULT;
-
+out:
kfree(kbuf);
return ret;
}