On Fri, May 18, 2018 at 11:27:56AM +0200, Florian Westphal wrote:

Greg Kroah-Hartman [off-list ref] wrote:

quoted

On Thu, May 17, 2018 at 12:42:00PM +0200, Jan Engelhardt wrote:

quoted

On Thursday 2018-05-17 12:09, Greg Kroah-Hartman wrote:

quoted

quoted

quoted

--- a/net/netfilter/x_tables.c
+++ b/net/netfilter/x_tables.c

@@ -1183,11 +1183,10 @@ struct xt_table_info *xt_alloc_table_info(unsigned int size)
 	 * than shoot all processes down before realizing there is nothing
 	 * more to reclaim.
 	 */
-	info = kvmalloc(sz, GFP_KERNEL | __GFP_NORETRY);
+	info = kvzalloc(sz, GFP_KERNEL | __GFP_NORETRY);
 	if (!info)
 		return NULL;

I am curious, what particular path does not later overwrite the whole zone ?

In do_ipt_get_ctl, the IPT_SO_GET_ENTRIES: option uses a len value that
can be larger than the size of the structure itself.

Then the data is copied to userspace in copy_entries_to_user() for ipv4
and v6, and that's where the "bad data"

If the kernel incorrectly copies more bytes than it should, isn't that
a sign that may be going going past the end of the info buffer?
(And thus, zeroing won't truly fix the issue)

No, the buffer size is correct, we just aren't filling up the whole
buffer as the data requested is smaller than the buffer size.

I have no objections to the patch but I'd like to understand what
problem its fixing.

Normal pattern is:
newinfo = xt_alloc_table_info(tmp.size);
copy_from_user(newinfo->entries, user + sizeof(tmp), tmp.size);

So inital value of the rule blob area should not matter.

Furthermore, when copying the rule blob back to userspace,
the kernel is not supposed to copy any padding back to userspace either,
since commit f32815d21d4d8287336fb9cef4d2d9e0866214c2 only the
user-relevant parts should be copied (some matches and targets allocate
kernel-private data such as pointers, and we did use to leak such pointer
values back to userspace).

Adding Peter to this thread, as he originally reported this issue to
Google back in February.

Peter, I know you reported this against the 4.4 kernel tree, but since
then, commit f32815d21d4d ("xtables: add xt_match, xt_target and data
copy_to_user functions") has been added to the kernel in release 4.11.
In digging through this crazy code path, I think the issue is still
there, but can not verify it for sure.

Is there any way you can run your tests on the 4.14 or newer kernel tree
to see if this issue really is fixed or not?

thanks,

greg k-h