Subject: Re: [Linux-kernel-mentees] [PATCH net] rds: Prevent kernel-infoleak in rds_notify_queue_get()

From: Jason Gunthorpe <[email-masked]>
Date: Fri, 31 Jul 2020 11:36:04 -0300

On Fri, Jul 31, 2020 at 04:21:48PM +0200, Greg Kroah-Hartman wrote:

> > The spec was updated in C11 to require zero'ing padding when doing
> > partial initialization of aggregates (eg = {})
> > 
> > """if it is an aggregate, every member is initialized (recursively)
> > according to these rules, and any padding is initialized to zero
> > bits;"""
> 
> But then why does the compilers not do this?

Do you have an example?

> > Considering we have thousands of aggregate initializers it
> > seems likely to me Linux also requires a compiler with this C11
> > behavior to operate correctly.
> 
> Note that this is not an "operate correctly" thing, it is a "zero out
> stale data in structure paddings so that data will not leak to
> userspace" thing.

Yes, not being insecure is "operate correctly", IMHO :)
 
> > Does this patch actually fix anything? My compiler generates identical
> > assembly code in either case.
> 
> What compiler version?

I tried clang 10 and gcc 9.3 for x86-64.

#include <string.h>

void test(void *out)
{
	struct rds_rdma_notify {
		unsigned long user_token;
		unsigned int status;
	} foo = {};
	memcpy(out, &foo, sizeof(foo));
}

$ gcc -mno-sse2 -O2 -Wall -std=c99 t.c -S

test:
	endbr64
	movq	$0, (%rdi)
	movq	$0, 8(%rdi)
	ret

Just did this same test with gcc 4.4 and it also gave the same output..

Made it more complex with this:

	struct rds_rdma_notify {
		unsigned long user_token;
		unsigned char status;
		unsigned long user_token1;
		unsigned char status1;
		unsigned long user_token2;
		unsigned char status2;
		unsigned long user_token3;
		unsigned char status3;
		unsigned long user_token4;
		unsigned char status4;
	} foo;

And still got the same assembly vs memset on gcc 4.4.

I tried for a bit and didn't find a way to get even old gcc 4.4 to not
initialize the holes.

Jason