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From: Dan Carpenter <dan.carpenter@oracle.com>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Leon Romanovsky <leon@kernel.org>,
	Peilin Ye <yepeilin.cs@gmail.com>,
	Santosh Shilimkar <santosh.shilimkar@oracle.com>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
	linux-kernel-mentees@lists.linuxfoundation.org,
	netdev@vger.kernel.org, linux-rdma@vger.kernel.org,
	rds-devel@oss.oracle.com, linux-kernel@vger.kernel.org
Subject: Re: [Linux-kernel-mentees] [PATCH net] rds: Prevent kernel-infoleak in rds_notify_queue_get()
Date: Sat, 1 Aug 2020 11:00:26 +0300
Message-ID: <20200801080026.GJ5493@kadam> (raw)
In-Reply-To: <20200731182712.GI24045@ziepe.ca>

On Fri, Jul 31, 2020 at 03:27:12PM -0300, Jason Gunthorpe wrote:
> On Fri, Jul 31, 2020 at 07:19:24PM +0200, Greg Kroah-Hartman wrote:
> 
> > > I tried for a bit and didn't find a way to get even old gcc 4.4 to not
> > > initialize the holes.
> > 
> > Odd, so it is just the "= {0};" that does not zero out the holes?
> 
> Nope, it seems to work fine too. I tried a number of situations and I
> could not get the compiler to not zero holes, even back to gcc 4.4
> 
> It is not just accidental either, take this:
> 
> 	struct rds_rdma_notify {
> 		unsigned long user_token;
> 		unsigned char status;
> 		unsigned long user_token1 __attribute__((aligned(32)));
> 	} foo = {0};
> 
> Which has quite a big hole, clang generates:
> 
> 	movq	$0, 56(%rdi)
> 	movq	$0, 48(%rdi)
> 	movq	$0, 40(%rdi)
> 	movq	$0, 32(%rdi)
> 	movq	$0, 24(%rdi)
> 	movq	$0, 16(%rdi)
> 	movq	$0, 8(%rdi)
> 	movq	$0, (%rdi)
> 
> Deliberate extra instructions to fill both holes. gcc 10 does the
> same, older gcc's do create a rep stosq over the whole thing.
> 
> Some fiddling with godbolt shows quite a variety of output, but I
> didn't see anything that looks like a compiler not filling
> padding. Even godbolt's gcc 4.1 filled the padding, which is super old.
> 
> In several cases it seems the aggregate initializer produced better
> code than memset, in other cases it didn't
> 
> Without an actual example where this doesn't work right it is hard to
> say anything more..

Here is the example that set off the recent patches:

https://lkml.org/lkml/2020/7/27/199

Another example is commit 5ff223e86f5a ("net: Zeroing the structure
ethtool_wolinfo in ethtool_get_wol()").  I tested this one with GCC 7.4
at the time and it was a real life bug.

The rest of these patches were based on static analysis from Smatch.
They're all "theoretical" bugs based on the C standard but it's
impossible to know if and when they'll turn into real life bugs.

It's not a super long list of code that's affected because we've known
that the bug was possible for a few years.  It was only last year when
I saw that it had become a real life bug.

regards,
dan carpenter


  reply index

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-30 19:20 Peilin Ye
2020-07-30 19:29 ` santosh.shilimkar
2020-07-31  4:53 ` Leon Romanovsky
2020-07-31  5:33   ` Greg Kroah-Hartman
2020-07-31  5:33     ` Greg Kroah-Hartman
     [not found]       ` <CAHp75Vdr2HC_ogNhBCxxGut9=Z6pQMFiA0w-268OQv+5unYOTg@mail.gmail.com>
2020-07-31  7:00         ` Leon Romanovsky
2020-07-31  7:05           ` Andy Shevchenko
2020-07-31 14:04       ` Jason Gunthorpe
2020-07-31 14:21         ` Greg Kroah-Hartman
2020-07-31 14:36           ` Jason Gunthorpe
2020-07-31 17:19             ` Greg Kroah-Hartman
2020-07-31 18:27               ` Jason Gunthorpe
2020-08-01  8:00                 ` Dan Carpenter [this message]
2020-08-01 14:40                   ` Jason Gunthorpe
2020-08-03  9:34                     ` Dan Carpenter
2020-08-01  5:38               ` Leon Romanovsky
2020-08-02 22:10                 ` Jason Gunthorpe
2020-08-02 22:23                   ` Joe Perches
2020-08-02 22:28                     ` Jason Gunthorpe
2020-08-02 22:45                       ` Joe Perches
2020-08-03  4:58                         ` Leon Romanovsky
2020-08-03 23:06                         ` Jason Gunthorpe
2020-08-08 22:57                           ` Jack Leadford
2020-08-09  7:04                             ` Leon Romanovsky
2020-08-14 17:07                             ` Jason Gunthorpe
2020-07-31  9:59   ` Dan Carpenter
2020-07-31 11:14     ` Håkon Bugge
2020-07-31 11:59       ` Greg Kroah-Hartman
2020-07-31 12:03         ` Håkon Bugge
2020-07-31 23:54 ` David Miller

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