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Date: Fri, 31 Jul 2020 11:04:52 -0300 From: Jason Gunthorpe <jgg@...pe.ca> To: Greg Kroah-Hartman <gregkh@...uxfoundation.org> Cc: Leon Romanovsky <leon@...nel.org>, Peilin Ye <yepeilin.cs@...il.com>, Santosh Shilimkar <santosh.shilimkar@...cle.com>, "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, Dan Carpenter <dan.carpenter@...cle.com>, Arnd Bergmann <arnd@...db.de>, linux-kernel-mentees@...ts.linuxfoundation.org, netdev@...r.kernel.org, linux-rdma@...r.kernel.org, rds-devel@....oracle.com, linux-kernel@...r.kernel.org Subject: Re: [Linux-kernel-mentees] [PATCH net] rds: Prevent kernel-infoleak in rds_notify_queue_get() On Fri, Jul 31, 2020 at 07:33:33AM +0200, Greg Kroah-Hartman wrote: > On Fri, Jul 31, 2020 at 07:33:06AM +0200, Greg Kroah-Hartman wrote: > > On Fri, Jul 31, 2020 at 07:53:01AM +0300, Leon Romanovsky wrote: > > > On Thu, Jul 30, 2020 at 03:20:26PM -0400, Peilin Ye wrote: > > > > rds_notify_queue_get() is potentially copying uninitialized kernel stack > > > > memory to userspace since the compiler may leave a 4-byte hole at the end > > > > of `cmsg`. > > > > > > > > In 2016 we tried to fix this issue by doing `= { 0 };` on `cmsg`, which > > > > unfortunately does not always initialize that 4-byte hole. Fix it by using > > > > memset() instead. > > > > > > Of course, this is the difference between "{ 0 }" and "{}" initializations. > > > > Really? Neither will handle structures with holes in it, try it and > > see. > > And if true, where in the C spec does it say that? 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;""" The difference between {0} and the {} extension is only that {} reliably triggers partial initialization for all kinds of aggregates, while {0} has a number of edge cases where it can fail to compile. IIRC gcc has cleared the padding during aggregate initialization for a long time. Considering we have thousands of aggregate initializers it seems likely to me Linux also requires a compiler with this C11 behavior to operate correctly. Does this patch actually fix anything? My compiler generates identical assembly code in either case. Jason
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