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Date: Fri, 31 Jul 2020 19:19:24 +0200 From: Greg Kroah-Hartman <gregkh@...uxfoundation.org> To: Jason Gunthorpe <jgg@...pe.ca> 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 11:36:04AM -0300, Jason Gunthorpe wrote: > 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? At the moment, no, but we have had them in the past due to security issues we have had to fix for this. > > > 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. Odd, so it is just the "= {0};" that does not zero out the holes? thanks, greg k-h
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