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Date: Fri, 31 Jul 2020 11:36:04 -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 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
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