From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Mon, 7 Oct 2019 16:22:30 +0200 From: Ingo Molnar Subject: Re: [PATCH v2 5.4 regression fix] x86/boot: Provide memzero_explicit Message-ID: <20191007142230.GA117630@gmail.com> References: <20191007134724.4019-1-hdegoede@redhat.com> <20191007140022.GA29008@gmail.com> <1dc3c53d-785e-f9a4-1b4c-3374c94ae0a7@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1dc3c53d-785e-f9a4-1b4c-3374c94ae0a7@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: To: Hans de Goede Cc: Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H . Peter Anvin" , Herbert Xu , Ard Biesheuvel , linux-crypto@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org, Arvind Sankar , Stephan Mueller * Hans de Goede wrote: > Hi, > > On 07-10-2019 16:00, Ingo Molnar wrote: > > > > * Hans de Goede wrote: > > > > > The purgatory code now uses the shared lib/crypto/sha256.c sha256 > > > implementation. This needs memzero_explicit, implement this. > > > > > > Reported-by: Arvind Sankar > > > Fixes: 906a4bb97f5d ("crypto: sha256 - Use get/put_unaligned_be32 to get input, memzero_explicit") > > > Signed-off-by: Hans de Goede > > > --- > > > Changes in v2: > > > - Add barrier_data() call after the memset, making the function really > > > explicit. Using barrier_data() works fine in the purgatory (build) > > > environment. > > > --- > > > arch/x86/boot/compressed/string.c | 6 ++++++ > > > 1 file changed, 6 insertions(+) > > > > > > diff --git a/arch/x86/boot/compressed/string.c b/arch/x86/boot/compressed/string.c > > > index 81fc1eaa3229..654a7164a702 100644 > > > --- a/arch/x86/boot/compressed/string.c > > > +++ b/arch/x86/boot/compressed/string.c > > > @@ -50,6 +50,12 @@ void *memset(void *s, int c, size_t n) > > > return s; > > > } > > > +void memzero_explicit(void *s, size_t count) > > > +{ > > > + memset(s, 0, count); > > > + barrier_data(s); > > > +} > > > > So the barrier_data() is only there to keep LTO from optimizing out the > > seemingly unused function? > > I believe that Stephan Mueller (who suggested adding the barrier) > was also worried about people using this as an example for other > "explicit" functions which actually might get inlined. > > This is not so much about protecting against LTO as it is against > protecting against inlining, which in this case boils down to the > same thing. Also this change makes the arch/x86/boot/compressed/string.c > and lib/string.c versions identical which seems like a good thing to me > (except for the code duplication part of it). > > But I agree a comment would be good, how about: > > void memzero_explicit(void *s, size_t count) > { > memset(s, 0, count); > /* Avoid the memset getting optimized away if we ever get inlined */ > barrier_data(s); > } Well, the standard construct for preventing inlining would be 'noinline', right? Any reason that wouldn't work? Thanks, Ingo