Date: Tue, 9 Jun 2020 11:39:11 -0700 From: Kees Cook <keescook@...omium.org> To: Alexander Popov <alex.popov@...ux.com> Cc: Jann Horn <jannh@...gle.com>, Elena Reshetova <elena.reshetova@...el.com>, Emese Revfy <re.emese@...il.com>, Miguel Ojeda <miguel.ojeda.sandonis@...il.com>, Masahiro Yamada <masahiroy@...nel.org>, Michal Marek <michal.lkml@...kovi.net>, Andrew Morton <akpm@...ux-foundation.org>, Masahiro Yamada <yamada.masahiro@...ionext.com>, Thiago Jung Bauermann <bauerman@...ux.ibm.com>, Luis Chamberlain <mcgrof@...nel.org>, Jessica Yu <jeyu@...nel.org>, Sven Schnelle <svens@...ckframe.org>, Iurii Zaikin <yzaikin@...gle.com>, Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, Vincenzo Frascino <vincenzo.frascino@....com>, Thomas Gleixner <tglx@...utronix.de>, Peter Collingbourne <pcc@...gle.com>, Naohiro Aota <naohiro.aota@....com>, Alexander Monakov <amonakov@...ras.ru>, Mathias Krause <minipli@...glemail.com>, PaX Team <pageexec@...email.hu>, Brad Spengler <spender@...ecurity.net>, Laura Abbott <labbott@...hat.com>, Florian Weimer <fweimer@...hat.com>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, linux-kbuild@...r.kernel.org, the arch/x86 maintainers <x86@...nel.org>, Linux ARM <linux-arm-kernel@...ts.infradead.org>, kernel list <linux-kernel@...r.kernel.org>, gcc@....gnu.org, notify@...nel.org Subject: Re: [PATCH 1/5] gcc-plugins/stackleak: Exclude alloca() from the instrumentation logic On Thu, Jun 04, 2020 at 06:23:38PM +0300, Alexander Popov wrote: > On 04.06.2020 17:01, Jann Horn wrote: > > On Thu, Jun 4, 2020 at 3:51 PM Alexander Popov <alex.popov@...ux.com> wrote: > >> Some time ago Variable Length Arrays (VLA) were removed from the kernel. > >> The kernel is built with '-Wvla'. Let's exclude alloca() from the > >> instrumentation logic and make it simpler. The build-time assertion > >> against alloca() is added instead. > > [...] > >> + /* Variable Length Arrays are forbidden in the kernel */ > >> + gcc_assert(!is_alloca(stmt)); > > > > There is a patch series from Elena and Kees on the kernel-hardening > > list that deliberately uses __builtin_alloca() in the syscall entry > > path to randomize the stack pointer per-syscall - see > > <https://email@example.com/>. > > Thanks, Jann. > > At first glance, leaving alloca() handling in stackleak instrumentation logic > would allow to integrate stackleak and this version of random_kstack_offset. Right, it seems there would be a need for this coverage to remain, otherwise the depth of stack erasure might be incorrect. It doesn't seem like the other patches strictly depend on alloca() support being removed, though? > Kees, Elena, did you try random_kstack_offset with upstream stackleak? I didn't try that combination yet, no. It seemed there would likely still be further discussion about the offset series first (though the thread has been silent -- I'll rebase and resend it after rc2). > It looks to me that without stackleak erasing random_kstack_offset can be > weaker. I mean, if next syscall has a bigger stack randomization gap, the data > on thread stack from the previous syscall is not overwritten and can be used. Am > I right? That's correct. I think the combination is needed, but I don't think they need to be strictly tied together. > Another aspect: CONFIG_STACKLEAK_METRICS can be used for guessing kernel stack > offset, which is bad. It should be disabled if random_kstack_offset is on. Agreed. -- Kees Cook
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