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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id x28sm12472404pfo.78.2019.06.01.22.06.11 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Sat, 01 Jun 2019 22:06:12 -0700 (PDT) Date: Sat, 1 Jun 2019 22:06:10 -0700 From: Kees Cook To: Catalin Marinas Cc: enh , Evgenii Stepanov , Andrey Konovalov , Khalid Aziz , Linux ARM , Linux Memory Management List , LKML , amd-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, linux-rdma@vger.kernel.org, linux-media@vger.kernel.org, kvm@vger.kernel.org, "open list:KERNEL SELFTEST FRAMEWORK" , Vincenzo Frascino , Will Deacon , Mark Rutland , Andrew Morton , Greg Kroah-Hartman , Yishai Hadas , Felix Kuehling , Alexander Deucher , Christian Koenig , Mauro Carvalho Chehab , Jens Wiklander , Alex Williamson , Leon Romanovsky , Dmitry Vyukov , Kostya Serebryany , Lee Smith , Ramana Radhakrishnan , Jacob Bramley , Ruben Ayrapetyan , Robin Murphy , Luc Van Oostenryck , Dave Martin , Kevin Brodsky , Szabolcs Nagy Subject: Re: [PATCH v15 00/17] arm64: untag user pointers passed to the kernel Message-ID: <201906012156.55E2C45@keescook> References: <201905211633.6C0BF0C2@keescook> <20190522101110.m2stmpaj7seezveq@mbp> <20190522163527.rnnc6t4tll7tk5zw@mbp> <201905221316.865581CF@keescook> <20190523144449.waam2mkyzhjpqpur@mbp> <201905230917.DEE7A75EF0@keescook> <20190523174345.6sv3kcipkvlwfmox@mbp> <201905231327.77CA8D0A36@keescook> <20190528170244.GF32006@arrakis.emea.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190528170244.GF32006@arrakis.emea.arm.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 28, 2019 at 06:02:45PM +0100, Catalin Marinas wrote: > On Thu, May 23, 2019 at 02:31:16PM -0700, Kees Cook wrote: > > syzkaller already attempts to randomly inject non-canonical and > > 0xFFFF....FFFF addresses for user pointers in syscalls in an effort to > > find bugs like CVE-2017-5123 where waitid() via unchecked put_user() was > > able to write directly to kernel memory[1]. > > > > It seems that using TBI by default and not allowing a switch back to > > "normal" ABI without a reboot actually means that userspace cannot inject > > kernel pointers into syscalls any more, since they'll get universally > > stripped now. Is my understanding correct, here? i.e. exploiting > > CVE-2017-5123 would be impossible under TBI? > > > > If so, then I think we should commit to the TBI ABI and have a boot > > flag to disable it, but NOT have a process flag, as that would allow > > attackers to bypass the masking. The only flag should be "TBI or MTE". > > > > If so, can I get top byte masking for other architectures too? Like, > > just to strip high bits off userspace addresses? ;) > > Just for fun, hack/attempt at your idea which should not interfere with > TBI. Only briefly tested on arm64 (and the s390 __TYPE_IS_PTR macro is > pretty weird ;)): OMG, this is amazing and bonkers. I love it. > --------------------------8<--------------------------------- > diff --git a/arch/s390/include/asm/compat.h b/arch/s390/include/asm/compat.h > index 63b46e30b2c3..338455a74eff 100644 > --- a/arch/s390/include/asm/compat.h > +++ b/arch/s390/include/asm/compat.h > @@ -11,9 +11,6 @@ > > #include > > -#define __TYPE_IS_PTR(t) (!__builtin_types_compatible_p( \ > - typeof(0?(__force t)0:0ULL), u64)) > - > #define __SC_DELOUSE(t,v) ({ \ > BUILD_BUG_ON(sizeof(t) > 4 && !__TYPE_IS_PTR(t)); \ > (__force t)(__TYPE_IS_PTR(t) ? ((v) & 0x7fffffff) : (v)); \ > diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h > index e2870fe1be5b..b1b9fe8502da 100644 > --- a/include/linux/syscalls.h > +++ b/include/linux/syscalls.h > @@ -119,8 +119,15 @@ struct io_uring_params; > #define __TYPE_IS_L(t) (__TYPE_AS(t, 0L)) > #define __TYPE_IS_UL(t) (__TYPE_AS(t, 0UL)) > #define __TYPE_IS_LL(t) (__TYPE_AS(t, 0LL) || __TYPE_AS(t, 0ULL)) > +#define __TYPE_IS_PTR(t) (!__builtin_types_compatible_p(typeof(0 ? (__force t)0 : 0ULL), u64)) > #define __SC_LONG(t, a) __typeof(__builtin_choose_expr(__TYPE_IS_LL(t), 0LL, 0L)) a > +#ifdef CONFIG_64BIT > +#define __SC_CAST(t, a) (__TYPE_IS_PTR(t) \ > + ? (__force t) ((__u64)a & ~(1UL << 55)) \ > + : (__force t) a) > +#else > #define __SC_CAST(t, a) (__force t) a > +#endif > #define __SC_ARGS(t, a) a > #define __SC_TEST(t, a) (void)BUILD_BUG_ON_ZERO(!__TYPE_IS_LL(t) && sizeof(t) > sizeof(long)) I'm laughing, I'm crying. Now I have to go look at the disassembly to see how this actually looks. (I mean, it _does_ solve my specific case of the waitid() flaw, but wouldn't help with pointers deeper in structs, etc, though TBI does, I think still help with that. I have to catch back up on the thread...) Anyway, if it's not too expensive it'd block reachability for those kinds of flaws. I wonder what my chances are of actually getting this landed? :) (Though, I guess I need to find a "VA width" macro instead of a raw 55.) Thanks for thinking of __SC_CAST() and __TYPE_IS_PTR() together. Really made my day. :) -- Kees Cook