From mboxrd@z Thu Jan  1 00:00:00 1970
From: Catalin Marinas <catalin.marinas@arm.com>
Subject: Re: [PATCH v15 00/17] arm64: untag user pointers passed to the kernel
Date: Tue, 28 May 2019 18:02:45 +0100
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To: Kees Cook <keescook@chromium.org>
Cc: enh <enh@google.com>, Evgenii Stepanov <eugenis@google.com>, Andrey Konovalov <andreyknvl@google.com>, Khalid Aziz <khalid.aziz@oracle.com>, Linux ARM <linux-arm-kernel@lists.infradead.org>, Linux Memory Management List <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>, 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" <linux-kselftest@vger.kernel.org>, Vincenzo Frascino <vincenzo.frascino@arm.com>, Will Deacon <will.deacon@arm.com>, Mark Rutland <mark.rutland@arm.com>, Andrew Morton <akpm@linux-foundation.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Yishai Hadas <yishaih@mellanox.com>Felix Kuehling <F>
List-Id: linux-rdma@vger.kernel.org

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 ;)):

--------------------------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 <asm-generic/compat.h>
 
-#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))
 

-- 
Catalin