Re: [PATCH v15 00/17] arm64: untag user pointers passed to the kernel

Catalin Marinas
2019-05-28T17:02:58Z

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