From: Kees Cook <keescook@chromium.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Kees Cook <keescook@chromium.org>,
Nick Desaulniers <ndesaulniers@google.com>,
Nathan Chancellor <nathan@kernel.org>,
Elena Reshetova <elena.reshetova@intel.com>,
David Laight <David.Laight@ACULAB.COM>,
Will Deacon <will@kernel.org>,
clang-built-linux@googlegroups.com, linux-kernel@vger.kernel.org
Subject: [PATCH] stack: replace "o" output with "r" input constraint
Date: Mon, 19 Apr 2021 16:17:41 -0700 [thread overview]
Message-ID: <20210419231741.4084415-1-keescook@chromium.org> (raw)
From: Nick Desaulniers <ndesaulniers@google.com>
"o" isn't a common asm() constraint to use; it triggers an assertion in
assert-enabled builds of LLVM that it's not recognized when targeting
aarch64 (though it appears to fall back to "m"). I've fixed this in LLVM
13 now, but there isn't really a good reason to be using "o" in particular
here. To avoid causing build issues for those using assert-enabled builds
of earlier LLVM versions, the constraint needs changing.
Instead, if the point is to retain the __builtin_alloca(), we can make ptr
appear to "escape" via being an input to an empty inline asm block. This
is preferable anyways, since otherwise this looks like a dead store.
While the use of "r" was considered in
https://lore.kernel.org/lkml/202104011447.2E7F543@keescook/
it was only tested as an output (which looks like a dead store, and
wasn't sufficient). Use "r" as an input constraint instead, which
behaves correctly across compilers and architectures:
https://godbolt.org/z/E9cd411ob
Link: https://reviews.llvm.org/D100412
Link: https://bugs.llvm.org/show_bug.cgi?id=49956
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Kees Cook <keescook@chromium.org>
Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall")
Signed-off-by: Kees Cook <keescook@chromium.org>
---
include/linux/randomize_kstack.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
index fd80fab663a9..bebc911161b6 100644
--- a/include/linux/randomize_kstack.h
+++ b/include/linux/randomize_kstack.h
@@ -38,7 +38,7 @@ void *__builtin_alloca(size_t size);
u32 offset = raw_cpu_read(kstack_offset); \
u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset)); \
/* Keep allocation even after "ptr" loses scope. */ \
- asm volatile("" : "=o"(*ptr) :: "memory"); \
+ asm volatile("" :: "r"(ptr) : "memory"); \
} \
} while (0)
--
2.25.1
next reply other threads:[~2021-04-19 23:17 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-19 23:17 Kees Cook [this message]
2021-04-23 1:02 ` [PATCH] stack: replace "o" output with "r" input constraint Nathan Chancellor
2021-05-05 21:12 ` Nathan Chancellor
2021-05-05 21:19 ` Kees Cook
2021-05-11 8:02 ` [tip: core/urgent] stack: Replace " tip-bot2 for Nick Desaulniers
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