linux-kernel - [PATCH] objtool: Permit __kasan_check

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Message-Id: <20200928224916.2101563-1-jannh@google.com>
Date:   Tue, 29 Sep 2020 00:49:16 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, kasan-dev@...glegroups.com,
        Dan Williams <dan.j.williams@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>
Subject: [PATCH] objtool: Permit __kasan_check_{read,write} under UACCESS

Building linux-next with JUMP_LABEL=n and KASAN=y, I got this objtool
warning:

arch/x86/lib/copy_mc.o: warning: objtool: copy_mc_to_user()+0x22: call to
__kasan_check_read() with UACCESS enabled

What happens here is that copy_mc_to_user() branches on a static key in a
UACCESS region:

        __uaccess_begin();
        if (static_branch_unlikely(&copy_mc_fragile_key))
                ret = copy_mc_fragile(to, from, len);
        ret = copy_mc_generic(to, from, len);
        __uaccess_end();

and the !CONFIG_JUMP_LABEL version of static_branch_unlikely() uses
static_key_enabled(), which uses static_key_count(), which uses
atomic_read(), which calls instrument_atomic_read(), which uses
kasan_check_read(), which is __kasan_check_read().

Let's permit these KASAN helpers in UACCESS regions - static keys should
probably work under UACCESS, I think.

Signed-off-by: Jann Horn <jannh@...gle.com>
---
Calling atomic_read() on a global under UACCESS should probably be fine,
right? The alternative to this patch would be to change
copy_mc_to_user()...

Note that copy_mc_to_user() does not exist in the tip tree yet; it
appeared in commit 0a78de3d4b7b1b80e5c1eead24ce11c4b3cc8791 in the
nvdimm tree.

 tools/objtool/check.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index a88fb05242d5..1141a8e26c1e 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -583,6 +583,8 @@ static const char *uaccess_safe_builtin[] = {
 	"__asan_store4_noabort",
 	"__asan_store8_noabort",
 	"__asan_store16_noabort",
+	"__kasan_check_read",
+	"__kasan_check_write",
 	/* KASAN in-line */
 	"__asan_report_load_n_noabort",
 	"__asan_report_load1_noabort",

base-commit: 0248dedd12d43035bf53c326633f0610a49d7134
-- 
2.28.0.709.gb0816b6eb0-goog