[tip: objtool/core] objtool: Permit __kasan_check_{read,write} under UACCESS

[tip-bot2 for Jann Horn]

The following commit has been merged into the objtool/core branch of tip:

Commit-ID:     b0b8e56b82c06b3bb6e5fb66d0e9c9c3fd3ce555
Gitweb:        https://git.kernel.org/tip/b0b8e56b82c06b3bb6e5fb66d0e9c9c3fd3ce555
Author:        Jann Horn <jannh@xxxxxxxxxx>
AuthorDate:    Tue, 29 Sep 2020 00:49:16 +02:00
Committer:     Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
CommitterDate: Fri, 02 Oct 2020 09:28:08 -05:00

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.

PeterZ adds:

  It's not a matter of permitting, it's a matter of being safe and
  correct. In this case it is, because it's a thin wrapper around
  check_memory_region() which was already marked safe.

  check_memory_region() is correct because the only thing it ends up
  calling is kasa_report() and that is also marked safe because that is
  annotated with user_access_save/restore() before it does anything else.

  On top of that, all of KASAN is noinstr, so nothing in here will end up
  in tracing and/or call schedule() before the user_access_save().

Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 tools/objtool/check.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 2df9f76..3d14134 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",