[PATCH] futex: Set USER_DS for the futex_detect_cmpxchg() test

Andy Lutomirski
2018-09-28T04:44:13Z

futex_detect_cmpxchg() checks whether cmpxchg is available by trying
it on the NULL pointer and seeing what the error code is (EFAULT vs
ENOSYS). This happens with KERNEL_DS set, which is impolite: while
the NULL *user* pointer is definitely invalid when there is no user
program running, the NULL *kernel* pointer seems more like a
programming error than a safe place to do an intentionally-failing
access. An upcoming hardening series I'm working on causes the
existing code to OOPS, because it considers any failed uaccess with
KERNEL_DS to be a sign of a bug.

Explicitly set USER_DS to avoid this problem.

Cc: linux-s390@vger.kernel.org
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Finn Thain <fthain@telegraphics.com.au>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
---

I have a couple questions here:

- Is this actually okay on all architectures? That is, are there
cases where we'll screw up if we fail a USER_DS access this early?
s390 stands out as the obvious special case (where USER_DS is not
than just a subset of KERNEL_DS), but s390 opts out.

- Why doesn't x86 set HAVE_FUTEX_CMPXCHG? Or do we still support
some 32-bit configurations that don't have cmpxchg and don't know
about it at compile time?

kernel/futex.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/kernel/futex.c b/kernel/futex.c
index 11fc3bb456d6..16bd3e72602a 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3593,6 +3593,7 @@ static void __init futex_detect_cmpxchg(void)
{
#ifndef CONFIG_HAVE_FUTEX_CMPXCHG
u32 curval;
+ mm_segment_t old_seg;

/*
* This will fail and we want it. Some arch implementations do
@@ -3604,8 +3605,11 @@ static void __init futex_detect_cmpxchg(void)
* implementation, the non-functional ones will return
* -ENOSYS.
*/
+ old_seg = get_fs();
+ set_fs(USER_DS);
if (cmpxchg_futex_value_locked(&curval, NULL, 0, 0) == -EFAULT)
futex_cmpxchg_enabled = 1;
+ set_fs(old_seg);
#endif
}

--
2.17.1