Re: [PATCH] futex: use fault_in to avoid infinite loop

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Dec 06, 2017 at 05:04:00PM +0100, Peter Zijlstra wrote:
> On Wed, Dec 06, 2017 at 10:21:07PM +0800, Cheng Jian wrote:
> > It will cause softlockup(infinite loop) in kernel
> > space when we use SYS_set_robust_list in futex which
> > incoming a misaligned address from user space.
> 
> Urgh, we should not allow that in the first place.
> 
> See how get_futex_key() does:
> 
>   if (unlikely(address % sizeof(u32)))
> 	return -EINVAL;
> 
> That same should also be true for the robust list. Using unaligned
> variables is insane.

Something a little like so perhaps..

---
Subject: futex: Sanitize user address in set_robust_list()

Passing in unaligned variables messes up cmpxchg on a whole bunch of
architectures. Also, not respecting the natural alignment of data
structures is pretty dumb to begin with.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 include/uapi/asm-generic/errno.h | 1 +
 kernel/futex.c                   | 5 +++++
 2 files changed, 6 insertions(+)

diff --git a/include/uapi/asm-generic/errno.h b/include/uapi/asm-generic/errno.h
index cf9c51ac49f9..4cb80d4ac160 100644
--- a/include/uapi/asm-generic/errno.h
+++ b/include/uapi/asm-generic/errno.h
@@ -119,5 +119,6 @@
 #define ERFKILL		132	/* Operation not possible due to RF-kill */
 
 #define EHWPOISON	133	/* Memory page has hardware error */
+#define EMORON		134	/* User did something particularly silly */
 
 #endif
diff --git a/kernel/futex.c b/kernel/futex.c
index 76ed5921117a..e2c1a818f88f 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3262,6 +3262,8 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
 SYSCALL_DEFINE2(set_robust_list, struct robust_list_head __user *, head,
 		size_t, len)
 {
+	unsigned long address = (unsigned long)head;
+
 	if (!futex_cmpxchg_enabled)
 		return -ENOSYS;
 	/*
@@ -3270,6 +3272,9 @@ SYSCALL_DEFINE2(set_robust_list, struct robust_list_head __user *, head,
 	if (unlikely(len != sizeof(*head)))
 		return -EINVAL;
 
+	if (unlikely(address % __alignof__(*head)))
+		return -EMORON;
+
 	current->robust_list = head;
 
 	return 0;