[PATCH v2] rseq: fix using an uninitialized stack variable in rseq_exit_user_update

[Qing Wang <wangqing7171@gmail.com>]

There is an bug which is an uninitialized stack variable use in
`rseq_exit_user_update()` reported by syzbot:

BUG: KMSAN: kernel-infoleak in rseq_set_ids_get_csaddr include/linux/rseq_entry.h:502 [inline]

The local variable:
```c
	struct rseq_ids ids = {
		.cpu_id	 = task_cpu(t),
		.mm_cid	 = task_mm_cid(t),
		.node_id = cpu_to_node(ids.cpu_id),
	};
```
According to the C standard, the evaluation order of expressions in an
initializer list is indeterminately sequenced. The compiler (Clang, in this
KMSAN build) evaluates `cpu_to_node(ids.cpu_id)` *before* `ids.cpu_id` is
initialized with `task_cpu(t)`.

This is fixed by moving the assignment of ids.node_id outside the structure
initialization.

Reported-by: syzbot+185a631927096f9da2fc@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=185a631927096f9da2fc
Fixes: 82f572449cfe ("rseq: Implement read only ABI enforcement for optimized RSEQ V2 mode")
Signed-off-by: Qing Wang <wangqing7171@gmail.com>
---
 include/linux/rseq_entry.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/linux/rseq_entry.h b/include/linux/rseq_entry.h
index 63bc72086e75..ed9da6e41a2a 100644
--- a/include/linux/rseq_entry.h
+++ b/include/linux/rseq_entry.h
@@ -635,10 +635,11 @@ static __always_inline bool rseq_exit_user_update(struct pt_regs *regs, struct t
 		return true;
 	}
 
+	int cpu = task_cpu(t);
 	struct rseq_ids ids = {
-		.cpu_id	 = task_cpu(t),
+		.cpu_id	 = cpu,
 		.mm_cid	 = task_mm_cid(t),
-		.node_id = cpu_to_node(ids.cpu_id),
+		.node_id = cpu_to_node(cpu),
 	};
 
 	return rseq_update_usr(t, regs, &ids);
-- 
2.34.1