rseq: Fix using an uninitialized stack variable in rseq_exit_user_update()

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

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

The local variable:

	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.

Fixes: 82f572449cfe ("rseq: Implement read only ABI enforcement for optimized RSEQ V2 mode")
Closes: https://syzkaller.appspot.com/bug?extid=185a631927096f9da2fc
Reported-by: syzbot+185a631927096f9da2fc@syzkaller.appspotmail.com
Signed-off-by: Qing Wang <wangqing7171@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://patch.msgid.link/20260602030854.574038-1-wangqing7171@gmail.com

diff --git a/include/linux/rseq_entry.h b/include/linux/rseq_entry.h
index 63bc72086e75bb9ac28a4233b4778ea6eb8610cb..ed9da6e41a2aad5b2253414fec4ff4d485f808bb 100644 (file)
--- 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);