Re: [RFC PATCH for 4.18 2/2] rseq: check that rseq->rseq_cs padding is zero

On 32-bit kernels, the rseq->rseq_cs_padding field is never read by the
kernel. However, 64-bit kernels dealing with 32-bit compat tasks read the
full 64-bit in its entirety, and terminates the offending process with
a segmentation fault if the upper 32 bits are set due to failure of
copy_from_user().

Ensure that both 32-bit and 64-bit kernels dealing with 32-bit tasks end
up terminating offending tasks with a segmentation fault if the upper
32-bit padding bits (rseq->rseq_cs_padding) are set by adding an explicit
check that padding is zero on 32-bit kernels.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC: "Paul E. McKenney" <redacted>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Paul Turner <redacted>
CC: Thomas Gleixner <redacted>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Andi Kleen <redacted>
CC: Dave Watson <redacted>
CC: Chris Lameter <redacted>
CC: Ingo Molnar <mingo@redhat.com>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: Ben Maurer <redacted>
CC: Steven Rostedt <rostedt@goodmis.org>
CC: Josh Triplett <josh@joshtriplett.org>
CC: Linus Torvalds <torvalds@linux-foundation.org>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: Russell King <redacted>
CC: Catalin Marinas <catalin.marinas@arm.com>
CC: Will Deacon <redacted>
CC: Michael Kerrisk <redacted>
CC: Boqun Feng <redacted>
CC: linux-api@vger.kernel.org
---
 kernel/rseq.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/kernel/rseq.c b/kernel/rseq.c
index 4ba582046fcd..b038f35a60d6 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c

@@ -112,6 +112,29 @@ static int rseq_reset_rseq_cpu_id(struct task_struct *t)
 	return 0;
 }
 
+#ifndef __LP64__
+/*
+ * Ensure that padding is zero.
+ */
+static int check_rseq_cs_padding(struct task_struct *t)
+{
+	unsigned long pad;
+	int ret;
+
+	ret = __get_user(pad, &t->rseq->rseq_cs_padding);
+	if (ret)
+		return ret;
+	if (pad)
+		return -EFAULT;
+	return 0;
+}
+#else
+static int check_rseq_cs_padding(struct task_struct *t)
+{
+	return 0;
+}
+#endif