[PATCH v10 5/9] riscv: mm: avoid enabling interrupts for nofault page faults

[Zhanpeng Zhang]

RISC-V enables interrupts in handle_page_fault() before checking whether
the fault occurred with fault handling disabled. A nofault access from an
atomic context can therefore run tracepoints and open an interrupt window
before reaching the exception-table fixup.

Handle an exception-table entry before entering the generic fault path when
fault handling is disabled. Also keep interrupts disabled until such a
fault has been resolved.

This makes RISC-V consistent with the expectation that an in-atomic
nofault access does not enter the normal fault-handling path. It also
removes one source of re-entry when perf sampling is delivered through an
SBI Supervisor Software Event (SSE).

Signed-off-by: Zhanpeng Zhang <zhangzhanpeng.jasper@xxxxxxxxxxxxx>
---
 arch/riscv/mm/fault.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c
index 04ed6f8acae4..520495420462 100644
--- a/arch/riscv/mm/fault.c
+++ b/arch/riscv/mm/fault.c
@@ -294,6 +294,13 @@ void handle_page_fault(struct pt_regs *regs)
 	if (kprobe_page_fault(regs, cause))
 		return;
 
+	/*
+	 * Nofault accesses must be resolved through the exception table before
+	 * entering the generic fault path or enabling interrupts.
+	 */
+	if (unlikely(faulthandler_disabled()) && fixup_exception(regs))
+		return;
+
 	if (user_mode(regs))
 		trace_page_fault_user(addr, regs, cause);
 	else
@@ -314,8 +321,8 @@ void handle_page_fault(struct pt_regs *regs)
 		return;
 	}
 
-	/* Enable interrupts if they were enabled in the parent context. */
-	if (!regs_irqs_disabled(regs))
+	/* Do not open an interrupt window before a nofault fixup completes. */
+	if (!regs_irqs_disabled(regs) && !faulthandler_disabled())
 		local_irq_enable();
 
 	/*
-- 
2.50.1 (Apple Git-155)