linux-kernel - [PATCH] xtensa: fix uaccess-related livelock in do_page

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Message-Id: <20210328031848.8755-1-jcmvbkbc@gmail.com>
Date:   Sat, 27 Mar 2021 20:18:48 -0700
From:   Max Filippov <jcmvbkbc@...il.com>
To:     linux-xtensa@...ux-xtensa.org
Cc:     Chris Zankel <chris@...kel.net>, linux-kernel@...r.kernel.org,
        Max Filippov <jcmvbkbc@...il.com>, stable@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>
Subject: [PATCH] xtensa: fix uaccess-related livelock in do_page_fault

If a uaccess (e.g. get_user()) triggers a fault and there's a
fault signal pending, the handler will return to the uaccess without
having performed a uaccess fault fixup, and so the CPU will immediately
execute the uaccess instruction again, whereupon it will livelock
bouncing between that instruction and the fault handler.

https://lore.kernel.org/lkml/20210121123140.GD48431@C02TD0UTHF1T.local/

Cc: stable@...r.kernel.org
Reported-by: Mark Rutland <mark.rutland@....com>
Signed-off-by: Max Filippov <jcmvbkbc@...il.com>
---
 arch/xtensa/mm/fault.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/xtensa/mm/fault.c b/arch/xtensa/mm/fault.c
index 7666408ce12a..95a74890c7e9 100644
--- a/arch/xtensa/mm/fault.c
+++ b/arch/xtensa/mm/fault.c
@@ -112,8 +112,11 @@ void do_page_fault(struct pt_regs *regs)
 	 */
 	fault = handle_mm_fault(vma, address, flags, regs);
 
-	if (fault_signal_pending(fault, regs))
+	if (fault_signal_pending(fault, regs)) {
+		if (!user_mode(regs))
+			goto bad_page_fault;
 		return;
+	}
 
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 		if (fault & VM_FAULT_OOM)
-- 
2.20.1