linux-kernel - [PATCH AUTOSEL 5.10 12/14] parisc: Clear stale IIR value on instruction access rights trap

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Message-Id: <20211227190452.1042714-12-sashal@kernel.org>
Date:   Mon, 27 Dec 2021 14:04:50 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Helge Deller <deller@....de>,
        John David Anglin <dave.anglin@...l.net>,
        Sasha Levin <sashal@...nel.org>,
        James.Bottomley@...senPartnership.com, svens@...ckframe.org,
        wangkefeng.wang@...wei.com, rmk+kernel@...linux.org.uk,
        akpm@...ux-foundation.org, linux-parisc@...r.kernel.org
Subject: [PATCH AUTOSEL 5.10 12/14] parisc: Clear stale IIR value on instruction access rights trap

From: Helge Deller <deller@....de>

[ Upstream commit 484730e5862f6b872dca13840bed40fd7c60fa26 ]

When a trap 7 (Instruction access rights) occurs, this means the CPU
couldn't execute an instruction due to missing execute permissions on
the memory region.  In this case it seems the CPU didn't even fetched
the instruction from memory and thus did not store it in the cr19 (IIR)
register before calling the trap handler. So, the trap handler will find
some random old stale value in cr19.

This patch simply overwrites the stale IIR value with a constant magic
"bad food" value (0xbaadf00d), in the hope people don't start to try to
understand the various random IIR values in trap 7 dumps.

Noticed-by: John David Anglin <dave.anglin@...l.net>
Signed-off-by: Helge Deller <deller@....de>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/parisc/kernel/traps.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/parisc/kernel/traps.c b/arch/parisc/kernel/traps.c
index a52c7abf2ca49..43f56335759a4 100644
--- a/arch/parisc/kernel/traps.c
+++ b/arch/parisc/kernel/traps.c
@@ -729,6 +729,8 @@ void notrace handle_interruption(int code, struct pt_regs *regs)
 			}
 			mmap_read_unlock(current->mm);
 		}
+		/* CPU could not fetch instruction, so clear stale IIR value. */
+		regs->iir = 0xbaadf00d;
 		fallthrough;
 	case 27: 
 		/* Data memory protection ID trap */
-- 
2.34.1