linux-kernel - [PATCH v2 1/6] MIPS: math-emu: CMP.Sxxx.<D|S>: Prevent occurrences of SIGILL crashes

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Date:   Mon, 21 Aug 2017 14:24:47 +0200
From:   Aleksandar Markovic <aleksandar.markovic@...rk.com>
To:     linux-mips@...ux-mips.org
Cc:     Aleksandar Markovic <aleksandar.markovic@...tec.com>,
        Miodrag Dinic <miodrag.dinic@...tec.com>,
        Goran Ferenc <goran.ferenc@...tec.com>,
        Douglas Leung <douglas.leung@...tec.com>,
        James Hogan <james.hogan@...tec.com>,
        linux-kernel@...r.kernel.org,
        "Maciej W. Rozycki" <macro@...tec.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Paul Burton <paul.burton@...tec.com>,
        Petar Jovanovic <petar.jovanovic@...tec.com>,
        Raghu Gandham <raghu.gandham@...tec.com>,
        Ralf Baechle <ralf@...ux-mips.org>
Subject: [PATCH v2 1/6] MIPS: math-emu: CMP.Sxxx.<D|S>: Prevent occurrences of SIGILL crashes

From: Aleksandar Markovic <aleksandar.markovic@...tec.com>

Fix CMP.Sxxx.<D|S> SIGILL crashes by fixing main switch/case statement
in fpu_emul() function so that inadvertent fall-troughs are prevented.

Consider, let's say, CMP.SAF.S instruction when one of inputs is zero
and another input is a signaling NaN. The desired output is zero, and
the exception flag "invalid operation" set. For such case, the main
portion of the implementation is within "d_fmt" case of the main
"switch/case" statement in fpu_emul() function. The execution will
follow one of "if-else" branches that doesn't contain "goto cop1scr;"
statement, and will therefore reach the end of "d_fmt" case. It will
subsequently fall through to the next case, "l_fmt". After following
similar pattern, the execution will fall through to the succeeding
case, which is "default". The "default" case contains "return SIGILL;"
statement only. This means that the caller application will crash
with "illegal instruction" message.

It is obvious that above described fall-throughs are unnecessary and
harmful. This patch rectifies that behavior by providing "break;"
statements at the end of cases "d_fmt" and "l_fmt".

There are 22 instructions affected by this problem:

CMP.<SAF|SEQ|SLE|SLT|SNE|SOR|SUEQ|SULE|SULT|SUN|SUNE>.<D|S>.

Signed-off-by: Miodrag Dinic <miodrag.dinic@...tec.com>
Signed-off-by: Goran Ferenc <goran.ferenc@...tec.com>
Signed-off-by: Aleksandar Markovic <aleksandar.markovic@...tec.com>
---
 arch/mips/math-emu/cp1emu.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/mips/math-emu/cp1emu.c b/arch/mips/math-emu/cp1emu.c
index f08a7b4..520a5ac 100644
--- a/arch/mips/math-emu/cp1emu.c
+++ b/arch/mips/math-emu/cp1emu.c
@@ -2394,6 +2394,7 @@ static int fpu_emu(struct pt_regs *xcp, struct mips_fpu_struct *ctx,
 			break;
 			}
 		}
+		break;
 	}

 	case l_fmt:
@@ -2468,6 +2469,8 @@ static int fpu_emu(struct pt_regs *xcp, struct mips_fpu_struct *ctx,
 			break;
 			}
 		}
+		break;
+
 	default:
 		return SIGILL;
 	}
-- 
2.9.3