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From:   Christophe Leroy <christophe.leroy@csgroup.eu>
To:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>
Cc:     Christophe Leroy <christophe.leroy@csgroup.eu>,
        linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        Stan Johnson <userm57@yahoo.com>,
        Finn Thain <fthain@linux-m68k.org>
Subject: [PATCH] powerpc/32s: Fix kuap_kernel_restore()
Date:   Wed, 15 Sep 2021 16:12:24 +0200
Message-Id: <0d0c4d0f050a637052287c09ba521bad960a2790.1631715131.git.christophe.leroy@csgroup.eu>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

At interrupt exit, kuap_kernel_restore() calls kuap_unclok() with the
value contained in regs->kuap. However, when regs->kuap contains
0xffffffff it means that KUAP was not unlocked so calling
kuap_unlock() is unrelevant and results in jeopardising the contents
of kernel space segment registers.

So check that regs->kuap doesn't contain KUAP_NONE before calling
kuap_unlock(). In the meantime it also means that if KUAP has not
been correcly locked back at interrupt exit, it must be locked
before continuing. This is done by checking the content of
current->thread.kuap which was returned by kuap_get_and_assert_locked()

Fixes: 16132529cee5 ("powerpc/32s: Rework Kernel Userspace Access Protection")
Reported-by: Stan Johnson <userm57@yahoo.com>
Cc: Finn Thain <fthain@linux-m68k.org>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
---
 arch/powerpc/include/asm/book3s/32/kup.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/powerpc/include/asm/book3s/32/kup.h b/arch/powerpc/include/asm/book3s/32/kup.h
index d4b145b279f6..9f38040f0641 100644
--- a/arch/powerpc/include/asm/book3s/32/kup.h
+++ b/arch/powerpc/include/asm/book3s/32/kup.h
@@ -136,6 +136,14 @@ static inline void kuap_kernel_restore(struct pt_regs *regs, unsigned long kuap)
 	if (kuap_is_disabled())
 		return;
 
+	if (unlikely(kuap != KUAP_NONE)) {
+		current->thread.kuap = KUAP_NONE;
+		kuap_lock(kuap, false);
+	}
+
+	if (likely(regs->kuap == KUAP_NONE))
+		return;
+
 	current->thread.kuap = regs->kuap;
 
 	kuap_unlock(regs->kuap, false);
-- 
2.31.1