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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>, stable@kernel.org
Subject: [PATCH 5.3 56/95] x86/cpu_entry_area: Add guard page for entry stack on 32bit
Date:   Wed, 27 Nov 2019 21:32:13 +0100
Message-Id: <20191127202923.382153935@linuxfoundation.org>
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From: Thomas Gleixner <tglx@linutronix.de>

commit 880a98c339961eaa074393e3a2117cbe9125b8bb upstream.

The entry stack in the cpu entry area is protected against overflow by the
readonly GDT on 64-bit, but on 32-bit the GDT needs to be writeable and
therefore does not trigger a fault on stack overflow.

Add a guard page.

Fixes: c482feefe1ae ("x86/entry/64: Make cpu_entry_area.tss read-only")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 arch/x86/include/asm/cpu_entry_area.h |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

--- a/arch/x86/include/asm/cpu_entry_area.h
+++ b/arch/x86/include/asm/cpu_entry_area.h
@@ -78,8 +78,12 @@ struct cpu_entry_area {
 
 	/*
 	 * The GDT is just below entry_stack and thus serves (on x86_64) as
-	 * a a read-only guard page.
+	 * a read-only guard page. On 32-bit the GDT must be writeable, so
+	 * it needs an extra guard page.
 	 */
+#ifdef CONFIG_X86_32
+	char guard_entry_stack[PAGE_SIZE];
+#endif
 	struct entry_stack_page entry_stack_page;
 
 	/*