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From:   Rick Edgecombe <rick.p.edgecombe@intel.com>
To:     Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@redhat.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org, hpa@zytor.com,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        Nadav Amit <nadav.amit@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>, linux_dti@icloud.com,
        linux-integrity@vger.kernel.org,
        linux-security-module@vger.kernel.org, akpm@linux-foundation.org,
        kernel-hardening@lists.openwall.com, linux-mm@kvack.org,
        will.deacon@arm.com, ard.biesheuvel@linaro.org,
        kristen@linux.intel.com, deneen.t.dock@intel.com,
        Nadav Amit <namit@vmware.com>,
        Rick Edgecombe <rick.p.edgecombe@intel.com>
Subject: [PATCH v2 09/20] x86/kprobes: instruction pages initialization enhancements
Date:   Mon, 28 Jan 2019 16:34:11 -0800
Message-Id: <20190129003422.9328-10-rick.p.edgecombe@intel.com>
In-Reply-To: <20190129003422.9328-1-rick.p.edgecombe@intel.com>
References: <20190129003422.9328-1-rick.p.edgecombe@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
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From: Nadav Amit <namit@vmware.com>

Make kprobes instruction pages read-only (and executable) after they are
set to prevent them from mistaken or malicious modifications.

This is a preparatory patch for a following patch that makes module
allocated pages non-executable and sets the page as executable after
allocation.

While at it, do some small cleanup of what appears to be unnecessary
masking.

Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Nadav Amit <namit@vmware.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
---
 arch/x86/kernel/kprobes/core.c | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index 4ba75afba527..fac692e36833 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -431,8 +431,20 @@ void *alloc_insn_page(void)
 	void *page;
 
 	page = module_alloc(PAGE_SIZE);
-	if (page)
-		set_memory_ro((unsigned long)page & PAGE_MASK, 1);
+	if (page == NULL)
+		return NULL;
+
+	/*
+	 * First make the page read-only, and then only then make it executable
+	 * to prevent it from being W+X in between.
+	 */
+	set_memory_ro((unsigned long)page, 1);
+
+	/*
+	 * TODO: Once additional kernel code protection mechanisms are set, ensure
+	 * that the page was not maliciously altered and it is still zeroed.
+	 */
+	set_memory_x((unsigned long)page, 1);
 
 	return page;
 }
@@ -440,8 +452,12 @@ void *alloc_insn_page(void)
 /* Recover page to RW mode before releasing it */
 void free_insn_page(void *page)
 {
-	set_memory_nx((unsigned long)page & PAGE_MASK, 1);
-	set_memory_rw((unsigned long)page & PAGE_MASK, 1);
+	/*
+	 * First make the page non-executable, and then only then make it
+	 * writable to prevent it from being W+X in between.
+	 */
+	set_memory_nx((unsigned long)page, 1);
+	set_memory_rw((unsigned long)page, 1);
 	module_memfree(page);
 }
 
-- 
2.17.1