linux-kernel - [PATCH v1 10/10] lkdtm: Fix execute_[user]

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Message-Id: <c551f3f4b803d1a4a184b0fa94475d405ba436d8.1633964380.git.christophe.leroy@csgroup.eu>
Date:   Mon, 11 Oct 2021 17:25:37 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
        Helge Deller <deller@....de>, Arnd Bergmann <arnd@...db.de>,
        Kees Cook <keescook@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-ia64@...r.kernel.org, linux-parisc@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH v1 10/10] lkdtm: Fix execute_[user]_location()

execute_location() and execute_user_location() intent
to copy do_nothing() text and execute it at a new location.
However, at the time being it doesn't copy do_nothing() function
but do_nothing() function descriptor which still points to the
original text. So at the end it still executes do_nothing() at
its original location allthough using a copied function descriptor.

So, fix that by really copying do_nothing() text and build a new
function descriptor by copying do_nothing() function descriptor and
updating the target address with the new location.

Also fix the displayed addresses by dereferencing do_nothing()
function descriptor.

Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
---
 drivers/misc/lkdtm/perms.c | 45 +++++++++++++++++++++++++++++++-------
 1 file changed, 37 insertions(+), 8 deletions(-)

diff --git a/drivers/misc/lkdtm/perms.c b/drivers/misc/lkdtm/perms.c
index da16564e1ecd..1d03cd44c21d 100644
--- a/drivers/misc/lkdtm/perms.c
+++ b/drivers/misc/lkdtm/perms.c
@@ -44,19 +44,42 @@ static noinline void do_overwritten(void)
 	return;
 }
 
+static void *setup_function_descriptor(funct_descr_t *fdesc, void *dst)
+{
+	int err;
+
+	if (!__is_defined(HAVE_DEREFERENCE_FUNCTION_DESCRIPTOR))
+		return dst;
+
+	err = copy_from_kernel_nofault(fdesc, do_nothing, sizeof(*fdesc));
+	if (err < 0)
+		return ERR_PTR(err);
+
+	fdesc->addr = (unsigned long)dst;
+	barrier();
+
+	return fdesc;
+}
+
 static noinline void execute_location(void *dst, bool write)
 {
-	void (*func)(void) = dst;
+	void (*func)(void);
+	funct_descr_t fdesc;
+	void *do_nothing_text = dereference_symbol_descriptor(do_nothing);
 
-	pr_info("attempting ok execution at %px\n", do_nothing);
+	pr_info("attempting ok execution at %px\n", do_nothing_text);
 	do_nothing();
 
 	if (write == CODE_WRITE) {
-		memcpy(dst, do_nothing, EXEC_SIZE);
+		memcpy(dst, do_nothing_text, EXEC_SIZE);
 		flush_icache_range((unsigned long)dst,
 				   (unsigned long)dst + EXEC_SIZE);
 	}
-	pr_info("attempting bad execution at %px\n", func);
+	func = setup_function_descriptor(&fdesc, dst);
+	if (IS_ERR(func))
+		return;
+
+	pr_info("attempting bad execution at %px\n", dst);
 	func();
 	pr_err("FAIL: func returned\n");
 }
@@ -66,16 +89,22 @@ static void execute_user_location(void *dst)
 	int copied;
 
 	/* Intentionally crossing kernel/user memory boundary. */
-	void (*func)(void) = dst;
+	void (*func)(void);
+	funct_descr_t fdesc;
+	void *do_nothing_text = dereference_symbol_descriptor(do_nothing);
 
-	pr_info("attempting ok execution at %px\n", do_nothing);
+	pr_info("attempting ok execution at %px\n", do_nothing_text);
 	do_nothing();
 
-	copied = access_process_vm(current, (unsigned long)dst, do_nothing,
+	copied = access_process_vm(current, (unsigned long)dst, do_nothing_text,
 				   EXEC_SIZE, FOLL_WRITE);
 	if (copied < EXEC_SIZE)
 		return;
-	pr_info("attempting bad execution at %px\n", func);
+	func = setup_function_descriptor(&fdesc, dst);
+	if (IS_ERR(func))
+		return;
+
+	pr_info("attempting bad execution at %px\n", dst);
 	func();
 	pr_err("FAIL: func returned\n");
 }
-- 
2.31.1