linux-kernel - [PATCH v4 3/7] kprobes: validate the symbol name provided during probe registration

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Message-Id: <20170421123234.6895-1-naveen.n.rao@linux.vnet.ibm.com>
Date:   Fri, 21 Apr 2017 18:02:33 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH v4 3/7] kprobes: validate the symbol name provided during probe registration

When a kprobe is being registered, we use the symbol_name field to
lookup the address where the probe should be placed. Since this is a
user-provided field, let's ensure that the length of the string is
within expected limits.

Signed-off-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
---
Masami, Michael,
Here's a simplified version that should hopefully be fine. I have
simplified the logic to keep the code compact and simple.

- Naveen


 include/linux/kprobes.h     |  1 +
 kernel/kprobes.c            | 30 ++++++++++++++++++++++++++++++
 kernel/trace/trace_kprobe.c |  4 ++++
 3 files changed, 35 insertions(+)

diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
index 1f82a3db00b1..4ee10fef5135 100644
--- a/include/linux/kprobes.h
+++ b/include/linux/kprobes.h
@@ -405,6 +405,7 @@ int disable_kprobe(struct kprobe *kp);
 int enable_kprobe(struct kprobe *kp);
 
 void dump_kprobe(struct kprobe *kp);
+bool is_valid_kprobe_symbol_name(const char *name);
 
 #else /* !CONFIG_KPROBES: */
 
diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index 6a128f3a7ed1..ff9b1ac72a38 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1383,6 +1383,34 @@ bool within_kprobe_blacklist(unsigned long addr)
 }
 
 /*
+ * We mainly want to ensure that the provided string is of a reasonable length
+ * and is of the form [<mod_name>:]<sym_name>, so that this is safe to process
+ * further.
+ * We don't worry about invalid characters as those will just prevent
+ * matching existing kallsyms.
+ */
+bool is_valid_kprobe_symbol_name(const char *name)
+{
+	size_t sym_len;
+	const char *s;
+
+	s = strnchr(name, ':', MODULE_NAME_LEN + KSYM_NAME_LEN + 1);
+	if (s) {
+		sym_len = (size_t)(s - name);
+		if (sym_len <= 0  || sym_len >= MODULE_NAME_LEN)
+			return false;
+		s++;
+	} else
+		s = name;
+
+	sym_len = strnlen(s, KSYM_NAME_LEN);
+	if (sym_len <= 0 || sym_len >= KSYM_NAME_LEN)
+		return false;
+
+	return true;
+}
+
+/*
  * If we have a symbol_name argument, look it up and add the offset field
  * to it. This way, we can specify a relative address to a symbol.
  * This returns encoded errors if it fails to look up symbol or invalid
@@ -1397,6 +1425,8 @@ static kprobe_opcode_t *kprobe_addr(struct kprobe *p)
 		goto invalid;
 
 	if (p->symbol_name) {
+		if (!is_valid_kprobe_symbol_name(p->symbol_name))
+			return ERR_PTR(-EINVAL);
 		addr = kprobe_lookup_name(p->symbol_name, p->offset);
 		if (!addr)
 			return ERR_PTR(-ENOENT);
diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
index 5f688cc724f0..bf73e5f31128 100644
--- a/kernel/trace/trace_kprobe.c
+++ b/kernel/trace/trace_kprobe.c
@@ -704,6 +704,10 @@ static int create_trace_kprobe(int argc, char **argv)
 			pr_info("Return probe must be used without offset.\n");
 			return -EINVAL;
 		}
+		if (!is_valid_kprobe_symbol_name(symbol)) {
+			pr_info("Symbol name is too long.\n");
+			return -EINVAL;
+		}
 	}
 	argc -= 2; argv += 2;
 
-- 
2.12.1