On Wed, 19 Apr 2017 16:38:22 +0000
"Naveen N. Rao" [off-list ref] wrote:

Excerpts from Masami Hiramatsu's message of April 19, 2017 20:07:

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On Wed, 19 Apr 2017 18:21:02 +0530
"Naveen N. Rao" [off-list ref] wrote:

quoted

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.

Would we really need this? Of course it may filter out longer
strings... anyway such name should be rejected by kallsyms.

I felt this would be good to have generically, as kallsyms does many 
string operations on the symbol name, including an unbounded strchr().

OK, so this is actually for performance reason.

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[...]

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diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index 6a128f3a7ed1..bb86681c8a10 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c

@@ -1382,6 +1382,28 @@ bool within_kprobe_blacklist(unsigned long addr)
 	return false;
 }
 
+bool is_valid_kprobe_symbol_name(const char *name)

This just check the length of symbol_name buffer, and can contain
some invalid chars.

Yes, I kept the function name generic incase we would like to do more 
validation in future, plus it's shorter than 
is_valid_kprobe_symbol_name_len() ;-)

OK, if this is enough general, we'd better define this in
kernel/kallsyms.c or in kallsyms.h. Of course the function
should be called is_valid_symbol_name(). :-)

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+{
+	size_t sym_len;
+	char *s;
+
+	s = strchr(name, ':');

Hmm.. this should be strnchr(). I re-factored the code that moved the 
strnlen() above this below. I'll fix this.

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+	if (s) {
+		sym_len = strnlen(s+1, KSYM_NAME_LEN);

If you use strnlen() here, you just need to ensure sym_len < KSYM_NAME_LEN.

Hmm.. not sure I follow. Are you saying the check for sym_len <= 0 is 
not needed?

You can check sym_len != 0, but anyway, here we concern about
"longer" string (for performance reason), we can focus on
such case.
(BTW, could you also check the name != NULL at first?)

So, what I think it can be;

if (strnlen(s+1, KSYM_NAME_LEN) == KSYM_NAME_LEN ||
    (size_t)(s - name) >= MODULE_NAME_LEN)
	return false;

Thanks,

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+		if (sym_len <= 0 || sym_len >= KSYM_NAME_LEN)
+			return false;
+		sym_len = (size_t)(s - name);
+		if (sym_len <= 0 || sym_len >= MODULE_NAME_LEN)
+			return false;
+	} else {
+		sym_len = strnlen(name, MODULE_NAME_LEN);
+		if (sym_len <= 0 || sym_len >= MODULE_NAME_LEN)

Would you mean KSYM_NAME_LEN here?

Oops... nice catch, Thanks!

- Naveen

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+			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.

@@ -1397,6 +1419,8 @@ 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

Thanks,

-- 
Masami Hiramatsu [off-list ref]

-- 
Masami Hiramatsu [off-list ref]