Re: [PATCH v3] exec: load_script: Do not exec truncated interpreter path

[Linus Torvalds]

On Thu, Feb 14, 2019 at 10:15 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> The only way we know the interpreter wasn't truncated in the
> no-newline case is if we see whitespace after first skipping any
> leading whitespace, and it seemed really ugly to add a special scan
> there.

No, much easier (and likely better code too), to just use 'memchr()'.

What's wrong with this simple and fairly self-describing patch?

And I'll rather add a few lines due to helper functions with names to
make it more legible, rather than code in that already fairly long
existing function.

          Linus
 fs/binfmt_script.c | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/fs/binfmt_script.c b/fs/binfmt_script.c
index 7cde3f46ad26..c9a7afd477a9 100644
--- a/fs/binfmt_script.c
+++ b/fs/binfmt_script.c
@@ -14,6 +14,24 @@
 #include <linux/err.h>
 #include <linux/fs.h>
 
+/*
+ * Do we have whitespace (ignoring leading) between first and last
+ * (inclusive). This is the "we are truncating the script command
+ * line" case.
+ */
+static inline bool tabspc(char c) { return c == ' ' || c == '\t'; }
+static inline bool no_tab_or_space(const char *first, const char *last)
+{
+	/* Skip leading space (and first non-space) */
+	while (tabspc(*first++))
+		if (first > last)
+			return false;
+	while (first <= last)
+		if (tabspc(*first++))
+			return true;
+	return false;
+}
+
 static int load_script(struct linux_binprm *bprm)
 {
 	const char *i_arg, *i_name;
@@ -42,9 +60,12 @@ static int load_script(struct linux_binprm *bprm)
 	fput(bprm->file);
 	bprm->file = NULL;
 
-	bprm->buf[BINPRM_BUF_SIZE - 1] = '\0';
-	if ((cp = strchr(bprm->buf, '\n')) == NULL)
+	cp = memchr(bprm->buf, BINPRM_BUF_SIZE, '\n');
+	if (!cp) {
 		cp = bprm->buf+BINPRM_BUF_SIZE-1;
+		if (no_tab_or_space(bprm->buf+2, cp))
+			return -ENOEXEC;
+	}
 	*cp = '\0';
 	while (cp > bprm->buf) {
 		cp--;