exec: load_script: Do not exec truncated interpreter path

#include <linux/err.h>

#include <linux/fs.h>

static inline bool spacetab(char c) { return c == ' ' || c == '\t'; }

static inline char *next_non_spacetab(char *first, const char *last)

{

	for (; first <= last; first++)

		if (!spacetab(*first))

			return first;

	return NULL;

}

static inline char *next_terminator(char *first, const char *last)

{

	for (; first <= last; first++)

		if (spacetab(*first) || !*first)

			return first;

	return NULL;

}

static int load_script(struct linux_binprm *bprm)

{

	const char *i_arg, *i_name;

	char *cp;

	char *cp, *buf_end;

	struct file *file;

	int retval;

	/* Not ours to exec if we don't start with "#!". */

	if ((bprm->buf[0] != '#') || (bprm->buf[1] != '!'))

		return -ENOEXEC;

	if (bprm->interp_flags & BINPRM_FLAGS_PATH_INACCESSIBLE)

		return -ENOENT;

	/*

	 * This section does the #! interpretation.

	 * Sorta complicated, but hopefully it will work.  -TYT

	 */

	/* Release since we are not mapping a binary into memory. */

	allow_write_access(bprm->file);

	fput(bprm->file);

	bprm->file = NULL;

	bprm->buf[BINPRM_BUF_SIZE - 1] = '\0';

	if ((cp = strchr(bprm->buf, '\n')) == NULL)

		cp = bprm->buf+BINPRM_BUF_SIZE-1;

	/*

	 * This section handles parsing the #! line into separate

	 * interpreter path and argument strings. We must be careful

	 * because bprm->buf is not yet guaranteed to be NUL-terminated

	 * (though the buffer will have trailing NUL padding when the

	 * file size was smaller than the buffer size).

	 *

	 * We do not want to exec a truncated interpreter path, so either

	 * we find a newline (which indicates nothing is truncated), or

	 * we find a space/tab/NUL after the interpreter path (which

	 * itself may be preceded by spaces/tabs). Truncating the

	 * arguments is fine: the interpreter can re-read the script to

	 * parse them on its own.

	 */

	buf_end = bprm->buf + sizeof(bprm->buf) - 1;

	cp = strnchr(bprm->buf, sizeof(bprm->buf), '\n');

	if (!cp) {

		cp = next_non_spacetab(bprm->buf + 2, buf_end);

		if (!cp)

			return -ENOEXEC; /* Entire buf is spaces/tabs */

		/*

		 * If there is no later space/tab/NUL we must assume the

		 * interpreter path is truncated.

		 */

		if (!next_terminator(cp, buf_end))

			return -ENOEXEC;

		cp = buf_end;

	}

	/* NUL-terminate the buffer and any trailing spaces/tabs. */

	*cp = '\0';

	while (cp > bprm->buf) {

		cp--;