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Date:   Mon, 12 Nov 2018 17:09:31 +0100
From:   Oleg Nesterov <oleg@redhat.com>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Ben Woodard <woodard@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Kees Cook <keescook@chromium.org>,
        Michal Hocko <mhocko@suse.com>, linux-kernel@vger.kernel.org
Subject: [PATCH 1/2] exec: load_script: don't blindly truncate shebang string
Message-ID: <20181112160931.GA28463@redhat.com>
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load_script() simply truncates bprm->buf and this is very wrong if the
length of shebang string exceeds BINPRM_BUF_SIZE-2. This can silently
truncate i_arg or (worse) we can execute the wrong binary if buf[2:126]
happens to be the valid executable path.

Change load_script() to return ENOEXEC if it can't find '\n' or zero in
bprm->buf. Note that '\0' can come from either prepare_binprm()->memset()
or from kernel_read(), we do not care.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 fs/binfmt_script.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/fs/binfmt_script.c b/fs/binfmt_script.c
index 7cde3f4..d0078cb 100644
--- a/fs/binfmt_script.c
+++ b/fs/binfmt_script.c
@@ -42,10 +42,14 @@ 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 = bprm->buf+BINPRM_BUF_SIZE-1;
+	for (cp = bprm->buf+2;; cp++) {
+		if (cp >= bprm->buf + BINPRM_BUF_SIZE)
+			return -ENOEXEC;
+		if (!*cp || (*cp == '\n'))
+			break;
+	}
 	*cp = '\0';
+
 	while (cp > bprm->buf) {
 		cp--;
 		if ((*cp == ' ') || (*cp == '\t'))
-- 
2.5.0