authentication: use hidden form instead of referer

This also gives us some CSRF protection. Note that we make use of the
hmac to protect the redirect value.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

3 files changed, 131 insertions, 94 deletions

diff --git a/cgit.c b/cgit.c
index c52ef33..be1265d 100644
--- a/cgit.c
+++ b/cgit.c
@@ -614,22 +614,19 @@ static inline void open_auth_filter(struct cgit_context *ctx, const char *functi
                 ctx->qry.url ? ctx->qry.url : "");
 }
 
+/* We intentionally keep this rather small, instead of looping and
+ * feeding it to the filter a couple bytes at a time. This way, the
+ * filter itself does not need to handle any denial of service or
+ * buffer bloat issues. If this winds up being too small, people
+ * will complain on the mailing list, and we'll increase it as needed. */
 #define MAX_AUTHENTICATION_POST_BYTES 4096
+/* The filter is expected to spit out "Status: " and all headers. */
 static inline void authenticate_post(struct cgit_context *ctx)
 {
-        if (ctx->env.http_referer && strlen(ctx->env.http_referer) > 0) {
-                html("Status: 302 Redirect\n");
-                html("Cache-Control: no-cache, no-store\n");
-                htmlf("Location: %s\n", ctx->env.http_referer);
-        } else {
-                html("Status: 501 Missing Referer\n");
-                html("Cache-Control: no-cache, no-store\n\n");
-                exit(0);
-        }
-
-        open_auth_filter(ctx, "authenticate-post");
         char buffer[MAX_AUTHENTICATION_POST_BYTES];
         int len;
+
+        open_auth_filter(ctx, "authenticate-post");
         len = ctx->env.content_length;
         if (len > MAX_AUTHENTICATION_POST_BYTES)
                 len = MAX_AUTHENTICATION_POST_BYTES;
@@ -637,10 +634,7 @@ static inline void authenticate_post(struct cgit_context *ctx)
                 die_errno("Could not read POST from stdin");
         if (write(STDOUT_FILENO, buffer, len) < 0)
                 die_errno("Could not write POST to stdout");
-        /* The filter may now spit out a Set-Cookie: ... */
         cgit_close_filter(ctx->cfg.auth_filter);
-
-        html("\n");
         exit(0);
 }
 
diff --git a/cgitrc.5.txt b/cgitrc.5.txt
index c45dbd3..682d8bb 100644
--- a/cgitrc.5.txt
+++ b/cgitrc.5.txt
@@ -662,7 +662,8 @@ auth filter::
         the http cookie and return a 0 if it is invalid or 1 if it is invalid,
         in the exit code / close function. If the filter action is
         "authenticate-post", this filter receives POST'd parameters on
-        standard input, and should write to output one or more "Set-Cookie"
+        standard input, and should write a complete CGI request, preferably
+        with a 302 redirect, and write to output one or more "Set-Cookie"
         HTTP headers, each followed by a newline.
 
         Please see `filters/simple-authentication.lua` for a clear example
diff --git a/filters/simple-authentication.lua b/filters/simple-authentication.lua
index 4cd4983..5935d08 100644
--- a/filters/simple-authentication.lua
+++ b/filters/simple-authentication.lua
@@ -33,15 +33,28 @@ local secret = "BE SURE TO CUSTOMIZE THIS STRING TO SOMETHING BIG AND RANDOM"
 --
 --
 
--- Sets HTTP cookie headers based on post
+-- Sets HTTP cookie headers based on post and sets up redirection.
 function authenticate_post()
         local password = users[post["username"]]
-        -- TODO: Implement time invariant string comparison function to mitigate against timing attack.
+        local redirect = validate_value(post["redirect"])
+
+        if redirect == nil then
+                not_found()
+                return 0
+        end
+
+        redirect_to(redirect)
+
+        -- TODO: Implement time invariant string comparison function to mitigate timing attack.
         if password == nil or password ~= post["password"] then
-                construct_cookie("", "cgitauth")
+                set_cookie("cgitauth", "")
         else
-                construct_cookie(post["username"], "cgitauth")
+                -- One week expiration time
+                local username = secure_value(post["username"], os.time() + 604800)
+                set_cookie("cgitauth", username)
         end
+
+        html("\n")
         return 0
 end
 
@@ -54,8 +67,8 @@ function authenticate_cookie()
                 return 1
         end
 
-        local username = validate_cookie(get_cookie(http["cookie"], "cgitauth"))
-        if username == nil or not accepted_users[username] then
+        local username = validate_value(get_cookie(http["cookie"], "cgitauth"))
+        if username == nil or not accepted_users[username:lower()] then
                 return 0
         else
                 return 1
@@ -68,6 +81,9 @@ function body()
         html("<form method='post' action='")
         html_attr(cgit["login"])
         html("'>")
+        html("<input type='hidden' name='redirect' value='")
+        html_attr(secure_value(cgit["url"], 0))
+        html("' />")
         html("<table>")
         html("<tr><td><label for='username'>Username:</label></td><td><input id='username' name='username' autofocus /></td></tr>")
         html("<tr><td><label for='password'>Password:</label></td><td><input id='password' name='password' type='password' /></td></tr>")
@@ -78,81 +94,10 @@ function body()
 end
 
 
---
---
--- Cookie construction and validation helpers.
---
---
-
-local crypto = require("crypto")
-
--- Returns username of cookie if cookie is valid. Otherwise returns nil.
-function validate_cookie(cookie)
-        local i = 0
-        local username = ""
-        local expiration = 0
-        local salt = ""
-        local hmac = ""
-
-        if cookie:len() < 3 or cookie:sub(1, 1) == "|" then
-                return nil
-        end
-
-        for component in string.gmatch(cookie, "[^|]+") do
-                if i == 0 then