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>

1 files changed, 8 insertions, 14 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);
 }