From 5e8d5c24b260b2e9e25a248a93eddc11bc9260ac Mon Sep 17 00:00:00 2001
From: "H. Peter Anvin" <hpa@zytor.com>
Date: Wed, 29 May 2024 18:10:41 -0700
Subject: [PATCH] tftpd: Use setres*id() if available

POSIX apparently doesn't clearly specify the behavior of the saved ID
when calling setre*id(). If the system has setres*id() then use it to
make absolutely sure that the ID changes cannot be undone.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
---
 configure.ac  |  2 ++
 tftpd/tftpd.c | 12 ++++++++----
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/configure.ac b/configure.ac
index 04b6f98..8770431 100644
--- a/configure.ac
+++ b/configure.ac
@@ -86,7 +86,9 @@ AC_CHECK_FUNCS(setsid)
 AC_CHECK_FUNCS(recvmsg)
 AC_CHECK_FUNCS(ftruncate)
 AC_CHECK_FUNCS(setreuid)
+AC_CHECK_FUNCS(setresuid)
 AC_CHECK_FUNCS(setregid)
+AC_CHECK_FUNCS(setresgid)
 AC_CHECK_FUNCS(initgroups)
 AC_CHECK_FUNCS(setgroups)
 AC_CHECK_FUNCS(sigaction)
diff --git a/tftpd/tftpd.c b/tftpd/tftpd.c
index e410d42..fcb29aa 100644
--- a/tftpd/tftpd.c
+++ b/tftpd/tftpd.c
@@ -1011,16 +1011,20 @@ int main(int argc, char **argv)
 #endif
     }
 
-#ifdef HAVE_SETREGID
+#ifdef HAVE_SETRESGID
+    setrv = setresgid(pw->pw_gid, pw->pw_gid, pw->pw_gid);
+#elif defined(HAVE_SETREGID)
     setrv = setregid(pw->pw_gid, pw->pw_gid);
 #else
     setrv = setegid(pw->pw_gid) || setgid(pw->pw_gid);
 #endif
     if (setrv && errno == EPERM) {
-	setrv = 0;		/* Already restricted */
+	setrv = 0;		/* Assume already restricted by system policy */
     }
 
-#ifdef HAVE_SETREUID
+#ifdef HAVE_SETRESUID
+    setrv = setrv || setresuid(pw->pw_uid, pw->pw_uid, pw->pw_uid);
+#elif defined(HAVE_SETREUID)
     setrv = setrv || setreuid(pw->pw_uid, pw->pw_uid);
 #else
     /* Important: setuid() must come first */
@@ -1028,7 +1032,7 @@ int main(int argc, char **argv)
         (geteuid() != pw->pw_uid && seteuid(pw->pw_uid));
 #endif
     if (setrv && errno == EPERM) {
-	setrv = 0;		/* Already restricted */
+	setrv = 0;		/* Assume already restricted by system policy */
     }
 
     if (setrv) {