Merge pull request #251095 from NixOS/revert-199599-suidwrapnoreal

Revert "nixos/security/wrappers: simplifications and a fix for #98863"

nixos/modules/security/wrappers/default.nix

···
 
   parentWrapperDir = dirOf wrapperDir;
 
-  securityWrapper = sourceProg : pkgs.callPackage ./wrapper.nix {
-    inherit sourceProg;
+  securityWrapper = pkgs.callPackage ./wrapper.nix {
+    inherit parentWrapperDir;
   };
 
   fileModeType =
···
     , ...
     }:
     ''
-      cp ${securityWrapper source}/bin/security-wrapper "$wrapperDir/${program}"
+      cp ${securityWrapper}/bin/security-wrapper "$wrapperDir/${program}"
+      echo -n "${source}" > "$wrapperDir/${program}.real"
 
       # Prevent races
       chmod 0000 "$wrapperDir/${program}"
···
     , ...
     }:
     ''
-      cp ${securityWrapper source}/bin/security-wrapper "$wrapperDir/${program}"
+      cp ${securityWrapper}/bin/security-wrapper "$wrapperDir/${program}"
+      echo -n "${source}" > "$wrapperDir/${program}.real"
 
       # Prevent races
       chmod 0000 "$wrapperDir/${program}"

nixos/modules/security/wrappers/wrapper.c

···
 #include <syscall.h>
 #include <byteswap.h>
 
-#ifndef SOURCE_PROG
-#error SOURCE_PROG should be defined via preprocessor commandline
-#endif
-
 // aborts when false, printing the failed expression
 #define ASSERT(expr) ((expr) ? (void) 0 : assert_failure(#expr))
 // aborts when returns non-zero, printing the failed expression and errno
 #define MUSTSUCCEED(expr) ((expr) ? print_errno_and_die(#expr) : (void) 0)
 
 extern char **environ;
+
+// The WRAPPER_DIR macro is supplied at compile time so that it cannot
+// be changed at runtime
+static char *wrapper_dir = WRAPPER_DIR;
 
 // Wrapper debug variable name
 static char *wrapper_debug = "WRAPPER_DEBUG";
···
     return 0;
 }
 
+int readlink_malloc(const char *p, char **ret) {
+    size_t l = FILENAME_MAX+1;
+    int r;
+
+    for (;;) {
+        char *c = calloc(l, sizeof(char));
+        if (!c) {
+            return -ENOMEM;
+        }
+
+        ssize_t n = readlink(p, c, l-1);
+        if (n < 0) {
+            r = -errno;
+            free(c);
+            return r;
+        }
+
+        if ((size_t) n < l-1) {
+            c[n] = 0;
+            *ret = c;
+            return 0;
+        }
+
+        free(c);
+        l *= 2;
+    }
+}
+
 int main(int argc, char **argv) {
     ASSERT(argc >= 1);
+    char *self_path = NULL;
+    int self_path_size = readlink_malloc("/proc/self/exe", &self_path);
+    if (self_path_size < 0) {
+        fprintf(stderr, "cannot readlink /proc/self/exe: %s", strerror(-self_path_size));
+    }
+
+    unsigned int ruid, euid, suid, rgid, egid, sgid;
+    MUSTSUCCEED(getresuid(&ruid, &euid, &suid));
+    MUSTSUCCEED(getresgid(&rgid, &egid, &sgid));
+
+    // If true, then we did not benefit from setuid privilege escalation,
+    // where the original uid is still in ruid and different from euid == suid.
+    int didnt_suid = (ruid == euid) && (euid == suid);
+    // If true, then we did not benefit from setgid privilege escalation
+    int didnt_sgid = (rgid == egid) && (egid == sgid);
+
+
+    // Make sure that we are being executed from the right location,
+    // i.e., `safe_wrapper_dir'.  This is to prevent someone from creating
+    // hard link `X' from some other location, along with a false
+    // `X.real' file, to allow arbitrary programs from being executed
+    // with elevated capabilities.
+    int len = strlen(wrapper_dir);
+    if (len > 0 && '/' == wrapper_dir[len - 1])
+      --len;
+    ASSERT(!strncmp(self_path, wrapper_dir, len));
+    ASSERT('/' == wrapper_dir[0]);
+    ASSERT('/' == self_path[len]);
+
+    // If we got privileges with the fs set[ug]id bit, check that the privilege we
+    // got matches the one one we expected, ie that our effective uid/gid
+    // matches the uid/gid of `self_path`. This ensures that we were executed as
+    // `self_path', and not, say, as some other setuid program.
+    // We don't check that if we did not benefit from the set[ug]id bit, as
+    // can be the case in nosuid mounts or user namespaces.
+    struct stat st;
+    ASSERT(lstat(self_path, &st) != -1);
+
+    // if the wrapper gained privilege with suid, check that we got the uid of the file owner
+    ASSERT(!((st.st_mode & S_ISUID) && !didnt_suid) || (st.st_uid == euid));
+    // if the wrapper gained privilege with sgid, check that we got the gid of the file group
+    ASSERT(!((st.st_mode & S_ISGID) && !didnt_sgid) || (st.st_gid == egid));
+    // same, but with suid instead of euid
+    ASSERT(!((st.st_mode & S_ISUID) && !didnt_suid) || (st.st_uid == suid));
+    ASSERT(!((st.st_mode & S_ISGID) && !didnt_sgid) || (st.st_gid == sgid));
+
+    // And, of course, we shouldn't be writable.
+    ASSERT(!(st.st_mode & (S_IWGRP | S_IWOTH)));
+
+    // Read the path of the real (wrapped) program from <self>.real.
+    char real_fn[PATH_MAX + 10];
+    int real_fn_size = snprintf(real_fn, sizeof(real_fn), "%s.real", self_path);
+    ASSERT(real_fn_size < sizeof(real_fn));
+
+    int fd_self = open(real_fn, O_RDONLY);
+    ASSERT(fd_self != -1);
+
+    char source_prog[PATH_MAX];
+    len = read(fd_self, source_prog, PATH_MAX);
+    ASSERT(len != -1);
+    ASSERT(len < sizeof(source_prog));
+    ASSERT(len > 0);
+    source_prog[len] = 0;
+
+    close(fd_self);
 
     // Read the capabilities set on the wrapper and raise them in to
     // the ambient set so the program we're wrapping receives the
     // capabilities too!
-    if (make_caps_ambient("/proc/self/exe") != 0) {
+    if (make_caps_ambient(self_path) != 0) {
+        free(self_path);
         return 1;
     }
+    free(self_path);
 
-    execve(SOURCE_PROG, argv, environ);
+    execve(source_prog, argv, environ);
     
     fprintf(stderr, "%s: cannot run `%s': %s\n",
-        argv[0], SOURCE_PROG, strerror(errno));
+        argv[0], source_prog, strerror(errno));
 
     return 1;
 }

nixos/modules/security/wrappers/wrapper.nix

···
-{ stdenv, linuxHeaders, sourceProg, debug ? false }:
+{ stdenv, linuxHeaders, parentWrapperDir, debug ? false }:
 # For testing:
 # $ nix-build -E 'with import <nixpkgs> {}; pkgs.callPackage ./wrapper.nix { parentWrapperDir = "/run/wrappers"; debug = true; }'
 stdenv.mkDerivation {
···
   dontUnpack = true;
   hardeningEnable = [ "pie" ];
   CFLAGS = [
-    ''-DSOURCE_PROG="${sourceProg}"''
+    ''-DWRAPPER_DIR="${parentWrapperDir}"''
   ] ++ (if debug then [
     "-Werror" "-Og" "-g"
   ] else [

nixos/tests/wrappers.nix

···
       test_as_regular_in_userns_mapped_as_root('/run/wrappers/bin/sgid_root_busybox id -g', '0')
       test_as_regular_in_userns_mapped_as_root('/run/wrappers/bin/sgid_root_busybox id -rg', '0')
 
-      # Test that in nonewprivs environment the wrappers simply exec their target.
-      test_as_regular('${pkgs.util-linux}/bin/setpriv --no-new-privs /run/wrappers/bin/suid_root_busybox id -u', '${toString userUid}')
-      test_as_regular('${pkgs.util-linux}/bin/setpriv --no-new-privs /run/wrappers/bin/suid_root_busybox id -ru', '${toString userUid}')
-      test_as_regular('${pkgs.util-linux}/bin/setpriv --no-new-privs /run/wrappers/bin/suid_root_busybox id -g', '${toString usersGid}')
-      test_as_regular('${pkgs.util-linux}/bin/setpriv --no-new-privs /run/wrappers/bin/suid_root_busybox id -rg', '${toString usersGid}')
-
-      test_as_regular('${pkgs.util-linux}/bin/setpriv --no-new-privs /run/wrappers/bin/sgid_root_busybox id -u', '${toString userUid}')
-      test_as_regular('${pkgs.util-linux}/bin/setpriv --no-new-privs /run/wrappers/bin/sgid_root_busybox id -ru', '${toString userUid}')
-      test_as_regular('${pkgs.util-linux}/bin/setpriv --no-new-privs /run/wrappers/bin/sgid_root_busybox id -g', '${toString usersGid}')
-      test_as_regular('${pkgs.util-linux}/bin/setpriv --no-new-privs /run/wrappers/bin/sgid_root_busybox id -rg', '${toString usersGid}')
-
       # We are only testing the permitted set, because it's easiest to look at with capsh.
       machine.fail(cmd_as_regular('${pkgs.libcap}/bin/capsh --has-p=CAP_CHOWN'))
       machine.fail(cmd_as_regular('${pkgs.libcap}/bin/capsh --has-p=CAP_SYS_ADMIN'))