Merge pull request #251770 from robryk/suidwrapapparm

nixos/security/wrappers: simplifications and a fix for #98863 (respin of #199599)

nixos/modules/security/wrappers/default.nix

@@ -5,8 +5,8 @@ let
 
   parentWrapperDir = dirOf wrapperDir;
 
-  securityWrapper = pkgs.callPackage ./wrapper.nix {
-    inherit parentWrapperDir;
+  securityWrapper = sourceProg : pkgs.callPackage ./wrapper.nix {
+    inherit sourceProg;
   };
 
   fileModeType =
@@ -91,8 +91,7 @@ let
     , ...
     }:
     ''
-      cp ${securityWrapper}/bin/security-wrapper "$wrapperDir/${program}"
-      echo -n "${source}" > "$wrapperDir/${program}.real"
+      cp ${securityWrapper source}/bin/security-wrapper "$wrapperDir/${program}"
 
       # Prevent races
       chmod 0000 "$wrapperDir/${program}"
@@ -119,8 +118,7 @@ let
     , ...
     }:
     ''
-      cp ${securityWrapper}/bin/security-wrapper "$wrapperDir/${program}"
-      echo -n "${source}" > "$wrapperDir/${program}.real"
+      cp ${securityWrapper source}/bin/security-wrapper "$wrapperDir/${program}"
 
       # Prevent races
       chmod 0000 "$wrapperDir/${program}"
@@ -248,11 +246,13 @@ in
       export PATH="${wrapperDir}:$PATH"
     '';
 
-    security.apparmor.includes."nixos/security.wrappers" = ''
-      include "${pkgs.apparmorRulesFromClosure { name="security.wrappers"; } [
-        securityWrapper
+    security.apparmor.includes = lib.mapAttrs' (wrapName: wrap: lib.nameValuePair
+     "nixos/security.wrappers/${wrapName}" ''
+      include "${pkgs.apparmorRulesFromClosure { name="security.wrappers.${wrapName}"; } [
+        (securityWrapper wrap.source)
       ]}"
-    '';
+      mrpx ${wrap.source},
+    '') wrappers;
 
     ###### wrappers activation script
     system.activationScripts.wrappers =

nixos/modules/security/wrappers/wrapper.c

@@ -17,6 +17,10 @@
 #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
@@ -24,10 +28,6 @@
 
 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";
 
@@ -151,115 +151,20 @@ static int make_caps_ambient(const char *self_path) {
     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(self_path) != 0) {
-        free(self_path);
+    if (make_caps_ambient("/proc/self/exe") != 0) {
         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

@@ -1,4 +1,4 @@
-{ stdenv, linuxHeaders, parentWrapperDir, debug ? false }:
+{ stdenv, linuxHeaders, sourceProg, debug ? false }:
 # For testing:
 # $ nix-build -E 'with import <nixpkgs> {}; pkgs.callPackage ./wrapper.nix { parentWrapperDir = "/run/wrappers"; debug = true; }'
 stdenv.mkDerivation {
@@ -7,7 +7,7 @@ stdenv.mkDerivation {
   dontUnpack = true;
   hardeningEnable = [ "pie" ];
   CFLAGS = [
-    ''-DWRAPPER_DIR="${parentWrapperDir}"''
+    ''-DSOURCE_PROG="${sourceProg}"''
   ] ++ (if debug then [
     "-Werror" "-Og" "-g"
   ] else [

nixos/modules/tasks/network-interfaces.nix

@@ -1396,14 +1396,12 @@ in
     security.apparmor.policies."bin.ping".profile = lib.mkIf config.security.apparmor.policies."bin.ping".enable (lib.mkAfter ''
       /run/wrappers/bin/ping {
         include <abstractions/base>
-        include <nixos/security.wrappers>
+        include <nixos/security.wrappers/ping>
         rpx /run/wrappers/wrappers.*/ping,
       }
       /run/wrappers/wrappers.*/ping {
         include <abstractions/base>
-        include <nixos/security.wrappers>
-        r /run/wrappers/wrappers.*/ping.real,
-        mrpx ${config.security.wrappers.ping.source},
+        include <nixos/security.wrappers/ping>
         capability net_raw,
         capability setpcap,
       }

nixos/tests/wrappers.nix

@@ -21,6 +21,8 @@ in
       };
     };
 
+    security.apparmor.enable = true;
+
     security.wrappers = {
       suidRoot = {
         owner = "root";
@@ -84,17 +86,27 @@ in
       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'))
       machine.succeed(cmd_as_regular('/run/wrappers/bin/capsh_with_chown --has-p=CAP_CHOWN'))
       machine.fail(cmd_as_regular('/run/wrappers/bin/capsh_with_chown --has-p=CAP_SYS_ADMIN'))
 
-      # test a few "attacks" against which the wrapper protects itself
-      machine.succeed("cp /run/wrappers/bin/suid_root_busybox{,.real} /tmp/")
-      machine.fail(cmd_as_regular("/tmp/suid_root_busybox id -u"))
-
-      machine.succeed("chmod u+s,a+w /run/wrappers/bin/suid_root_busybox")
-      machine.fail(cmd_as_regular("/run/wrappers/bin/suid_root_busybox id -u"))
+      # Test that the only user of apparmor policy includes generated by
+      # wrappers works. Ideally this'd be located in a test for the module that
+      # actually makes the apparmor policy for ping, but there's no convenient
+      # test for that one.
+      machine.succeed("ping -c 1 127.0.0.1")
     '';
 })