Redo emacsclient socket symlink-attack checking

* admin/merge-gnulib (GNULIB_MODULES): Add file-has-acl.
* lib/file-has-acl.c: New file, copied from Gnulib.
* lib/gnulib.mk.in, m4/gnulib-comp.m4: Regenerate.
* lib-src/emacsclient.c: Include acl.h, for file_has_acl.
(O_PATH): Default to O_SEARCH, which is good enough here.
(union local_sockaddr): New type.
(socket_status): Remove, replacing with ...
(connect_socket): New function.  All callers changed.
This function checks for ownership and permissions issues with the
parent directory of the socket file, instead of checking the
owner of the socket (which does not help security).
(socknamesize): Move to file scope.
(local_sockname): New arg S.  No need to pass socknamesize.
UID arg is now uid_t.  All callers changed.  Get file descriptor
of parent directory of socket, to foil some symlink attacks.
Do not follow symlinks to that directory.
(set_local_socket): Create the socket here instead of on
each attempt to connect it.  Fall back from XDG_RUNTIME_DIR
to /tmp only if the former fails due to ENOENT.  Adjust
permission-failure diagnostic to match changed behavior.

This addresses Bug#33847, which complained about emacsclient in a
safer XDG environment not connecting to an Emacs server running in
a less-safe enviroment outside XDG.  The patch fixes a
longstanding issue with emacsclient permission checking.
It’s ineffective to look at the permission of the socket file
itself; on some platforms, these permissions are ignored anyway.
What matters are the permissions on the parent directory of the
socket file, as these are what make symlink attacks possible.
Change the permissions check accordingly, and also refuse to
follow symlinks to that parent directory.  These changes make it
OK for emacsclient to fall back from XDG_RUNTIME_DIR to the
traditionally less-safe /tmp/emacsNNNN directories, since /tmp is
universally sticky nowadays.

2 files changed, 521 insertions, 0 deletions

diff --git a/lib/file-has-acl.c b/lib/file-has-acl.c
new file mode 100644
index 00000000000..c667ae9d24a
--- /dev/null
+++ b/lib/file-has-acl.c
@@ -0,0 +1,510 @@
+/* Test whether a file has a nontrivial ACL.  -*- coding: utf-8 -*-
+
+   Copyright (C) 2002-2003, 2005-2020 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+   Written by Paul Eggert, Andreas Grünbacher, and Bruno Haible.  */
+
+/* Without this pragma, gcc 4.7.0 20120126 may suggest that the
+   file_has_acl function might be candidate for attribute 'const'  */
+#if (__GNUC__ == 4 && 6 <= __GNUC_MINOR__) || 4 < __GNUC__
+# pragma GCC diagnostic ignored "-Wsuggest-attribute=const"
+#endif
+
+#include <config.h>
+
+#include "acl.h"
+
+#include "acl-internal.h"
+
+#if GETXATTR_WITH_POSIX_ACLS
+# include <sys/xattr.h>
+# include <linux/xattr.h>
+#endif
+
+/* Return 1 if NAME has a nontrivial access control list,
+   0 if ACLs are not supported, or if NAME has no or only a base ACL,
+   and -1 (setting errno) on error.  Note callers can determine
+   if ACLs are not supported as errno is set in that case also.
+   SB must be set to the stat buffer of NAME,
+   obtained through stat() or lstat().  */
+
+int
+file_has_acl (char const *name, struct stat const *sb)
+{
+#if USE_ACL
+  if (! S_ISLNK (sb->st_mode))
+    {
+
+# if GETXATTR_WITH_POSIX_ACLS
+
+      ssize_t ret;
+
+      ret = getxattr (name, XATTR_NAME_POSIX_ACL_ACCESS, NULL, 0);
+      if (ret < 0 && errno == ENODATA)
+        ret = 0;
+      else if (ret > 0)
+        return 1;
+
+      if (ret == 0 && S_ISDIR (sb->st_mode))
+        {
+          ret = getxattr (name, XATTR_NAME_POSIX_ACL_DEFAULT, NULL, 0);
+          if (ret < 0 && errno == ENODATA)
+            ret = 0;
+          else if (ret > 0)
+            return 1;
+        }
+
+      if (ret < 0)
+        return - acl_errno_valid (errno);
+      return ret;
+
+# elif HAVE_ACL_GET_FILE
+
+      /* POSIX 1003.1e (draft 17 -- abandoned) specific version.  */
+      /* Linux, FreeBSD, Mac OS X, IRIX, Tru64, Cygwin >= 2.5 */
+      int ret;
+
+      if (HAVE_ACL_EXTENDED_FILE) /* Linux */
+        {
+          /* On Linux, acl_extended_file is an optimized function: It only
+             makes two calls to getxattr(), one for ACL_TYPE_ACCESS, one for
+             ACL_TYPE_DEFAULT.  */
+          ret = acl_extended_file (name);
+        }
+      else /* FreeBSD, Mac OS X, IRIX, Tru64, Cygwin >= 2.5 */
+        {
+#  if HAVE_ACL_TYPE_EXTENDED /* Mac OS X */
+          /* On Mac OS X, acl_get_file (name, ACL_TYPE_ACCESS)
+             and acl_get_file (name, ACL_TYPE_DEFAULT)
+             always return NULL / EINVAL.  There is no point in making
+             these two useless calls.  The real ACL is retrieved through
+             acl_get_file (name, ACL_TYPE_EXTENDED).  */
+          acl_t acl = acl_get_file (name, ACL_TYPE_EXTENDED);
+          if (acl)
+            {
+              ret = acl_extended_nontrivial (acl);
+              acl_free (acl);
+            }
+          else
+            ret = -1;
+#  else /* FreeBSD, IRIX, Tru64, Cygwin >= 2.5 */
+          acl_t acl = acl_get_file (name, ACL_TYPE_ACCESS);
+          if (acl)
+            {
+              int saved_errno;
+
+              ret = acl_access_nontrivial (acl);
+              saved_errno = errno;
+              acl_free (acl);
+              errno = saved_errno;
+#   if HAVE_ACL_FREE_TEXT /* Tru64 */
+              /* On OSF/1, acl_get_file (name, ACL_TYPE_DEFAULT) always
+                 returns NULL with errno not set.  There is no point in
+                 making this call.  */
+#   else /* FreeBSD, IRIX, Cygwin >= 2.5 */
+              /* On Linux, FreeBSD, IRIX, acl_get_file (name, ACL_TYPE_ACCESS)
+                 and acl_get_file (name, ACL_TYPE_DEFAULT) on a directory
+                 either both succeed or both fail; it depends on the
+                 file system.  Therefore there is no point in making the second
+                 call if the first one already failed.  */
+              if (ret == 0 && S_ISDIR (sb->st_mode))
+                {
+                  acl = acl_get_file (name, ACL_TYPE_DEFAULT);
+                  if (acl)
+                    {
+#    ifdef __CYGWIN__ /* Cygwin >= 2.5 */
+                      ret = acl_access_nontrivial (acl);
+                      saved_errno = errno;
+                      acl_free (acl);
+                      errno = saved_errno;
+#    else
+                      ret = (0 < acl_entries (acl));
+                      acl_free (acl);
+#    endif
+                    }
+                  else
+                    ret = -1;
+                }
+#   endif
+            }
+          else
+            ret = -1;
+#  endif
+        }
+      if (ret < 0)
+        return - acl_errno_valid (errno);
+      return ret;
+
+# elif HAVE_FACL && defined GETACL /* Solaris, Cygwin < 2.5, not HP-UX */
+
+#  if defined ACL_NO_TRIVIAL
+
+      /* Solaris 10 (newer version), which has additional API declared in
+         <sys/acl.h> (acl_t) and implemented in libsec (acl_set, acl_trivial,
+         acl_fromtext, ...).  */
+      return acl_trivial (name);
+
+#  else /* Solaris, Cygwin, general case */
+
+      /* Solaris 2.5 through Solaris 10, Cygwin, and contemporaneous versions
+         of Unixware.  The acl() call returns the access and default ACL both
+         at once.  */
+      {
+        /* Initially, try to read the entries into a stack-allocated buffer.
+           Use malloc if it does not fit.  */
+        enum
+          {
+            alloc_init = 4000 / sizeof (aclent_t), /* >= 3 */
+            alloc_max = MIN (INT_MAX, SIZE_MAX / sizeof (aclent_t))
+          };
+        aclent_t buf[alloc_init];
+        size_t alloc = alloc_init;
+        aclent_t *entries = buf;
+        aclent_t *malloced = NULL;
+        int count;
+
+        for (;;)
+          {
+            count = acl (name, GETACL, alloc, entries);
+            if (count < 0 && errno == ENOSPC)
+              {
+                /* Increase the size of the buffer.  */
+                free (malloced);
+                if (alloc > alloc_max / 2)
+                  {
+                    errno = ENOMEM;
+                    return -1;
+                  }
+                alloc = 2 * alloc; /* <= alloc_max */
+                entries = malloced =
+                  (aclent_t *) malloc (alloc * sizeof (aclent_t));
+                if (entries == NULL)
+                  {
+                    errno = ENOMEM;
+                    return -1;
+                  }
+                continue;
+              }
+            break;
+          }
+        if (count < 0)
+          {
+            if (errno == ENOSYS || errno == ENOTSUP)
+              ;
+            else
+              {
+                int saved_errno = errno;
+                free (malloced);
+                errno = saved_errno;
+                return -1;
+              }
+          }
+        else if (count == 0)
+          ;
+        else
+          {
+            /* Don't use MIN_ACL_ENTRIES:  It's set to 4 on Cygwin, but Cygwin
+               returns only 3 entries for files with no ACL.  But this is safe:
+               If there are more than 4 entries, there cannot be only the
+               "user::", "group::", "other:", and "mask:" entries.  */
+            if (count > 4)
+              {
+                free (malloced);
+                return 1;
+              }
+
+            if (acl_nontrivial (count, entries))
+              {
+                free (malloced);
+                return 1;
+              }
+          }
+        free (malloced);
+      }
+
+#   ifdef ACE_GETACL
+      /* Solaris also has a different variant of ACLs, used in ZFS and NFSv4
+         file systems (whereas the other ones are used in UFS file systems).  */
+      {
+        /* Initially, try to read the entries into a stack-allocated buffer.
+           Use malloc if it does not fit.  */
+        enum
+          {
+            alloc_init = 4000 / sizeof (ace_t), /* >= 3 */
+            alloc_max = MIN (INT_MAX, SIZE_MAX / sizeof (ace_t))
+          };
+        ace_t buf[alloc_init];
+        size_t alloc = alloc_init;
+        ace_t *entries = buf;
+        ace_t *malloced = NULL;
+        int count;
+
+        for (;;)
+          {
+            count = acl (name, ACE_GETACL, alloc, entries);
+            if (count < 0 && errno == ENOSPC)
+              {
+                /* Increase the size of the buffer.  */
+                free (malloced);
+                if (alloc > alloc_max / 2)
+                  {
+                    errno = ENOMEM;
+                    return -1;
+                  }
+                alloc = 2 * alloc; /* <= alloc_max */
+                entries = malloced = (ace_t *) malloc (alloc * sizeof (ace_t));
+                if (entries == NULL)
+                  {
+                    errno = ENOMEM;
+                    return -1;
+                  }
+                continue;
+              }
+            break;
+          }
+        if (count < 0)
+          {
+            if (errno == ENOSYS || errno == EINVAL)
+              ;
+            else
+              {
+                int saved_errno = errno;
+                free (malloced);
+                errno = saved_errno;
+                return -1;
+              }
+          }
+        else if (count == 0)
+          ;
+        else
+          {
+            /* In the old (original Solaris 10) convention:
+               If there are more than 3 entries, there cannot be only the
+               ACE_OWNER, ACE_GROUP, ACE_OTHER entries.
+               In the newer Solaris 10 and Solaris 11 convention:
+               If there are more than 6 entries, there cannot be only the
+               ACE_OWNER, ACE_GROUP, ACE_EVERYONE entries, each once with
+               NEW_ACE_ACCESS_ALLOWED_ACE_TYPE and once with
+               NEW_ACE_ACCESS_DENIED_ACE_TYPE.  */
+            if (count > 6)
+              {
+                free (malloced);
+                return 1;
+              }
+
+            if (acl_ace_nontrivial (count, entries))
+              {
+                free (malloced);
+                return 1;
+              }
+          }
+        free (malloced);
+      }
+#   endif
+
+      return 0;
+#  endif
+
+# elif HAVE_GETACL /* HP-UX */
+
+      {
+        struct acl_entry entries[NACLENTRIES];
+        int count;
+
+        count = getacl (name, NACLENTRIES, entries);
+
+        if (count < 0)
+          {
+            /* ENOSYS is seen on newer HP-UX versions.
+               EOPNOTSUPP is typically seen on NFS mounts.
+               ENOTSUP was seen on Quantum StorNext file systems (cvfs).  */
+            if (errno == ENOSYS || errno == EOPNOTSUPP || errno == ENOTSUP)
+              ;
+            else
+              return -1;
+          }
+        else if (count == 0)
+          return 0;
+        else /* count > 0 */
+          {
+            if (count > NACLENTRIES)
+              /* If NACLENTRIES cannot be trusted, use dynamic memory
+                 allocation.  */
+              abort ();
+
+            /* If there are more than 3 entries, there cannot be only the
+               (uid,%), (%,gid), (%,%) entries.  */
+            if (count > 3)
+              return 1;
+
+            {
+              struct stat statbuf;
+
+              if (stat (name, &statbuf) < 0)
+                return -1;
+
+              return acl_nontrivial (count, entries);
+            }
+          }
+      }
+
+#  if HAVE_ACLV_H /* HP-UX >= 11.11 */
+
+      {
+        struct acl entries[NACLVENTRIES];
+        int count;
+
+        count = acl ((char *) name, ACL_GET, NACLVENTRIES, entries);
+
+        if (count < 0)
+          {
+            /* EOPNOTSUPP is seen on NFS in HP-UX 11.11, 11.23.
+               EINVAL is seen on NFS in HP-UX 11.31.  */
+            if (errno == ENOSYS || errno == EOPNOTSUPP || errno == EINVAL)
+              ;
+            else
+              return -1;
+          }
+        else if (count == 0)
+          return 0;
+        else /* count > 0 */
+          {
+            if (count > NACLVENTRIES)
+              /* If NACLVENTRIES cannot be trusted, use dynamic memory
+                 allocation.  */
+              abort ();
+
+            /* If there are more than 4 entries, there cannot be only the
+               four base ACL entries.  */
+            if (count > 4)
+              return 1;
+
+            return aclv_nontrivial (count, entries);
+          }
+      }
+
+#  endif
+
+# elif HAVE_ACLX_GET && defined ACL_AIX_WIP /* AIX */
+
+      acl_type_t type;
+      char aclbuf[1024];
+      void *acl = aclbuf;
+      size_t aclsize = sizeof (aclbuf);
+      mode_t mode;
+
+      for (;;)
+        {
+          /* The docs say that type being 0 is equivalent to ACL_ANY, but it
+             is not true, in AIX 5.3.  */
+          type.u64 = ACL_ANY;
+          if (aclx_get (name, 0, &type, aclbuf, &aclsize, &mode) >= 0)
+            break;
+          if (errno == ENOSYS)
+            return 0;
+          if (errno != ENOSPC)
+            {
+              if (acl != aclbuf)
+                {
+                  int saved_errno = errno;
+                  free (acl);
+                  errno = saved_errno;
+                }
+              return -1;
+            }
+          aclsize = 2 * aclsize;
+          if (acl != aclbuf)
+            free (acl);
+          acl = malloc (aclsize);
+          if (acl == NULL)
+            {
+              errno = ENOMEM;
+              return -1;
+            }
+        }
+
+      if (type.u64 == ACL_AIXC)
+        {
+          int result = acl_nontrivial ((struct acl *) acl);
+          if (acl != aclbuf)
+            free (acl);
+          return result;
+        }
+      else if (type.u64 == ACL_NFS4)
+        {
+          int result = acl_nfs4_nontrivial ((nfs4_acl_int_t *) acl);
+          if (acl != aclbuf)
+            free (acl);
+          return result;
+        }
+      else
+        {
+          /* A newer type of ACL has been introduced in the system.
+             We should better support it.  */
+          if (acl != aclbuf)
+            free (acl);
+          errno = EINVAL;
+          return -1;
+        }
+
+# elif HAVE_STATACL /* older AIX */
+
+      union { struct acl a; char room[4096]; } u;
+
+      if (statacl ((char *) name, STX_NORMAL, &u.a, sizeof (u)) < 0)
+        return -1;
+
+      return acl_nontrivial (&u.a);
+
+# elif HAVE_ACLSORT /* NonStop Kernel */
+
+      {
+        struct acl entries[NACLENTRIES];
+        int count;
+
+        count = acl ((char *) name, ACL_GET, NACLENTRIES, entries);
+
+        if (count < 0)
+          {
+            if (errno == ENOSYS || errno == ENOTSUP)
+              ;
+            else
+              return -1;
+          }
+        else if (count == 0)
+          return 0;
+        else /* count > 0 */
+          {
+            if (count > NACLENTRIES)
+              /* If NACLENTRIES cannot be trusted, use dynamic memory
+                 allocation.  */
+              abort ();
+
+            /* If there are more than 4 entries, there cannot be only the
+               four base ACL entries.  */
+            if (count > 4)
+              return 1;
+
+            return acl_nontrivial (count, entries);
+          }
+      }
+
+# endif
+    }
+#endif
+
+  return 0;
+}
diff --git a/lib/gnulib.mk.in b/lib/gnulib.mk.in
index 07736f9b8bc..0b9aaf6d9ea 100644
--- a/lib/gnulib.mk.in
+++ b/lib/gnulib.mk.in
@@ -98,6 +98,7 @@
 #  fcntl \
 #  fcntl-h \
 #  fdopendir \
+#  file-has-acl \
 #  filemode \
 #  filename \
 #  filevercmp \
@@ -1788,6 +1789,16 @@ EXTRA_libgnu_a_SOURCES += fdopendir.c
 endif
 ## end   gnulib module fdopendir
 
+## begin gnulib module file-has-acl
+ifeq (,$(OMIT_GNULIB_MODULE_file-has-acl))
+
+libgnu_a_SOURCES += file-has-acl.c
+
+EXTRA_DIST += acl-internal.h
+
+endif
+## end   gnulib module file-has-acl
+
 ## begin gnulib module filemode
 ifeq (,$(OMIT_GNULIB_MODULE_filemode))