• Christian Brauner's avatar
    getxattr: use correct xattr length · 118cb277
    Christian Brauner authored
    commit 82c9a927 upstream.
    
    When running in a container with a user namespace, if you call getxattr
    with name = "system.posix_acl_access" and size % 8 != 4, then getxattr
    silently skips the user namespace fixup that it normally does resulting in
    un-fixed-up data being returned.
    This is caused by posix_acl_fix_xattr_to_user() being passed the total
    buffer size and not the actual size of the xattr as returned by
    vfs_getxattr().
    This commit passes the actual length of the xattr as returned by
    vfs_getxattr() down.
    
    A reproducer for the issue is:
    
      touch acl_posix
    
      setfacl -m user:0:rwx acl_posix
    
    and the compile:
    
      #define _GNU_SOURCE
      #include <errno.h>
      #include <stdio.h>
      #include <stdlib.h>
      #include <string.h>
      #include <sys/types.h>
      #include <unistd.h>
      #include <attr/xattr.h>
    
      /* Run in user namespace with nsuid 0 mapped to uid != 0 on the host. */
      int main(int argc, void **argv)
      {
              ssize_t ret1, ret2;
              char buf1[128], buf2[132];
              int fret = EXIT_SUCCESS;
              char *file;
    
              if (argc < 2) {
                      fprintf(stderr,
                              "Please specify a file with "
                              "\"system.posix_acl_access\" permissions set\n");
                      _exit(EXIT_FAILURE);
              }
              file = argv[1];
    
              ret1 = getxattr(file, "system.posix_acl_access",
                              buf1, sizeof(buf1));
              if (ret1 < 0) {
                      fprintf(stderr, "%s - Failed to retrieve "
                                      "\"system.posix_acl_access\" "
                                      "from \"%s\"\n", strerror(errno), file);
                      _exit(EXIT_FAILURE);
              }
    
              ret2 = getxattr(file, "system.posix_acl_access",
                              buf2, sizeof(buf2));
              if (ret2 < 0) {
                      fprintf(stderr, "%s - Failed to retrieve "
                                      "\"system.posix_acl_access\" "
                                      "from \"%s\"\n", strerror(errno), file);
                      _exit(EXIT_FAILURE);
              }
    
              if (ret1 != ret2) {
                      fprintf(stderr, "The value of \"system.posix_acl_"
                                      "access\" for file \"%s\" changed "
                                      "between two successive calls\n", file);
                      _exit(EXIT_FAILURE);
              }
    
              for (ssize_t i = 0; i < ret2; i++) {
                      if (buf1[i] == buf2[i])
                              continue;
    
                      fprintf(stderr,
                              "Unexpected different in byte %zd: "
                              "%02x != %02x\n", i, buf1[i], buf2[i]);
                      fret = EXIT_FAILURE;
              }
    
              if (fret == EXIT_SUCCESS)
                      fprintf(stderr, "Test passed\n");
              else
                      fprintf(stderr, "Test failed\n");
    
              _exit(fret);
      }
    and run:
    
      ./tester acl_posix
    
    On a non-fixed up kernel this should return something like:
    
      root@c1:/# ./t
      Unexpected different in byte 16: ffffffa0 != 00
      Unexpected different in byte 17: ffffff86 != 00
      Unexpected different in byte 18: 01 != 00
    
    and on a fixed kernel:
    
      root@c1:~# ./t
      Test passed
    
    Cc: stable@vger.kernel.org
    Fixes: 2f6f0654 ("userns: Convert vfs posix_acl support to use kuids and kgids")
    Link: https://bugzilla.kernel.org/show_bug.cgi?id=199945Reported-by: 's avatarColin Watson <cjwatson@ubuntu.com>
    Signed-off-by: 's avatarChristian Brauner <christian@brauner.io>
    Acked-by: 's avatarSerge Hallyn <serge@hallyn.com>
    Signed-off-by: 's avatarEric W. Biederman <ebiederm@xmission.com>
    Signed-off-by: 's avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    118cb277