vim-patch:8.0.1263: others can read the swap file if a user is careless

Problem:    Others can read the swap file if a user is careless with his
            primary group.
Solution:   If the group permission allows for reading but the world
            permissions doesn't, make sure the group is right.
5a73e0ca54

src/nvim/fileio.c

@@ -310,6 +310,7 @@ readfile (
 #endif
   int fileformat = 0;                   /* end-of-line format */
   int keep_fileformat = FALSE;
+  FileInfo file_info;
   int file_readonly;
   linenr_T skip_count = 0;
   linenr_T read_count = 0;
@@ -481,7 +482,6 @@ readfile (
 
   if (newfile && !read_stdin && !read_buffer && !read_fifo) {
     // Remember time of file.
-    FileInfo file_info;
     if (os_fileinfo((char *)fname, &file_info)) {
       buf_store_file_info(curbuf, &file_info);
       curbuf->b_mtime_read = curbuf->b_mtime;
@@ -627,8 +627,25 @@ readfile (
     // Set swap file protection bits after creating it.
     if (swap_mode > 0 && curbuf->b_ml.ml_mfp != NULL
         && curbuf->b_ml.ml_mfp->mf_fname != NULL) {
-      (void)os_setperm((const char *)curbuf->b_ml.ml_mfp->mf_fname,
+      const char *swap_fname = (const char *)curbuf->b_ml.ml_mfp->mf_fname;
-                       (long)swap_mode);
+
+      // If the group-read bit is set but not the world-read bit, then
+      // the group must be equal to the group of the original file.  If
+      // we can't make that happen then reset the group-read bit.  This
+      // avoids making the swap file readable to more users when the
+      // primary group of the user is too permissive.
+      if ((swap_mode & 044) == 040) {
+        FileInfo swap_info;
+
+        if (os_fileinfo(swap_fname, &swap_info)
+            && file_info.stat.st_gid != swap_info.stat.st_gid
+            && os_fchown(curbuf->b_ml.ml_mfp->mf_fd, -1, file_info.stat.st_gid)
+            == -1) {
+          swap_mode &= 0600;
+        }
+      }
+
+      (void)os_setperm(swap_fname, swap_mode);
     }
 #endif
   }

src/nvim/testdir/test_swap.vim

@@ -51,6 +51,40 @@ func Test_swap_directory()
   call delete("Xtest.je", "rf")
 endfunc
 
+func Test_swap_group()
+  if !has("unix")
+    return
+  endif
+  let groups = split(system('groups'))
+  if len(groups) <= 1
+    throw 'Skipped: need at least two groups, got ' . groups
+  endif
+
+  call delete('Xtest')
+  split Xtest
+  call setline(1, 'just some text')
+  wq
+  if system('ls -l Xtest') !~ ' ' . groups[0] . ' \d'
+    throw 'Skipped: test file does not have the first group'
+  else
+    silent !chmod 640 Xtest
+    call system('chgrp ' . groups[1] . ' Xtest')
+    if system('ls -l Xtest') !~ ' ' . groups[1] . ' \d'
+      throw 'Skipped: cannot set second group on test file'
+    else
+      split Xtest
+      let swapname = substitute(execute('swapname'), '[[:space:]]', '', 'g')
+      call assert_match('Xtest', swapname)
+      " Group of swapfile must now match original file.
+      call assert_match(' ' . groups[1] . ' \d', system('ls -l ' . swapname))
+
+      bwipe!
+    endif
+  endif
+
+  call delete('Xtest')
+endfunc
+
 func Test_missing_dir()
   call mkdir('Xswapdir')
   exe 'set directory=' . getcwd() . '/Xswapdir'