From 5a73e0ca54c77e067c3b12ea6f35e3e8681e8cf8 Mon Sep 17 00:00:00 2001
From: Bram Moolenaar <Bram@vim.org>
Date: Sat, 4 Nov 2017 21:35:01 +0100
Subject: [PATCH] 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.
---
 src/Makefile              |   1 +
 src/fileio.c              |  24 +++++++++-
 src/testdir/test_swap.vim | 112 ++++++++++++++++++++++++++++++----------------
 src/version.c             |   2 +
 4 files changed, 99 insertions(+), 40 deletions(-)

diff --git a/src/fileio.c b/src/fileio.c
index 87b85cf..34dcdb6 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -716,7 +716,29 @@ 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)mch_setperm(curbuf->b_ml.ml_mfp->mf_fname, (long)swap_mode);
+	{
+	    char_u *swap_fname = curbuf->b_ml.ml_mfp->mf_fname;
+
+	    /*
+	     * 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)
+	    {
+		stat_T	swap_st;
+
+		if (mch_stat((char *)swap_fname, &swap_st) >= 0
+			&& st.st_gid != swap_st.st_gid
+			&& fchown(curbuf->b_ml.ml_mfp->mf_fd, -1, st.st_gid)
+									 == -1)
+		    swap_mode &= 0600;
+	    }
+
+	    (void)mch_setperm(swap_fname, (long)swap_mode);
+	}
 #endif
     }