From 5e7a3c2e7e118c7f12d5dfda9f9140f638976aa2 Mon Sep 17 00:00:00 2001
From: Benno Schulenberg <bensberg@telfort.nl>
Date: Sun, 28 Apr 2024 10:51:52 +0200
Subject: files: run `chmod` and `chown` on the descriptor, not on the filename

This closes a window of opportunity where the emergency file could be
replaced by a malicious symlink.

The issue was reported by `MartinJM` and `InvisibleMeerkat`.

Problem existed since version 2.2.0, commit 123110c5, when chmodding
and chowning of the emergency .save file was added.
---
 src/definitions.h |  2 +-
 src/files.c       | 13 ++++++++++++-
 src/nano.c        | 12 +-----------
 3 files changed, 14 insertions(+), 13 deletions(-)

diff --git a/src/definitions.h b/src/definitions.h
index af3a793..55d8235 100644
--- a/src/definitions.h
+++ b/src/definitions.h
@@ -288,7 +288,7 @@ typedef enum {
 } message_type;
 
 typedef enum {
-	OVERWRITE, APPEND, PREPEND
+	OVERWRITE, APPEND, PREPEND, EMERGENCY
 } kind_of_writing_type;
 
 typedef enum {
diff --git a/src/files.c b/src/files.c
index 57c2001..584b579 100644
--- a/src/files.c
+++ b/src/files.c
@@ -1763,6 +1763,8 @@ bool write_file(const char *name, FILE *thefile, bool normal,
 #endif
 	char *realname = real_dir_from_tilde(name);
 		/* The filename after tilde expansion. */
+	int fd = 0;
+		/* The descriptor that is assigned when opening the file. */
 	char *tempname = NULL;
 		/* The name of the temporary file we use when prepending. */
 	linestruct *line = openfile->filetop;
@@ -1846,7 +1848,6 @@ bool write_file(const char *name, FILE *thefile, bool normal,
 	 * For an emergency file, access is restricted to just the owner. */
 	if (thefile == NULL) {
 		mode_t permissions = (normal ? RW_FOR_ALL : S_IRUSR|S_IWUSR);
-		int fd;
 
 #ifndef NANO_TINY
 		block_sigwinch(TRUE);
@@ -1972,6 +1973,16 @@ bool write_file(const char *name, FILE *thefile, bool normal,
 		}
 #endif
 
+#if !defined(NANO_TINY) && defined(HAVE_CHMOD) && defined(HAVE_CHOWN)
+	/* Change permissions and owner of an emergency save file to the values
+	 * of the original file, but ignore any failure as we are in a hurry. */
+	if (method == EMERGENCY && fd && openfile->statinfo) {
+		IGNORE_CALL_RESULT(fchmod(fd, openfile->statinfo->st_mode));
+		IGNORE_CALL_RESULT(fchown(fd, openfile->statinfo->st_uid,
+											openfile->statinfo->st_gid));
+	}
+#endif
+
 	if (fclose(thefile) != 0) {
 		statusline(ALERT, _("Error writing %s: %s"), realname, strerror(errno));
 
diff --git a/src/nano.c b/src/nano.c
index 90b4a0b..973054f 100644
--- a/src/nano.c
+++ b/src/nano.c
@@ -337,18 +337,8 @@ void emergency_save(const char *filename)
 
 	if (*targetname == '\0')
 		fprintf(stderr, _("\nToo many .save files\n"));
-	else if (write_file(targetname, NULL, SPECIAL, OVERWRITE, NONOTES)) {
+	else if (write_file(targetname, NULL, SPECIAL, EMERGENCY, NONOTES))
 		fprintf(stderr, _("\nBuffer written to %s\n"), targetname);
-#if !defined(NANO_TINY) && defined(HAVE_CHMOD) && defined(HAVE_CHOWN)
-		/* Try to chmod/chown the saved file to the values of the original file,
-		 * but ignore any failure as we are in a hurry to get out. */
-		if (openfile->statinfo) {
-			IGNORE_CALL_RESULT(chmod(targetname, openfile->statinfo->st_mode));
-			IGNORE_CALL_RESULT(chown(targetname, openfile->statinfo->st_uid,
-													openfile->statinfo->st_gid));
-		}
-#endif
-	}
 
 	free(targetname);
 	free(plainname);
-- 
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