[Nano-devel] [PATCH v2] pull in the futimens module from gnulib

[Kamil Dudka]

... and use futimens() instead of utime() to prevent a symlink attack
while creating a backup file.  Otherwise, a non-privileged user could
create an arbitrary symlink with name of the backup file and this way
fool a privileged user to call utime() on the attacker-chosen file.
---
 autogen.sh  |  1 +
 src/files.c | 25 +++++++++++++++----------
 src/proto.h |  2 +-
 3 files changed, 17 insertions(+), 11 deletions(-)

diff --git a/autogen.sh b/autogen.sh
index 617705c..de9f3bb 100755
--- a/autogen.sh
+++ b/autogen.sh
@@ -5,6 +5,7 @@ gnulib_url="git://git.sv.gnu.org/gnulib.git"
 gnulib_hash="4084b3a1094372b960ce4a97634e08f4538c8bdd"
 
 modules="
+       futimens
        getdelim
        getline
        getopt-gnu
diff --git a/src/files.c b/src/files.c
index 033b963..38c1cbc 100644
--- a/src/files.c
+++ b/src/files.c
@@ -1541,12 +1541,14 @@ void init_backup_dir(void)
 
 /* Read from inn, write to out.  We assume inn is opened for reading,
  * and out for writing.  We return 0 on success, -1 on read error, or -2
- * on write error. */
-int copy_file(FILE *inn, FILE *out)
+ * on write error.  inn is always closed by this function, out is closed
+ * only if close_out is true. */
+int copy_file(FILE *inn, FILE *out, bool close_out)
 {
     int retval = 0;
     char buf[BUFSIZ];
     size_t charsread;
+    int (*flush_out_fnc)(FILE *) = (close_out) ? fclose : fflush;
 
     assert(inn != NULL && out != NULL && inn != out);
 
@@ -1564,7 +1566,7 @@ int copy_file(FILE *inn, FILE *out)
 
     if (fclose(inn) == EOF)
        retval = -1;
-    if (fclose(out) == EOF)
+    if (flush_out_fnc(out) == EOF)
        retval = -2;
 
     return retval;
@@ -1655,13 +1657,13 @@ bool write_file(const char *name, FILE *f_open, bool 
tmp,
        int backup_fd;
        FILE *backup_file;
        char *backupname;
-       struct utimbuf filetime;
+       static struct timespec filetime[2];
        int copy_status;
        int backup_cflags;
 
        /* Save the original file's access and modification times. */
-       filetime.actime = openfile->current_stat->st_atime;
-       filetime.modtime = openfile->current_stat->st_mtime;
+       filetime[/* atime */ 0].tv_sec = openfile->current_stat->st_atime;
+       filetime[/* mtime */ 1].tv_sec = openfile->current_stat->st_mtime;
 
        if (f_open == NULL) {
            /* Open the original file to copy to the backup. */
@@ -1790,7 +1792,7 @@ bool write_file(const char *name, FILE *f_open, bool tmp,
 #endif
 
        /* Copy the file. */
-       copy_status = copy_file(f, backup_file);
+       copy_status = copy_file(f, backup_file, /* close_out */ false);
 
        if (copy_status != 0) {
            statusline(ALERT, _("Error reading %s: %s"), realname,
@@ -1799,7 +1801,8 @@ bool write_file(const char *name, FILE *f_open, bool tmp,
        }
 
        /* And set its metadata. */
-       if (utime(backupname, &filetime) == -1 && !ISSET(INSECURE_BACKUP)) {
+       if (futimens(backup_fd, filetime) == -1 && !ISSET(INSECURE_BACKUP)) {
+           fclose(backup_file);
            if (prompt_failed_backupwrite(backupname))
                goto skip_backup;
            statusline(HUSH, _("Error writing backup file %s: %s"),
@@ -1811,6 +1814,7 @@ bool write_file(const char *name, FILE *f_open, bool tmp,
            goto cleanup_and_exit;
        }
 
+       fclose(backup_file);
        free(backupname);
     }
 
@@ -1867,7 +1871,8 @@ bool write_file(const char *name, FILE *f_open, bool tmp,
            }
        }
 
-       if (f_source == NULL || copy_file(f_source, f) != 0) {
+       if (f_source == NULL
+               || copy_file(f_source, f, /* close_out */ true) != 0) {
            statusline(ALERT, _("Error writing temp file: %s"),
                        strerror(errno));
            unlink(tempname);
@@ -1975,7 +1980,7 @@ bool write_file(const char *name, FILE *f_open, bool tmp,
            goto cleanup_and_exit;
        }
 
-       if (copy_file(f_source, f) == -1) {
+       if (copy_file(f_source, f, /* close_out */ true) == -1) {
            statusline(ALERT, _("Error writing %s: %s"), realname,
                        strerror(errno));
            goto cleanup_and_exit;
diff --git a/src/proto.h b/src/proto.h
index 0250ad6..d8255a9 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -298,7 +298,7 @@ void init_backup_dir(void);
 int delete_lockfile(const char *lockfilename);
 int write_lockfile(const char *lockfilename, const char *origfilename, bool 
modified);
 #endif
-int copy_file(FILE *inn, FILE *out);
+int copy_file(FILE *inn, FILE *out, bool close_out);
 bool write_file(const char *name, FILE *f_open, bool tmp,
        kind_of_writing_type method, bool nonamechange);
 #ifndef NANO_TINY
-- 
2.9.3