s3:passdb: smbpasswd reset permissions only if not 0600

Browsing files or download files from samba server, smbd would check user's
id to decide whether this user could access these files, by lookup user's
information from the password file (e.g. /usr/local/samba/private/smbpasswd).
smbd might goes through startsmbfilepwent(), this api calls [f]chmod() to
make sure the password file has valid permissions 0600.

Consider a scenario: we are doing a read performance benchmark about
downloading a bunch of files (e.g. a thousand files) from a samba server,
monitoring file system i/o activities counters, and expecting that should
be only read operations on file system because this is just downloading, no
uploading is involved. But actually found that still write operations on file
system, because smbd lookup user and always reset 0600 permissions on password
file while access each file, it makes dirty pages (inode modification) in ram,
later triggered a kernel journal daemon to sync dirty pages into back storage
(e.g. ext3 kjournald, or ext4 jbd2).
This looks like not friendly for read performance benchmark if it happened on
an entry-level systems with much less memory and limited computation power,
because dirty pages syncing in the meantime slows down read performance.

This patch adds fstat() before [f]chmod(), it would check whether password
file has valid permissions 0600 or not. If 0600 smbd would bypass [f]chmod()
to avoid making dirty pages on file systems. If not 0600 smbd would warn and
go through [f]chmod() to set valid permissions 0600 to password file as
earlier days.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15555

Signed-off-by: Jones Syue <jonessyue@qnap.com>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>
Autobuild-User(master): Volker Lendecke <vl@samba.org>
Autobuild-Date(master): Thu Jan 18 10:28:19 UTC 2024 on atb-devel-224

(cherry picked from commit c82a267b2a1b7617e818548aa486b7cfbda74657)

Autobuild-User(v4-18-test): Jule Anger <janger@samba.org>
Autobuild-Date(v4-18-test): Mon Jan 22 10:09:52 UTC 2024 on atb-devel-224

diff --git a/source3/passdb/pdb_smbpasswd.c b/source3/passdb/pdb_smbpasswd.c
index 515e5f9f84d321ce770fc8cfe1fcec744576e338..fd611f614b8dd8cc256ab97aa1ad6da1db0d1cc4 100644 (file)
--- a/source3/passdb/pdb_smbpasswd.c
+++ b/source3/passdb/pdb_smbpasswd.c
@@ -192,6 +192,7 @@ static FILE *startsmbfilepwent(const char *pfile, enum pwf_access_type type, int
        const char *open_mode = NULL;
        int race_loop = 0;
        int lock_type = F_RDLCK;
+       struct stat st;
 
        if (!*pfile) {
                DEBUG(0, ("startsmbfilepwent: No SMB password file set\n"));
@@ -324,19 +325,38 @@ Error was %s\n", pfile, strerror(errno)));
        /* Set a buffer to do more efficient reads */
        setvbuf(fp, (char *)NULL, _IOFBF, 1024);
 
-       /* Make sure it is only rw by the owner */
-#ifdef HAVE_FCHMOD
-       if(fchmod(fileno(fp), S_IRUSR|S_IWUSR) == -1) {
-#else
-       if(chmod(pfile, S_IRUSR|S_IWUSR) == -1) {
-#endif
-               DEBUG(0, ("startsmbfilepwent_internal: failed to set 0600 permissions on password file %s. \
-Error was %s\n.", pfile, strerror(errno) ));
+       /* Ensure we have a valid stat. */
+       if (fstat(fileno(fp), &st) != 0) {
+               DBG_ERR("Unable to fstat file %s. Error was %s\n",
+                       pfile,
+                       strerror(errno));
                pw_file_unlock(fileno(fp), lock_depth);
                fclose(fp);
                return NULL;
        }
 
+       /* If file has invalid permissions != 0600, then [f]chmod(). */
+       if ((st.st_mode & 0777) != (S_IRUSR|S_IWUSR)) {
+               DBG_WARNING("file %s has invalid permissions 0%o should "
+                           "be 0600.\n",
+                           pfile,
+                           (unsigned int)st.st_mode & 0777);
+               /* Make sure it is only rw by the owner */
+#ifdef HAVE_FCHMOD
+               if (fchmod(fileno(fp), S_IRUSR|S_IWUSR) == -1) {
+#else
+               if (chmod(pfile, S_IRUSR|S_IWUSR) == -1) {
+#endif
+                       DBG_ERR("Failed to set 0600 permissions on password file %s. "
+                               "Error was %s\n.",
+                               pfile,
+                               strerror(errno));
+                       pw_file_unlock(fileno(fp), lock_depth);
+                       fclose(fp);
+                       return NULL;
+               }
+       }
+
        /* We have a lock on the file. */
        return fp;
 }