Re: [PATCH] libsemanage: fsync before rename

[Stephen Smalley <stephen.smalley.work@gmail.com>]

On Wed, May 13, 2020 at 2:52 PM Nicolas Iooss <nicolas.iooss@m4x.org> wrote:
>
> On Wed, May 13, 2020 at 8:46 PM Smalley <stephen.smalley.work@gmail.com> wrote:
> >
> > From: Stephen Smalley <stephen.smalley.work@gmail.com>
> >
> > Prior to rename(2)'ing new files into place, fsync(2) them to ensure
> > the contents will be fully written prior to rename.  While we are here,
> > also fix checking of write(2) to detect short writes.  This code could
> > be more generally improved but keeping to the minimal changes required
> > to fix this bug.
> >
> > Fixes: https://github.com/SELinuxProject/selinux/issues/237
> > Signed-off-by: Stephen Smalley <stephen.smalley.work@gmail.com>
> > ---
> >  libsemanage/src/semanage_store.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/libsemanage/src/semanage_store.c b/libsemanage/src/semanage_store.c
> > index 859c0a22..3cac36d4 100644
> > --- a/libsemanage/src/semanage_store.c
> > +++ b/libsemanage/src/semanage_store.c
> > @@ -735,7 +735,7 @@ int semanage_copy_file(const char *src, const char *dst, mode_t mode)
> >         }
> >         umask(mask);
> >         while (retval == 0 && (amount_read = read(in, buf, sizeof(buf))) > 0) {
> > -               if (write(out, buf, amount_read) < 0) {
> > +               if (write(out, buf, amount_read) != amount_read) {
> >                         errsv = errno;
> >                         retval = -1;
> >                 }
>
> If I remember correctly, errno is not defined if a short write
> happens. If this is confirmed and if you want to keep the patch short,
> you could for example use errsv = EIO if write() returned a value
> different from -1 and from amount_read.

True.  It also occurred to me that this is too heavyweight given how
widely semanage_copy_file() is used within libsemanage; performing a
fsync(2) for every one of these file copies will be very expensive.
We only really care about it when copying the final files to
/etc/selinux, not for the copying under /var/lib/selinux IIUC.  So I
guess I need a bool argument or similar to semanage_copy_file() to
indicate when a fsync is required.  Even with this change, installing
the final SELinux policy files under /etc/selinux won't be fully
atomic; one can still end up with a mix of old and new (e.g. new
policy.32 with old file_contexts).  But that's out of scope for this
particular bug.