On Fri, Sep 01, 2023 at 11:34:32AM -0700, Kees Cook wrote: > On Fri, Sep 01, 2023 at 04:50:53PM +0200, Michał Cłapiński wrote: > > On Fri, Sep 1, 2023 at 2:56 PM Christian Brauner <brauner@xxxxxxxxxx> wrote: > > > > > > On Thu, Aug 31, 2023 at 10:36:46PM +0200, Michal Clapinski wrote: > > > > Add a way to check if an fd points to the memfd's original open fd > > > > (the one created by memfd_create). > > > > Useful because only the original open fd can be both writable and > > > > executable. > > > > > > > > Signed-off-by: Michal Clapinski <mclapinski@xxxxxxxxxx> > > > > --- > > > > fs/fcntl.c | 3 +++ > > > > include/uapi/linux/fcntl.h | 9 +++++++++ > > > > 2 files changed, 12 insertions(+) > > > > > > > > diff --git a/fs/fcntl.c b/fs/fcntl.c > > > > index e871009f6c88..301527e07a4d 100644 > > > > --- a/fs/fcntl.c > > > > +++ b/fs/fcntl.c > > > > @@ -419,6 +419,9 @@ static long do_fcntl(int fd, unsigned int cmd, unsigned long arg, > > > > case F_SET_RW_HINT: > > > > err = fcntl_rw_hint(filp, cmd, arg); > > > > break; > > > > + case F_CHECK_ORIGINAL_MEMFD: > > > > + err = !(filp->f_mode & FMODE_WRITER); > > > > + break; > > > > > > Honestly, make this an ioctl on memfds. This is so specific that it > > > really doesn't belong into fcntl(). > > > > I've never touched ioctls but if I'm correct, I can't just add it to > > memfd. I would have to add it to the underlying fs, so hugetlbfs and > > shmem (which I think can be defined as ramfs so also there). File > > sealing fcntl is already memfd specific. Are you sure ioctl will be a > > better idea? fcntl() should be generic. Frankly, the sealing stuff should've gone into an ioctl as well and only upgraded to a fcntl() once multiple fd types support it. > > Does this check "mean" anything for other files? Because if it's > generically useful (and got renamed) it maybe would be right for > fcntl... For regular files it just means that the file has gotten write access to the underlying fs and we use this flag to release the necessary refcounts/protections once the file is closed. If this check has any meaning beyond that than it only has meaning for memfd. I'm also not sure why this checks FMODE_WRITER and not FMODE_WRITE as FMODE_WRITER is almost an entirely internal thing that only very specific codepaths need to know about.
