Re: [PATCH] SELinux: Introduce security_file_ioctl_compat hook

From: Stephen Smalley <stephen.smalley.work@gmail.com>
To: Alfred Piccioni <alpic@google.com>
Cc: Paul Moore <paul@paul-moore.com>,
	Eric Paris <eparis@parisplace.org>,
	stable@vger.kernel.org,  selinux@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] SELinux: Introduce security_file_ioctl_compat hook
Date: Mon, 18 Dec 2023 08:50:10 -0500	[thread overview]
Message-ID: <CAEjxPJ5mpMbTpnP1tbrFs9o2F0ymtaRbBPMMT4xogRmKbtZ2AA@mail.gmail.com> (raw)
In-Reply-To: <CAEjxPJ6G7gHi=kLmgT3m1kDQeTpT1xLexDXbZRBMjJdiWrx=mw@mail.gmail.com>

On Mon, Dec 18, 2023 at 8:46 AM Stephen Smalley
<stephen.smalley.work@gmail.com> wrote:
>
> On Mon, Dec 18, 2023 at 7:43 AM Alfred Piccioni <alpic@google.com> wrote:
> >
> > Some ioctl commands do not require ioctl permission, but are routed to
> > other permissions such as FILE_GETATTR or FILE_SETATTR. This routing is
> > done by comparing the ioctl cmd to a set of 64-bit flags (FS_IOC_*).
> >
> > However, if a 32-bit process is running on a 64-bit kernel, it emmits
>
> s/emmits/emits/
>
> > 32-bit flags (FS_IOC32_*) for certain ioctl operations. These flags are
> > being checked erroneously, which leads to these ioctl operations being
> > routed to the ioctl permission, rather than the correct file
> > permissions.
> >
> > This was also noted in a RED-PEN finding from a while back -
> > "/* RED-PEN how should LSM module know it's handling 32bit? */".
> >
> > This patch introduces a new hook, security_file_ioctl_compat, that
> > replaces security_file_ioctl if the CONFIG_COMPAT flag is on. All
> > current LSMs have been changed to hook into the compat flag.
>
> It doesn't (or shouldn't) replace security_file_ioctl, and the hook
> doesn't appear to be conditional on CONFIG_COMPAT per se.
> It is a new hook that is called from the compat ioctl syscall. The old
> hook continues to be used from the regular ioctl syscall and
> elsewhere.
>
> > Reviewing the three places where we are currently using
> > security_file_ioctl, it appears that only SELinux needs a dedicated
> > compat change; TOMOYO and SMACK appear to be functional without any
> > change.
> >
> > Fixes: 0b24dcb7f2f7 ("Revert "selinux: simplify ioctl checking"")
> > Signed-off-by: Alfred Piccioni <alpic@google.com>
> > Cc: stable@vger.kernel.org
> > ---
> > diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> > index 83ef66644c21..170687b5985b 100644
> > --- a/fs/overlayfs/inode.c
> > +++ b/fs/overlayfs/inode.c
> > @@ -751,7 +751,11 @@ static int ovl_security_fileattr(const struct path *realpath, struct fileattr *f
> >         else
> >                 cmd = fa->fsx_valid ? FS_IOC_FSGETXATTR : FS_IOC_GETFLAGS;
> >
> > +#ifdef CONFIG_COMPAT
> > +       err = security_file_ioctl_compat(file, cmd, 0);
> > +# else
> >         err = security_file_ioctl(file, cmd, 0);
> > +#endif
>
> I don't understand why you made this change, possibly a leftover of an
> earlier version of the patch that tried to replace
> security_file_ioctl() everywhere?

By the way, for extra credit, you could augment the ioctl tests in the
selinux-testsuite to also exercise this new hook and confirm that it
works correctly. See
https://github.com/SELinuxProject/selinux-testsuite particularly
tests/ioctl and policy/test_ioctl.te. Feel free to ask for help on
that.