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Message-ID: <CAEjxPJ6gFh7h5MnEEqTL34_dVEoAmoGfqa01eeYHnOECjkYxBg@mail.gmail.com>
Date: Tue, 12 Sep 2023 08:00:12 -0400
From: Stephen Smalley <stephen.smalley.work@...il.com>
To: Alfred Piccioni <alpic@...gle.com>
Cc: kernel test robot <lkp@...el.com>,
Paul Moore <paul@...l-moore.com>,
Eric Paris <eparis@...isplace.org>,
oe-kbuild-all@...ts.linux.dev, stable@...r.kernel.org,
selinux@...r.kernel.org, linux-kernel@...r.kernel.org,
LSM List <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH V2] SELinux: Check correct permissions for FS_IOC32_*
On Tue, Sep 12, 2023 at 5:00 AM Alfred Piccioni <alpic@...gle.com> wrote:
>
> On Mon, Sep 11, 2023 at 3:49 PM Stephen Smalley
> <stephen.smalley.work@...il.com> wrote:
> >
> > On Mon, Sep 11, 2023 at 9:19 AM Stephen Smalley
> > <stephen.smalley.work@...il.com> wrote:
> > >
> > > On Fri, Sep 8, 2023 at 6:54 PM kernel test robot <lkp@...el.com> wrote:
> > > >
> > > > Hi Alfred,
> > > >
> > > > kernel test robot noticed the following build errors:
> > > >
> > > > [auto build test ERROR on 50a510a78287c15cee644f345ef8bac8977986a7]
> > > >
> > > > url: https://github.com/intel-lab-lkp/linux/commits/Alfred-Piccioni/SELinux-Check-correct-permissions-for-FS_IOC32_/20230906-200131
> > > > base: 50a510a78287c15cee644f345ef8bac8977986a7
> > > > patch link: https://lore.kernel.org/r/20230906115928.3749928-1-alpic%40google.com
> > > > patch subject: [PATCH V2] SELinux: Check correct permissions for FS_IOC32_*
> > > > config: i386-debian-10.3 (https://download.01.org/0day-ci/archive/20230909/202309090600.NSyo7d2q-lkp@intel.com/config)
> > > > compiler: gcc-12 (Debian 12.2.0-14) 12.2.0
> > > > reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20230909/202309090600.NSyo7d2q-lkp@intel.com/reproduce)
> > > >
> > > > If you fix the issue in a separate patch/commit (i.e. not just a new version of
> > > > the same patch/commit), kindly add following tags
> > > > | Reported-by: kernel test robot <lkp@...el.com>
> > > > | Closes: https://lore.kernel.org/oe-kbuild-all/202309090600.NSyo7d2q-lkp@intel.com/
> > > >
> > > > All errors (new ones prefixed by >>):
> > > >
> > > > security/selinux/hooks.c: In function 'selinux_file_ioctl':
> > > > >> security/selinux/hooks.c:3647:9: error: duplicate case value
> > > > 3647 | case FS_IOC32_GETFLAGS:
> > > > | ^~~~
> > > > security/selinux/hooks.c:3645:9: note: previously used here
> > > > 3645 | case FS_IOC_GETFLAGS:
> > > > | ^~~~
> > > > security/selinux/hooks.c:3648:9: error: duplicate case value
> > > > 3648 | case FS_IOC32_GETVERSION:
> > > > | ^~~~
> > > > security/selinux/hooks.c:3646:9: note: previously used here
> > > > 3646 | case FS_IOC_GETVERSION:
> > > > | ^~~~
> > > > security/selinux/hooks.c:3654:9: error: duplicate case value
> > > > 3654 | case FS_IOC32_SETFLAGS:
> > > > | ^~~~
> > > > security/selinux/hooks.c:3652:9: note: previously used here
> > > > 3652 | case FS_IOC_SETFLAGS:
> > > > | ^~~~
> > > > security/selinux/hooks.c:3655:9: error: duplicate case value
> > > > 3655 | case FS_IOC32_SETVERSION:
> > > > | ^~~~
> > > > security/selinux/hooks.c:3653:9: note: previously used here
> > > > 3653 | case FS_IOC_SETVERSION:
> > > > | ^~~~
> > >
> > > Not sure of the right way to fix this while addressing the original
> > > issue that this patch was intended to fix. Looking in fs/ioctl.c, I
> > > see that the some FS_IOC32 values are remapped to the corresponding
> > > FS_IOC values by the compat ioctl syscall entrypoint. Also notice this
> > > comment there:
> > >
> > > /* RED-PEN how should LSM module know it's handling 32bit? */
> > > error = security_file_ioctl(f.file, cmd, arg);
> > > if (error)
> > > goto out;
> > >
> > > So perhaps this is a defect in LSM that needs to be addressed?
> >
> > Note btw that some of the 32-bit ioctl commands are only handled in
> > the fs-specific compat_ioctl routines, e.g. ext4_compat_ioctl()
> > handles EXT4_IOC32_GETVERSION == FS_IOC32_GETVERSION and ditto for
> > _SETVERSION.
> >
> > >
> > >
> > > >
> > > >
> > > > vim +3647 security/selinux/hooks.c
> > > >
> > > > 3634
> > > > 3635 static int selinux_file_ioctl(struct file *file, unsigned int cmd,
> > > > 3636 unsigned long arg)
> > > > 3637 {
> > > > 3638 const struct cred *cred = current_cred();
> > > > 3639 int error = 0;
> > > > 3640
> > > > 3641 switch (cmd) {
> > > > 3642 case FIONREAD:
> > > > 3643 case FIBMAP:
> > > > 3644 case FIGETBSZ:
> > > > 3645 case FS_IOC_GETFLAGS:
> > > > 3646 case FS_IOC_GETVERSION:
> > > > > 3647 case FS_IOC32_GETFLAGS:
> > > > 3648 case FS_IOC32_GETVERSION:
> > > > 3649 error = file_has_perm(cred, file, FILE__GETATTR);
> > > > 3650 break;
> > > > 3651
> > > > 3652 case FS_IOC_SETFLAGS:
> > > > 3653 case FS_IOC_SETVERSION:
> > > > 3654 case FS_IOC32_SETFLAGS:
> > > > 3655 case FS_IOC32_SETVERSION:
> > > > 3656 error = file_has_perm(cred, file, FILE__SETATTR);
> > > > 3657 break;
> > > > 3658
> > > > 3659 /* sys_ioctl() checks */
> > > > 3660 case FIONBIO:
> > > > 3661 case FIOASYNC:
> > > > 3662 error = file_has_perm(cred, file, 0);
> > > > 3663 break;
> > > > 3664
> > > > 3665 case KDSKBENT:
> > > > 3666 case KDSKBSENT:
> > > > 3667 error = cred_has_capability(cred, CAP_SYS_TTY_CONFIG,
> > > > 3668 CAP_OPT_NONE, true);
> > > > 3669 break;
> > > > 3670
> > > > 3671 case FIOCLEX:
> > > > 3672 case FIONCLEX:
> > > > 3673 if (!selinux_policycap_ioctl_skip_cloexec())
> > > > 3674 error = ioctl_has_perm(cred, file, FILE__IOCTL, (u16) cmd);
> > > > 3675 break;
> > > > 3676
> > > > 3677 /* default case assumes that the command will go
> > > > 3678 * to the file's ioctl() function.
> > > > 3679 */
> > > > 3680 default:
> > > > 3681 error = ioctl_has_perm(cred, file, FILE__IOCTL, (u16) cmd);
> > > > 3682 }
> > > > 3683 return error;
> > > > 3684 }
> > > > 3685
>
> Hey Stephen,
>
> Thanks for looking into it a bit deeper! This seems a bit of a pickle.
> I can think of a few somewhat hacky ways to fix this.
>
> I can just set the flags to check `if FS_IOC32_*; set FS_IOC_*;`,
> which is the quickest but kinda hacky.
>
> I can go with the other plan of dropping the irrelevant bytes from the
> cmd code, so all codes will be read as u16. This effectively does the
> same thing, but may be unclear.
>
> I can also look into whether this can be solved at the LSM or a higher
> level. Perhaps the filesystems setting `if FS_IOC32_*; set FS_IOC_*;`
> is a hint that something else interesting is going wrong.
>
> I'll spend a little time thinking and investigating and get back with
> a more concrete solution. I'll also need to do a bit more robust
> testing; it built on my machine!
Likewise for me; I don't generally try building for 32-bit systems.
Remapping FS_IOC32_* to FS_IOC_* in selinux_file_ioctl() seems
reasonable to me although optimally that would be conditional on
whether selinux_file_ioctl() is being called from the compat ioctl
syscall (e.g. adding a flag to the LSM hook to indicate this or using
a separate hook for it). Otherwise we might misinterpret some other
ioctl on 64-bit.
If we didn't have compatibility requirements, it would be tempting to
just get rid of all the special case ioctl command handling in
selinux_file_ioctl() and let ioctl_has_perm() handle them all with the
extended ioctl permissions support. But that would require a SELinux
policy cap to switch it on conditionally for compatibility at least
and not sure anyone is willing to refactor their policies accordingly.
>
> Thanks!
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