On Wed, Nov 21, 2018 at 1:41 PM Ondrej Mosnacek <omosnace@xxxxxxxxxx> wrote: > On Tue, Nov 20, 2018 at 11:09 PM Paul Moore <paul@xxxxxxxxxxxxxx> wrote: > > On Fri, Nov 16, 2018 at 8:12 AM Ondrej Mosnacek <omosnace@xxxxxxxxxx> wrote: > > > If a superblock has the MS_SUBMOUNT flag set, we should always allow > > > mounting it. These mounts are done automatically by the kernel either as > > > part of mounting some parent mount (e.g. debugfs always mounts tracefs > > > under "tracing" for compatibility) or they are mounted automatically as > > > needed on subdirectory accesses (e.g. NFS crossmnt mounts). Since such > > > automounts are either an implicit consequence of the parent mount (which > > > is already checked) or they can happen during regular accesses (where it > > > doesn't make sense to check against the current task's context), the > > > mount permission check should be skipped for them. > > > > > > Without this patch, attempts to access contents of an automounted > > > directory can cause unexpected SELinux denials. > > > > > > In the current kernel tree, the MS_SUBMOUNT flag is set only via > > > vfs_submount(), which is called only from the following places: > > > - AFS, when automounting special "symlinks" referencing other cells > > > - CIFS, when automounting "referrals" > > > - NFS, when automounting subtrees > > > - debugfs, when automounting tracefs > > > > > > In all cases the submounts are meant to be transparent to the user and > > > it makes sense that if mounting the master is allowed, then so should be > > > the automounts. Note that CAP_SYS_ADMIN capability checking is already > > > skipped for (SB_KERNMOUNT|SB_SUBMOUNT) in: > > > - sget_userns() in fs/super.c: > > > if (!(flags & (SB_KERNMOUNT|SB_SUBMOUNT)) && > > > !(type->fs_flags & FS_USERNS_MOUNT) && > > > !capable(CAP_SYS_ADMIN)) > > > return ERR_PTR(-EPERM); > > > - sget() in fs/super.c: > > > /* Ensure the requestor has permissions over the target filesystem */ > > > if (!(flags & (SB_KERNMOUNT|SB_SUBMOUNT)) && !ns_capable(user_ns, CAP_SYS_ADMIN)) > > > return ERR_PTR(-EPERM); > > > > > > Verified internally on patched RHEL 7.6 with a reproducer using > > > NFS+httpd and selinux-tesuite. > > > > I think this all sounds reasonable, but please verify this with an > > upstream kernel. Upstream our focus is on the upstream kernel > > (surprise!), downstream RHEL is your responsibility, not ours :) > > I tested on RHEL because that's what I can do most conveniently. I > don't have a very good workflow/environment for complex testing on > upstream right now. I don't expect the results to be any different on > the upstream kernel, but I understand your concern. I have been > thinking about some patch testing automation using Fedora Rawhide (I > hope that's close enough to upstream at least :), so I guess it's time > to get scriptin'... I have now tested it on Fedora Rawhide with a scratch kernel with this patch applied [1] (x86_64 only). I ran the whole selinux-testsuite with the submount test [2] and everything passed (except for the known overlay failures and skipped binder test): domain_trans/test ........... ok entrypoint/test ............. ok execshare/test .............. ok exectrace/test .............. ok execute_no_trans/test ....... ok fdreceive/test .............. ok inherit/test ................ ok link/test ................... ok mkdir/test .................. ok msg/test .................... ok open/test ................... ok ptrace/test ................. ok readlink/test ............... ok relabel/test ................ ok rename/test ................. ok rxdir/test .................. ok sem/test .................... ok setattr/test ................ ok setnice/test ................ ok shm/test .................... ok sigkill/test ................ ok stat/test ................... ok sysctl/test ................. ok task_create/test ............ ok task_setnice/test ........... ok task_setscheduler/test ...... ok task_getscheduler/test ...... ok task_getsid/test ............ ok task_getpgid/test ........... ok task_setpgid/test ........... ok file/test ................... ok ioctl/test .................. ok capable_file/test ........... ok capable_net/test ............ ok capable_sys/test ............ ok dyntrans/test ............... ok dyntrace/test ............... ok bounds/test ................. ok nnp_nosuid/test ............. ok mmap/test ................... ok unix_socket/test ............ ok inet_socket/test ............ ok overlay/test ................ 62/119 # Failed test at overlay/test line 275. # Failed test at overlay/test line 293. overlay/test ................ 97/119 # Failed test at overlay/test line 547. # Failed test at overlay/test line 622. # Looks like you failed 4 tests of 119. overlay/test ................ Dubious, test returned 4 (wstat 1024, 0x400) Failed 4/119 subtests checkreqprot/test ........... ok mqueue/test ................. ok mac_admin/test .............. ok atsecure/test ............... ok submount/test ............... ok cap_userns/test ............. ok extended_socket_class/test .. ok sctp/test ................... ok netlink_socket/test ......... ok prlimit/test ................ ok binder/test ................. skipped: Binder not supported by kernel Test Summary Report ------------------- overlay/test (Wstat: 1024 Tests: 119 Failed: 4) Failed tests: 81, 83, 107, 112 Non-zero exit status: 4 Files=54, Tests=615, 133 wallclock secs ( 0.45 usr 0.28 sys + 3.37 cusr 11.14 csys = 15.24 CPU) Result: FAIL Failed 1/54 test programs. 4/615 subtests failed. [1] https://koji.fedoraproject.org/koji/taskinfo?taskID=31037298 [2] https://github.com/SELinuxProject/selinux-testsuite/pull/43 > > I was hoping to get some independent testing after porting the test to > the selinux-testsuite... (But I get it, the burden of proof is on my > side...) > > > > > > Fixes: 93faccbbfa95 ("fs: Better permission checking for submounts") > > > Signed-off-by: Ondrej Mosnacek <omosnace@xxxxxxxxxx> > > > --- > > > security/selinux/hooks.c | 2 +- > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c > > > index 7ce683259357..7ce012d9ec51 100644 > > > --- a/security/selinux/hooks.c > > > +++ b/security/selinux/hooks.c > > > @@ -2934,7 +2934,7 @@ static int selinux_sb_kern_mount(struct super_block *sb, int flags, void *data) > > > return rc; > > > > > > /* Allow all mounts performed by the kernel */ > > > - if (flags & MS_KERNMOUNT) > > > + if (flags & (MS_KERNMOUNT | MS_SUBMOUNT)) > > > return 0; > > > > > > ad.type = LSM_AUDIT_DATA_DENTRY; > > > > -- > > paul moore > > www.paul-moore.com > -- > Ondrej Mosnacek <omosnace at redhat dot com> > Associate Software Engineer, Security Technologies > Red Hat, Inc. -- Ondrej Mosnacek <omosnace at redhat dot com> Associate Software Engineer, Security Technologies Red Hat, Inc.
