From: Ondrej Mosnacek omosnace@redhat.com
[ Upstream commit 2cbdcb882f97a45f7475c67ac6257bbc16277dfe ]
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.
Fixes: 93faccbbfa95 ("fs: Better permission checking for submounts") Signed-off-by: Ondrej Mosnacek omosnace@redhat.com Signed-off-by: Paul Moore paul@paul-moore.com Signed-off-by: Sasha Levin sashal@kernel.org --- 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 f5d304736852..d6b9ed34ceae 100644 --- a/security/selinux/hooks.c +++ b/security/selinux/hooks.c @@ -2820,7 +2820,7 @@ 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;