From: Tejun Heo <tj@xxxxxxxxxx> commit e57457641613fef0d147ede8bd6a3047df588b95 upstream. cgroup process migration permission checks are performed at write time as whether a given operation is allowed or not is dependent on the content of the write - the PID. This currently uses current's cgroup namespace which is a potential security weakness as it may allow scenarios where a less privileged process tricks a more privileged one into writing into a fd that it created. This patch makes cgroup remember the cgroup namespace at the time of open and uses it for migration permission checks instad of current's. Note that this only applies to cgroup2 as cgroup1 doesn't have namespace support. This also fixes a use-after-free bug on cgroupns reported in https://lore.kernel.org/r/00000000000048c15c05d0083397@xxxxxxxxxx Note that backporting this fix also requires the preceding patch. Reported-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxx> Cc: Michal Koutný <mkoutny@xxxxxxxx> Cc: Oleg Nesterov <oleg@xxxxxxxxxx> Reviewed-by: Michal Koutný <mkoutny@xxxxxxxx> Reported-by: syzbot+50f5cf33a284ce738b62@xxxxxxxxxxxxxxxxxxxxxxxxx Link: https://lore.kernel.org/r/00000000000048c15c05d0083397@xxxxxxxxxx Fixes: 5136f6365ce3 ("cgroup: implement "nsdelegate" mount option") Signed-off-by: Tejun Heo <tj@xxxxxxxxxx> [mkoutny: v5.10: duplicate ns check in procs/threads write handler, adjust context] Signed-off-by: Michal Koutný <mkoutny@xxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- kernel/cgroup/cgroup-internal.h | 2 ++ kernel/cgroup/cgroup.c | 32 ++++++++++++++++++++++---------- 2 files changed, 24 insertions(+), 10 deletions(-) --- a/kernel/cgroup/cgroup-internal.h +++ b/kernel/cgroup/cgroup-internal.h @@ -68,6 +68,8 @@ static inline struct cgroup_fs_context * struct cgroup_pidlist; struct cgroup_file_ctx { + struct cgroup_namespace *ns; + struct { void *trigger; } psi; --- a/kernel/cgroup/cgroup.c +++ b/kernel/cgroup/cgroup.c @@ -3700,14 +3700,19 @@ static int cgroup_file_open(struct kernf ctx = kzalloc(sizeof(*ctx), GFP_KERNEL); if (!ctx) return -ENOMEM; + + ctx->ns = current->nsproxy->cgroup_ns; + get_cgroup_ns(ctx->ns); of->priv = ctx; if (!cft->open) return 0; ret = cft->open(of); - if (ret) + if (ret) { + put_cgroup_ns(ctx->ns); kfree(ctx); + } return ret; } @@ -3718,13 +3723,14 @@ static void cgroup_file_release(struct k if (cft->release) cft->release(of); + put_cgroup_ns(ctx->ns); kfree(ctx); } static ssize_t cgroup_file_write(struct kernfs_open_file *of, char *buf, size_t nbytes, loff_t off) { - struct cgroup_namespace *ns = current->nsproxy->cgroup_ns; + struct cgroup_file_ctx *ctx = of->priv; struct cgroup *cgrp = of->kn->parent->priv; struct cftype *cft = of->kn->priv; struct cgroup_subsys_state *css; @@ -3741,7 +3747,7 @@ static ssize_t cgroup_file_write(struct */ if ((cgrp->root->flags & CGRP_ROOT_NS_DELEGATE) && !(cft->flags & CFTYPE_NS_DELEGATABLE) && - ns != &init_cgroup_ns && ns->root_cset->dfl_cgrp == cgrp) + ctx->ns != &init_cgroup_ns && ctx->ns->root_cset->dfl_cgrp == cgrp) return -EPERM; if (cft->write) @@ -4726,9 +4732,9 @@ static int cgroup_may_write(const struct static int cgroup_procs_write_permission(struct cgroup *src_cgrp, struct cgroup *dst_cgrp, - struct super_block *sb) + struct super_block *sb, + struct cgroup_namespace *ns) { - struct cgroup_namespace *ns = current->nsproxy->cgroup_ns; struct cgroup *com_cgrp = src_cgrp; int ret; @@ -4757,11 +4763,12 @@ static int cgroup_procs_write_permission static int cgroup_attach_permissions(struct cgroup *src_cgrp, struct cgroup *dst_cgrp, - struct super_block *sb, bool threadgroup) + struct super_block *sb, bool threadgroup, + struct cgroup_namespace *ns) { int ret = 0; - ret = cgroup_procs_write_permission(src_cgrp, dst_cgrp, sb); + ret = cgroup_procs_write_permission(src_cgrp, dst_cgrp, sb, ns); if (ret) return ret; @@ -4778,6 +4785,7 @@ static int cgroup_attach_permissions(str static ssize_t cgroup_procs_write(struct kernfs_open_file *of, char *buf, size_t nbytes, loff_t off) { + struct cgroup_file_ctx *ctx = of->priv; struct cgroup *src_cgrp, *dst_cgrp; struct task_struct *task; ssize_t ret; @@ -4798,7 +4806,8 @@ static ssize_t cgroup_procs_write(struct spin_unlock_irq(&css_set_lock); ret = cgroup_attach_permissions(src_cgrp, dst_cgrp, - of->file->f_path.dentry->d_sb, true); + of->file->f_path.dentry->d_sb, true, + ctx->ns); if (ret) goto out_finish; @@ -4820,6 +4829,7 @@ static void *cgroup_threads_start(struct static ssize_t cgroup_threads_write(struct kernfs_open_file *of, char *buf, size_t nbytes, loff_t off) { + struct cgroup_file_ctx *ctx = of->priv; struct cgroup *src_cgrp, *dst_cgrp; struct task_struct *task; ssize_t ret; @@ -4843,7 +4853,8 @@ static ssize_t cgroup_threads_write(stru /* thread migrations follow the cgroup.procs delegation rule */ ret = cgroup_attach_permissions(src_cgrp, dst_cgrp, - of->file->f_path.dentry->d_sb, false); + of->file->f_path.dentry->d_sb, false, + ctx->ns); if (ret) goto out_finish; @@ -6023,7 +6034,8 @@ static int cgroup_css_set_fork(struct ke goto err; ret = cgroup_attach_permissions(cset->dfl_cgrp, dst_cgrp, sb, - !(kargs->flags & CLONE_THREAD)); + !(kargs->flags & CLONE_THREAD), + current->nsproxy->cgroup_ns); if (ret) goto err;
