From: Tadeusz Struk tadeusz.struk@linaro.org
commit 4c46091ee985ae84c60c5e95055d779fcd291d87 upstream.
Syzbot found a Use After Free bug in compute_effective_progs(). The reproducer creates a number of BPF links, and causes a fault injected alloc to fail, while calling bpf_link_detach on them. Link detach triggers the link to be freed by bpf_link_free(), which calls __cgroup_bpf_detach() and update_effective_progs(). If the memory allocation in this function fails, the function restores the pointer to the bpf_cgroup_link on the cgroup list, but the memory gets freed just after it returns. After this, every subsequent call to update_effective_progs() causes this already deallocated pointer to be dereferenced in prog_list_length(), and triggers KASAN UAF error.
To fix this issue don't preserve the pointer to the prog or link in the list, but remove it and replace it with a dummy prog without shrinking the table. The subsequent call to __cgroup_bpf_detach() or __cgroup_bpf_detach() will correct it.
Fixes: af6eea57437a ("bpf: Implement bpf_link-based cgroup BPF program attachment") Reported-by: syzbot+f264bffdfbd5614f3bb2@syzkaller.appspotmail.com Signed-off-by: Tadeusz Struk tadeusz.struk@linaro.org Signed-off-by: Andrii Nakryiko andrii@kernel.org Cc: stable@vger.kernel.org Link: https://syzkaller.appspot.com/bug?id=8ebf179a95c2a2670f7cf1ba62429ec044369db... Link: https://lore.kernel.org/bpf/20220517180420.87954-1-tadeusz.struk@linaro.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- kernel/bpf/cgroup.c | 70 ++++++++++++++++++++++++++++++++++++++++++++-------- 1 file changed, 60 insertions(+), 10 deletions(-)
--- a/kernel/bpf/cgroup.c +++ b/kernel/bpf/cgroup.c @@ -748,6 +748,60 @@ static struct bpf_prog_list *find_detach }
/** + * purge_effective_progs() - After compute_effective_progs fails to alloc new + * cgrp->bpf.inactive table we can recover by + * recomputing the array in place. + * + * @cgrp: The cgroup which descendants to travers + * @prog: A program to detach or NULL + * @link: A link to detach or NULL + * @atype: Type of detach operation + */ +static void purge_effective_progs(struct cgroup *cgrp, struct bpf_prog *prog, + struct bpf_cgroup_link *link, + enum cgroup_bpf_attach_type atype) +{ + struct cgroup_subsys_state *css; + struct bpf_prog_array *progs; + struct bpf_prog_list *pl; + struct list_head *head; + struct cgroup *cg; + int pos; + + /* recompute effective prog array in place */ + css_for_each_descendant_pre(css, &cgrp->self) { + struct cgroup *desc = container_of(css, struct cgroup, self); + + if (percpu_ref_is_zero(&desc->bpf.refcnt)) + continue; + + /* find position of link or prog in effective progs array */ + for (pos = 0, cg = desc; cg; cg = cgroup_parent(cg)) { + if (pos && !(cg->bpf.flags[atype] & BPF_F_ALLOW_MULTI)) + continue; + + head = &cg->bpf.progs[atype]; + list_for_each_entry(pl, head, node) { + if (!prog_list_prog(pl)) + continue; + if (pl->prog == prog && pl->link == link) + goto found; + pos++; + } + } +found: + BUG_ON(!cg); + progs = rcu_dereference_protected( + desc->bpf.effective[atype], + lockdep_is_held(&cgroup_mutex)); + + /* Remove the program from the array */ + WARN_ONCE(bpf_prog_array_delete_safe_at(progs, pos), + "Failed to purge a prog from array at index %d", pos); + } +} + +/** * __cgroup_bpf_detach() - Detach the program or link from a cgroup, and * propagate the change to descendants * @cgrp: The cgroup which descendants to traverse @@ -766,7 +820,6 @@ static int __cgroup_bpf_detach(struct cg struct bpf_prog_list *pl; struct list_head *progs; u32 flags; - int err;
atype = to_cgroup_bpf_attach_type(type); if (atype < 0) @@ -788,9 +841,12 @@ static int __cgroup_bpf_detach(struct cg pl->prog = NULL; pl->link = NULL;
- err = update_effective_progs(cgrp, atype); - if (err) - goto cleanup; + if (update_effective_progs(cgrp, atype)) { + /* if update effective array failed replace the prog with a dummy prog*/ + pl->prog = old_prog; + pl->link = link; + purge_effective_progs(cgrp, old_prog, link, atype); + }
/* now can actually delete it from this cgroup list */ list_del(&pl->node); @@ -802,12 +858,6 @@ static int __cgroup_bpf_detach(struct cg bpf_prog_put(old_prog); static_branch_dec(&cgroup_bpf_enabled_key[atype]); return 0; - -cleanup: - /* restore back prog or link */ - pl->prog = old_prog; - pl->link = link; - return err; }
static int cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,