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. Cc: "Alexei Starovoitov" <ast@xxxxxxxxxx> Cc: "Daniel Borkmann" <daniel@xxxxxxxxxxxxx> Cc: "Andrii Nakryiko" <andrii@xxxxxxxxxx> Cc: "Martin KaFai Lau" <kafai@xxxxxx> Cc: "Song Liu" <songliubraving@xxxxxx> Cc: "Yonghong Song" <yhs@xxxxxx> Cc: "John Fastabend" <john.fastabend@xxxxxxxxx> Cc: "KP Singh" <kpsingh@xxxxxxxxxx> Cc: <netdev@xxxxxxxxxxxxxxx> Cc: <bpf@xxxxxxxxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Cc: <linux-kernel@xxxxxxxxxxxxxxx> Link: https://syzkaller.appspot.com/bug?id=8ebf179a95c2a2670f7cf1ba62429ec044369db4 Fixes: af6eea57437a ("bpf: Implement bpf_link-based cgroup BPF program attachment") Reported-by: <syzbot+f264bffdfbd5614f3bb2@xxxxxxxxxxxxxxxxxxxxxxxxx> Signed-off-by: Tadeusz Struk <tadeusz.struk@xxxxxxxxxx> --- v2: Add a fall back path that removes a prog from the effective progs table in case detach fails to allocate memory in compute_effective_progs(). v3: Implement the fallback in a separate function purge_effective_progs v4: Changed purge_effective_progs() to manipulate the array in a similar way how replace_effective_prog() does it. --- kernel/bpf/cgroup.c | 68 +++++++++++++++++++++++++++++++++++++++------ 1 file changed, 60 insertions(+), 8 deletions(-) diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c index 128028efda64..6f1a6160c99e 100644 --- a/kernel/bpf/cgroup.c +++ b/kernel/bpf/cgroup.c @@ -681,6 +681,60 @@ static struct bpf_prog_list *find_detach_entry(struct list_head *progs, return ERR_PTR(-ENOENT); } +/** + * 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 @@ -723,8 +777,12 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog, pl->link = NULL; err = update_effective_progs(cgrp, atype); - if (err) - goto cleanup; + if (err) { + /* If update affective 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); @@ -736,12 +794,6 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog, 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, -- 2.36.1
