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 don't preserve the pointer to the link on the cgroup list in __cgroup_bpf_detach(), but proceed with the cleanup and retry calling update_effective_progs() again afterwards. 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> --- kernel/bpf/cgroup.c | 25 ++++++++++++++----------- 1 file changed, 14 insertions(+), 11 deletions(-) diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c index 128028efda64..b6307337a3c7 100644 --- a/kernel/bpf/cgroup.c +++ b/kernel/bpf/cgroup.c @@ -723,10 +723,11 @@ 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; - - /* now can actually delete it from this cgroup list */ + /* + * Proceed regardless of error. The link and/or prog will be freed + * just after this function returns so just delete it from this + * cgroup list and retry calling update_effective_progs again later. + */ list_del(&pl->node); kfree(pl); if (list_empty(progs)) @@ -735,12 +736,11 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog, if (old_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; + /* In case of error call update_effective_progs again */ + if (err) + err = update_effective_progs(cgrp, atype); + return err; } @@ -881,6 +881,7 @@ static void bpf_cgroup_link_release(struct bpf_link *link) struct bpf_cgroup_link *cg_link = container_of(link, struct bpf_cgroup_link, link); struct cgroup *cg; + int err; /* link might have been auto-detached by dying cgroup already, * in that case our work is done here @@ -896,8 +897,10 @@ static void bpf_cgroup_link_release(struct bpf_link *link) return; } - WARN_ON(__cgroup_bpf_detach(cg_link->cgroup, NULL, cg_link, - cg_link->type)); + err = __cgroup_bpf_detach(cg_link->cgroup, NULL, cg_link, + cg_link->type); + if (err) + pr_warn("cgroup_bpf_detach() failed, err %d\n", err); cg = cg_link->cgroup; cg_link->cgroup = NULL; -- 2.35.1
