Re: [PATCH] bpf: Fix KASAN use-after-free Read in compute_effective_progs

On 4/5/22 10:03, Tadeusz Struk wrote:

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@kernel.org Cc: "Daniel Borkmann" daniel@iogearbox.net Cc: "Andrii Nakryiko" andrii@kernel.org Cc: "Martin KaFai Lau" kafai@fb.com Cc: "Song Liu" songliubraving@fb.com Cc: "Yonghong Song" yhs@fb.com Cc: "John Fastabend" john.fastabend@gmail.com Cc: "KP Singh" kpsingh@kernel.org Cc: netdev@vger.kernel.org Cc: bpf@vger.kernel.org Cc: stable@vger.kernel.org Cc: linux-kernel@vger.kernel.org

Link: https://syzkaller.appspot.com/bug?id=8ebf179a95c2a2670f7cf1ba62429ec044369db... 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

---

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;

Hi, Any feedback/comments on this one?