Re: [PATCH v4] bpf: Fix KASAN use-after-free Read in compute_effective_progs

On Mon, May 23, 2022 at 2:36 PM Tadeusz Struk tadeusz.struk@linaro.org wrote:

On 5/17/22 11:04, 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 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@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

---

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,

Hi Andrii, Do you have any more feedback? Does it look better to you now?

Hi, this is on my TODO list, but I need a bit more focused time to think all this through and I haven't managed to get it in last week. I'm worried about the percpu_ref_is_zero(&desc->bpf.refcnt) portion and whether it can cause some skew in the calculated array index, I need to look at this a bit more in depth. Sorry for the delay.

-- Thanks, Tadeusz