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