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@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;
Hi,
Any feedback/comments on this one?
--
Thanks,
Tadeusz
