On Fri, Apr 15, 2022 at 7:14 AM Tadeusz Struk <tadeusz.struk@xxxxxxxxxx> 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 rearrange the effective table without > shrinking it. 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(). > --- > kernel/bpf/cgroup.c | 55 +++++++++++++++++++++++++++++++++++++++------ > 1 file changed, 48 insertions(+), 7 deletions(-) > > diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c > index 128028efda64..5a64cece09f3 100644 > --- a/kernel/bpf/cgroup.c > +++ b/kernel/bpf/cgroup.c > @@ -723,10 +723,8 @@ 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 */ > + /* now can delete it from this cgroup list */ > list_del(&pl->node); > kfree(pl); > if (list_empty(progs)) > @@ -735,12 +733,55 @@ 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; > + > + if (!err) > + return 0; > > cleanup: > - /* restore back prog or link */ > - pl->prog = old_prog; > - pl->link = link; > + /* > + * If compute_effective_progs failed with -ENOMEM, i.e. alloc for > + * cgrp->bpf.inactive table failed, we can recover by removing > + * the detached prog from effective table and rearranging it. > + */ > + if (err == -ENOMEM) { > + struct bpf_prog_array_item *item; > + struct bpf_prog *prog_tmp, *prog_detach, *prog_last; > + struct bpf_prog_array *array; > + int index = 0, index_detach = -1; > + > + array = cgrp->bpf.effective[atype]; > + item = &array->items[0]; > + > + if (prog) > + prog_detach = prog; > + else > + prog_detach = link->link.prog; > + > + if (!prog_detach) > + return -EINVAL; > + > + while ((prog_tmp = READ_ONCE(item->prog))) { > + if (prog_tmp == prog_detach) > + index_detach = index; > + item++; > + index++; > + prog_last = prog_tmp; > + } > + > + /* Check if we found what's needed for removing the prog */ > + if (index_detach == -1 || index_detach == index-1) > + return -EINVAL; > + > + /* Remove the last program in the array */ > + if (bpf_prog_array_delete_safe_at(array, index-1)) > + return -EINVAL; > + > + /* and update the detached with the last just removed */ > + if (bpf_prog_array_update_at(array, index_detach, prog_last)) > + return -EINVAL; > + > + err = 0; > + } There are a bunch of problems with this implementation. 1. We should do this fallback right after update_effective_progs() returns error, before we get to list_del(&pl->node) and subsequent code that does some additional things (like clearing flags and stuff). This additional code needs to run even if update_effective_progs() fails. So I suggest to extract the logic of removing program from effective prog arrays into a helper function and doing err = update_effective_progs(...); if (err) purge_effective_progs(); where purge_effective_progs() will be the logic you are adding. And it will be void function because it can't fail. 2. We have to update not just cgrp->bpf.effective array, but all the descendants' lists as well. See what update_effective_progs() is doing, it has css_for_each_descendant_pre() iteration. You need to do it here as well. But instead of doing compute_effective_progs() which allocates a new copy of an array we'll need to update existing array in place. 3. Not clear why you need to do both bpf_prog_array_delete_safe_at() and bpf_prog_array_update_at(), isn't delete_safe_at() enought? > return err; > } > > -- > 2.35.1 >
