On Fri, May 13, 2022 at 12:08 PM 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 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 > --- > kernel/bpf/cgroup.c | 64 +++++++++++++++++++++++++++++++++++++++------ > 1 file changed, 56 insertions(+), 8 deletions(-) > > diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c > index 128028efda64..9d3af4d6c055 100644 > --- a/kernel/bpf/cgroup.c > +++ b/kernel/bpf/cgroup.c > @@ -681,6 +681,57 @@ 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 traverse > + * @link: A link to detach > + * @atype: Type of detach operation > + */ > +static void purge_effective_progs(struct cgroup *cgrp, struct bpf_prog *prog, > + enum cgroup_bpf_attach_type atype) > +{ > + struct cgroup_subsys_state *css; > + struct bpf_prog_array_item *item; > + struct bpf_prog *tmp; > + struct bpf_prog_array *array; > + int index = 0, index_purge = -1; > + > + if (!prog) > + return; > + > + /* recompute effective prog array in place */ > + css_for_each_descendant_pre(css, &cgrp->self) { > + struct cgroup *desc = container_of(css, struct cgroup, self); > + > + array = desc->bpf.effective[atype]; ../kernel/bpf/cgroup.c:748:23: warning: incorrect type in assignment (different address spaces) ../kernel/bpf/cgroup.c:748:23: expected struct bpf_prog_array *array ../kernel/bpf/cgroup.c:748:23: got struct bpf_prog_array [noderef] __rcu * you need rcu_dereference here? but also see suggestions below to avoid iterating effective directly (it's ambiguous to search by prog only) > + item = &array->items[0]; > + > + /* Find the index of the prog to purge */ > + while ((tmp = READ_ONCE(item->prog))) { > + if (tmp == prog) { I think comparing just prog isn't always correct, as the same program can be in effective array multiple times if attached through bpf_link. Looking at replace_effective_prog() I think we can do a very similar (and tested) approach: 1. restore original pl state in __cgroup_bpf_detach (so we can find it by comparing pl->prog == prog && pl->link == link) 2. use replace_effective_prog's approach to find position of pl in effective array (using this nested for loop over cgroup parents and list_for_each_entry inside) 3. then instead of replacing one prog with another do bpf_prog_array_delete_safe_at ? I'd feel more comfortable using the same tested overall approach of replace_effective_prog. > + index_purge = index; > + break; > + } > + item++; > + index++; > + } > + > + /* Check if we found what's needed for removing the prog */ > + if (index_purge == -1 || index_purge == index - 1) > + continue; the search shouldn't fail, should it? > + > + /* Remove the program from the array */ > + WARN_ONCE(bpf_prog_array_delete_safe_at(array, index_purge), > + "Failed to purge a prog from array at index %d", index_purge); > + > + index = 0; > + index_purge = -1; > + } > +} > + > /** > * __cgroup_bpf_detach() - Detach the program or link from a cgroup, and > * propagate the change to descendants > @@ -723,8 +774,11 @@ 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; > + if (err) { > + struct bpf_prog *prog_purge = prog ? prog : link->link.prog; > + so here we shouldn't forget link, instead pass both link and prog (one of them will have to be NULL) into purge_effective_progs > + purge_effective_progs(cgrp, prog_purge, atype); > + } > > /* now can actually delete it from this cgroup list */ > list_del(&pl->node); > @@ -736,12 +790,6 @@ 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, > -- > 2.36.1 >
