Messages in this thread | | | Date | Mon, 16 May 2022 16:35:06 -0700 | From | Tadeusz Struk <> | Subject | Re: [PATCH v3] bpf: Fix KASAN use-after-free Read in compute_effective_progs |
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On 5/16/22 16:16, Andrii Nakryiko wrote: > On Fri, May 13, 2022 at 12:08 PM Tadeusz Struk <tadeusz.struk@linaro.org> wrote: >> 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)
I didn't check it with sparse so I didn't see this warning. Will fix in the next version.
> >> + 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.
Ok, I can try that.
> >> + 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?
I wasn't if the prog will be present in all parents so I decided to add this check to make sure it is found.
> >> + >> + /* 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
ok, I will pass in both.
-- Thanks, Tadeusz
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