Re: [PATCH v3] bpf: Fix KASAN use-after-free Read in compute_effective_progs
From: Tadeusz Struk
Date: Mon May 16 2022 - 19:35:15 EST
On 5/16/22 16:16, Andrii Nakryiko wrote:
On Fri, May 13, 2022 at 12:08 PM Tadeusz Struk <tadeusz.struk@xxxxxxxxxx> 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