[PATCH 5.18 0086/1095] bpf: Fix KASAN use-after-free Read in compute_effective_progs

Greg Kroah-Hartman
2022-08-15T20:17:05Z

From: Tadeusz Struk <tadeusz.struk@linaro.org>

commit 4c46091ee985ae84c60c5e95055d779fcd291d87 upstream.

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.

Fixes: af6eea57437a ("bpf: Implement bpf_link-based cgroup BPF program attachment")
Reported-by: <syzbot+f264bffdfbd5614f3bb2@syzkaller.appspotmail.com>
Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Cc: <stable@vger.kernel.org>
Link: https://syzkaller.appspot.com/bug?id=8ebf179a95c2a2670f7cf1ba62429ec044369db4
Link: https://lore.kernel.org/bpf/20220517180420.87954-1-tadeusz.struk@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/bpf/cgroup.c | 70 ++++++++++++++++++++++++++++++++++++++++++++--------
1 file changed, 60 insertions(+), 10 deletions(-)

--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -748,6 +748,60 @@ static struct bpf_prog_list *find_detach
}

/**
+ * 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 travers
+ * @prog: A program to detach or NULL
+ * @link: A link to detach or NULL
+ * @atype: Type of detach operation
+ */
+static void purge_effective_progs(struct cgroup *cgrp, struct bpf_prog *prog,
+ struct bpf_cgroup_link *link,
+ enum cgroup_bpf_attach_type atype)
+{
+ struct cgroup_subsys_state *css;
+ struct bpf_prog_array *progs;
+ struct bpf_prog_list *pl;
+ struct list_head *head;
+ struct cgroup *cg;
+ int pos;
+
+ /* recompute effective prog array in place */
+ css_for_each_descendant_pre(css, &cgrp->self) {
+ struct cgroup *desc = container_of(css, struct cgroup, self);
+
+ if (percpu_ref_is_zero(&desc->bpf.refcnt))
+ continue;
+
+ /* find position of link or prog in effective progs array */
+ for (pos = 0, cg = desc; cg; cg = cgroup_parent(cg)) {
+ if (pos && !(cg->bpf.flags[atype] & BPF_F_ALLOW_MULTI))
+ continue;
+
+ head = &cg->bpf.progs[atype];
+ list_for_each_entry(pl, head, node) {
+ if (!prog_list_prog(pl))
+ continue;
+ if (pl->prog == prog && pl->link == link)
+ goto found;
+ pos++;
+ }
+ }
+found:
+ BUG_ON(!cg);
+ progs = rcu_dereference_protected(
+ desc->bpf.effective[atype],
+ lockdep_is_held(&cgroup_mutex));
+
+ /* Remove the program from the array */
+ WARN_ONCE(bpf_prog_array_delete_safe_at(progs, pos),
+ "Failed to purge a prog from array at index %d", pos);
+ }
+}
+
+/**
* __cgroup_bpf_detach() - Detach the program or link from a cgroup, and
* propagate the change to descendants
* @cgrp: The cgroup which descendants to traverse
@@ -766,7 +820,6 @@ static int __cgroup_bpf_detach(struct cg
struct bpf_prog_list *pl;
struct list_head *progs;
u32 flags;
- int err;

atype = to_cgroup_bpf_attach_type(type);
if (atype < 0)
@@ -788,9 +841,12 @@ static int __cgroup_bpf_detach(struct cg
pl->prog = NULL;
pl->link = NULL;

- err = update_effective_progs(cgrp, atype);
- if (err)
- goto cleanup;
+ if (update_effective_progs(cgrp, atype)) {
+ /* if update effective array failed replace the prog with a dummy prog*/
+ pl->prog = old_prog;
+ pl->link = link;
+ purge_effective_progs(cgrp, old_prog, link, atype);
+ }

/* now can actually delete it from this cgroup list */
list_del(&pl->node);
@@ -802,12 +858,6 @@ static int __cgroup_bpf_detach(struct cg
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,