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Subject[PATCH v3] bpf: Fix KASAN use-after-free Read in compute_effective_progs
Date
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@kernel.org>
Cc: "Daniel Borkmann" <daniel@iogearbox.net>
Cc: "Andrii Nakryiko" <andrii@kernel.org>
Cc: "Martin KaFai Lau" <kafai@fb.com>
Cc: "Song Liu" <songliubraving@fb.com>
Cc: "Yonghong Song" <yhs@fb.com>
Cc: "John Fastabend" <john.fastabend@gmail.com>
Cc: "KP Singh" <kpsingh@kernel.org>
Cc: <netdev@vger.kernel.org>
Cc: <bpf@vger.kernel.org>
Cc: <stable@vger.kernel.org>
Cc: <linux-kernel@vger.kernel.org>

Link: https://syzkaller.appspot.com/bug?id=8ebf179a95c2a2670f7cf1ba62429ec044369db4
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>
---
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];
+ item = &array->items[0];
+
+ /* Find the index of the prog to purge */
+ while ((tmp = READ_ONCE(item->prog))) {
+ if (tmp == 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;
+
+ /* 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;
+
+ 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
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