[PATCH v2] bpf: Fix KASAN use-after-free Read in compute_effective_progs

[Tadeusz Struk <tadeusz.struk@linaro.org>]

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 rearrange the effective table without
shrinking it. 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().
---
 kernel/bpf/cgroup.c | 55 +++++++++++++++++++++++++++++++++++++++------
 1 file changed, 48 insertions(+), 7 deletions(-)

diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index 128028efda64..5a64cece09f3 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -723,10 +723,8 @@ 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;
 
-	/* now can actually delete it from this cgroup list */
+	/* now can delete it from this cgroup list */
 	list_del(&pl->node);
 	kfree(pl);
 	if (list_empty(progs))
@@ -735,12 +733,55 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,
 	if (old_prog)
 		bpf_prog_put(old_prog);
 	static_branch_dec(&cgroup_bpf_enabled_key[atype]);
-	return 0;
+
+	if (!err)
+		return 0;
 
 cleanup:
-	/* restore back prog or link */
-	pl->prog = old_prog;
-	pl->link = link;
+	/*
+	 * If compute_effective_progs failed with -ENOMEM, i.e. alloc for
+	 * cgrp->bpf.inactive table failed, we can recover by removing
+	 * the detached prog from effective table and rearranging it.
+	 */
+	if (err == -ENOMEM) {
+		struct bpf_prog_array_item *item;
+		struct bpf_prog *prog_tmp, *prog_detach, *prog_last;
+		struct bpf_prog_array *array;
+		int index = 0, index_detach = -1;
+
+		array = cgrp->bpf.effective[atype];
+		item = &array->items[0];
+
+		if (prog)
+			prog_detach = prog;
+		else
+			prog_detach = link->link.prog;
+
+		if (!prog_detach)
+			return -EINVAL;
+
+		while ((prog_tmp = READ_ONCE(item->prog))) {
+			if (prog_tmp == prog_detach)
+				index_detach = index;
+			item++;
+			index++;
+			prog_last = prog_tmp;
+		}
+
+		/* Check if we found what's needed for removing the prog */
+		if (index_detach == -1 || index_detach == index-1)
+			return -EINVAL;
+
+		/* Remove the last program in the array */
+		if (bpf_prog_array_delete_safe_at(array, index-1))
+			return -EINVAL;
+
+		/* and update the detached with the last just removed */
+		if (bpf_prog_array_update_at(array, index_detach, prog_last))
+			return -EINVAL;
+
+		err = 0;
+	}
 	return err;
 }
 
-- 
2.35.1