[PATCH AUTOSEL 5.7 100/388] apparmor: check/put label on apparmor_sk_clone_security()

Sasha Levin
2020-06-18T02:47:34Z

From: Mauricio Faria de Oliveira <mfo@canonical.com>

[ Upstream commit 3b646abc5bc6c0df649daea4c2c976bd4d47e4c8 ]

Currently apparmor_sk_clone_security() does not check for existing
label/peer in the 'new' struct sock; it just overwrites it, if any
(with another reference to the label of the source sock.)

static void apparmor_sk_clone_security(const struct sock *sk,
struct sock *newsk)
{
struct aa_sk_ctx *ctx = SK_CTX(sk);
struct aa_sk_ctx *new = SK_CTX(newsk);

new->label = aa_get_label(ctx->label);
new->peer = aa_get_label(ctx->peer);
}

This might leak label references, which might overflow under load.
Thus, check for and put labels, to prevent such errors.

Note this is similarly done on:

static int apparmor_socket_post_create(struct socket *sock, ...)
...
if (sock->sk) {
struct aa_sk_ctx *ctx = SK_CTX(sock->sk);

aa_put_label(ctx->label);
ctx->label = aa_get_label(label);
}
...

Context:
-------

The label reference count leak is observed if apparmor_sock_graft()
is called previously: this sets the 'ctx->label' field by getting
a reference to the current label (later overwritten, without put.)

static void apparmor_sock_graft(struct sock *sk, ...)
{
struct aa_sk_ctx *ctx = SK_CTX(sk);

if (!ctx->label)
ctx->label = aa_get_current_label();
}

And that is the case on crypto/af_alg.c:af_alg_accept():

int af_alg_accept(struct sock *sk, struct socket *newsock, ...)
...
struct sock *sk2;
...
sk2 = sk_alloc(...);
...
security_sock_graft(sk2, newsock);
security_sk_clone(sk, sk2);
...

Apparently both calls are done on their own right, especially for
other LSMs, being introduced in 2010/2014, before apparmor socket
mediation in 2017 (see commits [1,2,3,4]).

So, it looks OK there! Let's fix the reference leak in apparmor.

Test-case:
---------

Exercise that code path enough to overflow label reference count.

$ cat aa-refcnt-af_alg.c
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <linux/if_alg.h>

int main() {
int sockfd;
struct sockaddr_alg sa;

/* Setup the crypto API socket */
sockfd = socket(AF_ALG, SOCK_SEQPACKET, 0);
if (sockfd < 0) {
perror("socket");
return 1;
}

memset(&sa, 0, sizeof(sa));
sa.salg_family = AF_ALG;
strcpy((char *) sa.salg_type, "rng");
strcpy((char *) sa.salg_name, "stdrng");

if (bind(sockfd, (struct sockaddr *) &sa, sizeof(sa)) < 0) {
perror("bind");
return 1;
}

/* Accept a "connection" and close it; repeat. */
while (!close(accept(sockfd, NULL, 0)));

return 0;
}

$ gcc -o aa-refcnt-af_alg aa-refcnt-af_alg.c

$ ./aa-refcnt-af_alg
<a few hours later>

[ 9928.475953] refcount_t overflow at apparmor_sk_clone_security+0x37/0x70 in aa-refcnt-af_alg[1322], uid/euid: 1000/1000
...
[ 9928.507443] RIP: 0010:apparmor_sk_clone_security+0x37/0x70
...
[ 9928.514286] security_sk_clone+0x33/0x50
[ 9928.514807] af_alg_accept+0x81/0x1c0 [af_alg]
[ 9928.516091] alg_accept+0x15/0x20 [af_alg]
[ 9928.516682] SYSC_accept4+0xff/0x210
[ 9928.519609] SyS_accept+0x10/0x20
[ 9928.520190] do_syscall_64+0x73/0x130
[ 9928.520808] entry_SYSCALL_64_after_hwframe+0x3d/0xa2

Note that other messages may be seen, not just overflow, depending on
the value being incremented by kref_get(); on another run:

[ 7273.182666] refcount_t: saturated; leaking memory.
...
[ 7273.185789] refcount_t: underflow; use-after-free.

Kprobes:
-------

Using kprobe events to monitor sk -> sk_security -> label -> count (kref):

Original v5.7 (one reference leak every iteration)

... (af_alg_accept+0x0/0x1c0) label=0xffff8a0f36c25eb0 label_refcnt=0x11fd2
... (af_alg_release_parent+0x0/0xd0) label=0xffff8a0f36c25eb0 label_refcnt=0x11fd4
... (af_alg_accept+0x0/0x1c0) label=0xffff8a0f36c25eb0 label_refcnt=0x11fd3
... (af_alg_release_parent+0x0/0xd0) label=0xffff8a0f36c25eb0 label_refcnt=0x11fd5
... (af_alg_accept+0x0/0x1c0) label=0xffff8a0f36c25eb0 label_refcnt=0x11fd4
... (af_alg_release_parent+0x0/0xd0) label=0xffff8a0f36c25eb0 label_refcnt=0x11fd6

Patched v5.7 (zero reference leak per iteration)

... (af_alg_accept+0x0/0x1c0) label=0xffff9ff376c25eb0 label_refcnt=0x593
... (af_alg_release_parent+0x0/0xd0) label=0xffff9ff376c25eb0 label_refcnt=0x594
... (af_alg_accept+0x0/0x1c0) label=0xffff9ff376c25eb0 label_refcnt=0x593
... (af_alg_release_parent+0x0/0xd0) label=0xffff9ff376c25eb0 label_refcnt=0x594
... (af_alg_accept+0x0/0x1c0) label=0xffff9ff376c25eb0 label_refcnt=0x593
... (af_alg_release_parent+0x0/0xd0) label=0xffff9ff376c25eb0 label_refcnt=0x594

Commits:
-------

[1] commit 507cad355fc9 ("crypto: af_alg - Make sure sk_security is initialized on accept()ed sockets")
[2] commit 4c63f83c2c2e ("crypto: af_alg - properly label AF_ALG socket")
[3] commit 2acce6aa9f65 ("Networking") a.k.a ("crypto: af_alg - Avoid sock_graft call warning)
[4] commit 56974a6fcfef ("apparmor: add base infastructure for socket mediation")

Fixes: 56974a6fcfef ("apparmor: add base infastructure for socket mediation")
Reported-by: Brian Moyles <bmoyles@netflix.com>
Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
security/apparmor/lsm.c | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
index b621ad74f54a..66a8504c8bea 100644
--- a/security/apparmor/lsm.c
+++ b/security/apparmor/lsm.c
@@ -804,7 +804,12 @@ static void apparmor_sk_clone_security(const struct sock *sk,
struct aa_sk_ctx *ctx = SK_CTX(sk);
struct aa_sk_ctx *new = SK_CTX(newsk);

+ if (new->label)
+ aa_put_label(new->label);
new->label = aa_get_label(ctx->label);
+
+ if (new->peer)
+ aa_put_label(new->peer);
new->peer = aa_get_label(ctx->peer);
}

--
2.25.1