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 ec3a928af829..e31965dc6dd1 100644 --- a/security/apparmor/lsm.c +++ b/security/apparmor/lsm.c @@ -791,7 +791,12 @@ 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); }