[PATCH 5.4 11/18] selinux: fix race condition when computing ocontext SIDs

From: Ondrej Mosnacek omosnace@redhat.com

commit cbfcd13be5cb2a07868afe67520ed181956579a7 upstream.

Current code contains a lot of racy patterns when converting an ocontext's context structure to an SID. This is being done in a "lazy" fashion, such that the SID is looked up in the SID table only when it's first needed and then cached in the "sid" field of the ocontext structure. However, this is done without any locking or memory barriers and is thus unsafe.

Between commits 24ed7fdae669 ("selinux: use separate table for initial SID lookup") and 66f8e2f03c02 ("selinux: sidtab reverse lookup hash table"), this race condition lead to an actual observable bug, because a pointer to the shared sid field was passed directly to sidtab_context_to_sid(), which was using this location to also store an intermediate value, which could have been read by other threads and interpreted as an SID. In practice this caused e.g. new mounts to get a wrong (seemingly random) filesystem context, leading to strange denials. This bug has been spotted in the wild at least twice, see [1] and [2].

Fix the race condition by making all the racy functions use a common helper that ensures the ocontext::sid accesses are made safely using the appropriate SMP constructs.

Note that security_netif_sid() was populating the sid field of both contexts stored in the ocontext, but only the first one was actually used. The SELinux wiki's documentation on the "netifcon" policy statement [3] suggests that using only the first context is intentional. I kept only the handling of the first context here, as there is really no point in doing the SID lookup for the unused one.

I wasn't able to reproduce the bug mentioned above on any kernel that includes commit 66f8e2f03c02, even though it has been reported that the issue occurs with that commit, too, just less frequently. Thus, I wasn't able to verify that this patch fixes the issue, but it makes sense to avoid the race condition regardless.

[1] https://github.com/containers/container-selinux/issues/89 [2] https://lists.fedoraproject.org/archives/list/selinux@lists.fedoraproject.or... [3] https://selinuxproject.org/page/NetworkStatements#netifcon

Cc: stable@vger.kernel.org Cc: Xinjie Zheng xinjie@google.com Reported-by: Sujithra Periasamy sujithra@google.com Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Signed-off-by: Ondrej Mosnacek omosnace@redhat.com Signed-off-by: Paul Moore paul@paul-moore.com [vijayb: Backport contextual differences are due to v5.10 RCU related changes are not in 5.4] Signed-off-by: Vijay Balakrishna vijayb@linux.microsoft.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- security/selinux/ss/services.c | 159 ++++++++++++++++++++++------------------- 1 file changed, 87 insertions(+), 72 deletions(-)

--- a/security/selinux/ss/services.c +++ b/security/selinux/ss/services.c @@ -2251,6 +2251,43 @@ size_t security_policydb_len(struct seli }

/** + * ocontext_to_sid - Helper to safely get sid for an ocontext + * @sidtab: SID table + * @c: ocontext structure + * @index: index of the context entry (0 or 1) + * @out_sid: pointer to the resulting SID value + * + * For all ocontexts except OCON_ISID the SID fields are populated + * on-demand when needed. Since updating the SID value is an SMP-sensitive + * operation, this helper must be used to do that safely. + * + * WARNING: This function may return -ESTALE, indicating that the caller + * must retry the operation after re-acquiring the policy pointer! + */ +static int ocontext_to_sid(struct sidtab *sidtab, struct ocontext *c, + size_t index, u32 *out_sid) +{ + int rc; + u32 sid; + + /* Ensure the associated sidtab entry is visible to this thread. */ + sid = smp_load_acquire(&c->sid[index]); + if (!sid) { + rc = sidtab_context_to_sid(sidtab, &c->context[index], &sid); + if (rc) + return rc; + + /* + * Ensure the new sidtab entry is visible to other threads + * when they see the SID. + */ + smp_store_release(&c->sid[index], sid); + } + *out_sid = sid; + return 0; +} + +/** * security_port_sid - Obtain the SID for a port. * @protocol: protocol number * @port: port number @@ -2262,10 +2299,12 @@ int security_port_sid(struct selinux_sta struct policydb *policydb; struct sidtab *sidtab; struct ocontext *c; - int rc = 0; + int rc;

read_lock(&state->ss->policy_rwlock);

+retry: + rc = 0; policydb = &state->ss->policydb; sidtab = state->ss->sidtab;

@@ -2279,14 +2318,11 @@ int security_port_sid(struct selinux_sta }

if (c) { - if (!c->sid[0]) { - rc = sidtab_context_to_sid(sidtab, - &c->context[0], - &c->sid[0]); - if (rc) - goto out; - } - *out_sid = c->sid[0]; + rc = ocontext_to_sid(sidtab, c, 0, out_sid); + if (rc == -ESTALE) + goto retry; + if (rc) + goto out; } else { *out_sid = SECINITSID_PORT; } @@ -2308,10 +2344,12 @@ int security_ib_pkey_sid(struct selinux_ struct policydb *policydb; struct sidtab *sidtab; struct ocontext *c; - int rc = 0; + int rc;

read_lock(&state->ss->policy_rwlock);

+retry: + rc = 0; policydb = &state->ss->policydb; sidtab = state->ss->sidtab;

@@ -2326,14 +2364,11 @@ int security_ib_pkey_sid(struct selinux_ }

if (c) { - if (!c->sid[0]) { - rc = sidtab_context_to_sid(sidtab, - &c->context[0], - &c->sid[0]); - if (rc) - goto out; - } - *out_sid = c->sid[0]; + rc = ocontext_to_sid(sidtab, c, 0, out_sid); + if (rc == -ESTALE) + goto retry; + if (rc) + goto out; } else *out_sid = SECINITSID_UNLABELED;

@@ -2354,10 +2389,12 @@ int security_ib_endport_sid(struct selin struct policydb *policydb; struct sidtab *sidtab; struct ocontext *c; - int rc = 0; + int rc;

read_lock(&state->ss->policy_rwlock);

+retry: + rc = 0; policydb = &state->ss->policydb; sidtab = state->ss->sidtab;

@@ -2373,14 +2410,11 @@ int security_ib_endport_sid(struct selin }

if (c) { - if (!c->sid[0]) { - rc = sidtab_context_to_sid(sidtab, - &c->context[0], - &c->sid[0]); - if (rc) - goto out; - } - *out_sid = c->sid[0]; + rc = ocontext_to_sid(sidtab, c, 0, out_sid); + if (rc == -ESTALE) + goto retry; + if (rc) + goto out; } else *out_sid = SECINITSID_UNLABELED;

@@ -2399,11 +2433,13 @@ int security_netif_sid(struct selinux_st { struct policydb *policydb; struct sidtab *sidtab; - int rc = 0; + int rc; struct ocontext *c;

read_lock(&state->ss->policy_rwlock);

+retry: + rc = 0; policydb = &state->ss->policydb; sidtab = state->ss->sidtab;

@@ -2415,19 +2451,11 @@ int security_netif_sid(struct selinux_st }

if (c) { - if (!c->sid[0] || !c->sid[1]) { - rc = sidtab_context_to_sid(sidtab, - &c->context[0], - &c->sid[0]); - if (rc) - goto out; - rc = sidtab_context_to_sid(sidtab, - &c->context[1], - &c->sid[1]); - if (rc) - goto out; - } - *if_sid = c->sid[0]; + rc = ocontext_to_sid(sidtab, c, 0, if_sid); + if (rc == -ESTALE) + goto retry; + if (rc) + goto out; } else *if_sid = SECINITSID_NETIF;

@@ -2469,6 +2497,7 @@ int security_node_sid(struct selinux_sta

read_lock(&state->ss->policy_rwlock);

+retry: policydb = &state->ss->policydb; sidtab = state->ss->sidtab;

@@ -2511,14 +2540,11 @@ int security_node_sid(struct selinux_sta }

if (c) { - if (!c->sid[0]) { - rc = sidtab_context_to_sid(sidtab, - &c->context[0], - &c->sid[0]); - if (rc) - goto out; - } - *out_sid = c->sid[0]; + rc = ocontext_to_sid(sidtab, c, 0, out_sid); + if (rc == -ESTALE) + goto retry; + if (rc) + goto out; } else { *out_sid = SECINITSID_NODE; } @@ -2677,7 +2703,7 @@ static inline int __security_genfs_sid(s u16 sclass; struct genfs *genfs; struct ocontext *c; - int rc, cmp = 0; + int cmp = 0;

while (path[0] == '/' && path[1] == '/') path++; @@ -2691,9 +2717,8 @@ static inline int __security_genfs_sid(s break; }

- rc = -ENOENT; if (!genfs || cmp) - goto out; + return -ENOENT;

for (c = genfs->head; c; c = c->next) { len = strlen(c->u.name); @@ -2702,20 +2727,10 @@ static inline int __security_genfs_sid(s break; }

- rc = -ENOENT; if (!c) - goto out; + return -ENOENT;

- if (!c->sid[0]) { - rc = sidtab_context_to_sid(sidtab, &c->context[0], &c->sid[0]); - if (rc) - goto out; - } - - *sid = c->sid[0]; - rc = 0; -out: - return rc; + return ocontext_to_sid(sidtab, c, 0, sid); }

/** @@ -2750,13 +2765,15 @@ int security_fs_use(struct selinux_state { struct policydb *policydb; struct sidtab *sidtab; - int rc = 0; + int rc; struct ocontext *c; struct superblock_security_struct *sbsec = sb->s_security; const char *fstype = sb->s_type->name;

read_lock(&state->ss->policy_rwlock);

+retry: + rc = 0; policydb = &state->ss->policydb; sidtab = state->ss->sidtab;

@@ -2769,13 +2786,11 @@ int security_fs_use(struct selinux_state

if (c) { sbsec->behavior = c->v.behavior; - if (!c->sid[0]) { - rc = sidtab_context_to_sid(sidtab, &c->context[0], - &c->sid[0]); - if (rc) - goto out; - } - sbsec->sid = c->sid[0]; + rc = ocontext_to_sid(sidtab, c, 0, &sbsec->sid); + if (rc == -ESTALE) + goto retry; + if (rc) + goto out; } else { rc = __security_genfs_sid(state, fstype, "/", SECCLASS_DIR, &sbsec->sid);