[RFC][PATCH 00/15] KEYS: Fixes
David Howells
dhowells at redhat.com
Fri Oct 13 15:39:28 UTC 2017
Eric Biggers <ebiggers3 at gmail.com> wrote:
> > static inline bool key_is_instantiated(const struct key *key)
> ...
> This should use 'smp_load_acquire(&key->state) == KEY_IS_INSTANTIATED',
I made a key_read_state() wrapper function that does this and then used it in
a bunch of places that read it, including this one.
> > + short state = READ_ONCE(key->state);
>
> Here the key has no more references, so nothing can be changing the state.
> Thus, the READ_ONCE() isn't actually needed.
Changed.
> > +static void mark_key_instantiated(struct key *key, int reject_error)
> > +{
> > + smp_wmb(); /* Commit the payload before setting the state */
> > + key->state = (reject_error < 0) ? reject_error : KEY_IS_INSTANTIATED;
> > +}
> > +
>
> smp_store_release() would make this simpler as well as guarantee that the
> write is atomic.
Changed.
> > + ret = key->state;
> > + if (ret < 0)
> > + goto error2; /* Negatively instantiated */
>
> Not too important in practice (as this is constantly gotten wrong all over
> the kernel, and compilers play nice enough to make it not a huge deal), but
> READ_ONCE(key->state) will guarantee that the read of 'key->state' is atomic
> and not e.g. done byte-by-byte.
Changed to use key_read_state().
> > - if (key->type->describe)
> > + if (key->type->describe) {
> > + smp_rmb(); /* Order access to payload after state set. */
> > key->type->describe(key, m);
> > + }
>
> This is the wrong place for this memory barrier. The state is checked
> separately in ->describe() and it may have changed between when it was shown
> in proc_keys_show() vs. when it is checked in ->describe(). We can't
> actually make these two access to ->state consistent with respect to each
> other right now. The most we can do is use smp_load_acquire() in
> key_is_instantiated() so that at least ->describe() isn't broken by itself.
> So the change here is pointless.
Feasibly. I've changed the preceding:
state = READ_ONCE(key->state);
into:
state = key_read_state(key);
and got rid of smp_rmb() anyway, but you're right, ->describe() needs to
recheck the state if it might want to touch it.
I wonder if I should pass the state into ->describe()?
Possibly I shouldn't be calling ->describe() if the key isn't instantiated.
> > + if (!(lflags & KEY_LOOKUP_PARTIAL)) {
> > + if (key->state != KEY_IS_INSTANTIATED)
> > + goto invalid_key;
> > + smp_rmb(); /* Order access to payload after state set. */
> > + }
>
> This should be simply:
>
> if (!(lflags & KEY_LOOKUP_PARTIAL) && !key_is_instantiated(key))
> goto invalid_key;
Changed.
> > + ret = READ_ONCE(key->state);
> > + if (ret < 0)
> > + return ret;
> > return key_validate(key);
> > }
> > EXPORT_SYMBOL(wait_for_key_construction);
>
> smp_load_acquire() rather than READ_ONCE(), in case the caller uses this as an
> indication that it is safe to access the payload.
Changed to use key_read_state().
> >- if (!test_bit(KEY_FLAG_NEGATIVE, &key->flags))
> >+ if (key->state < 0)
> > zap = dereference_key_locked(key);
> > rcu_assign_keypointer(key, prep->payload.data[0]);
> > prep->payload.data[0] = NULL;
>
> This is backwards; it should be 'if (!key_is_negative(key))'.
Actually, I think it should be 'if (key_is_instantiated())'
> > + if (test_bit(KEY_FLAG_USER_CONSTRUCT, &key->flags)) {
> ...
> 'key' is NULL here. It should be 'key_ref_to_ptr(key_ref)'.
Fixed.
David
---
commit 21fe21748b709710fbc78536a6d68bb4df8297ad
Author: David Howells <dhowells at redhat.com>
Date: Wed Oct 4 16:43:25 2017 +0100
KEYS: Fix race between updating and finding a negative key
Consolidate KEY_FLAG_INSTANTIATED, KEY_FLAG_NEGATIVE and the rejection
error into one field such that:
(1) The instantiation state can be modified/read atomically.
(2) The error can be accessed atomically with the state.
(3) The error isn't stored unioned with the payload pointers.
This deals with the problem that the state is spread over three different
objects (two bits and a separate variable) and reading or updating them
atomically isn't practical, given that not only can uninstantiated keys
change into instantiated or rejected keys, but rejected keys can also turn
into instantiated keys - and someone accessing the key might not be using
any locking.
The main side effect of this problem is that what was held in the payload
may change, depending on the state. For instance, you might observe the
key to be in the rejected state. You then read the cached error, but if
the key semaphore wasn't locked, the key might've become instantiated
between the two reads - and you might now have something in hand that isn't
actually an error code.
Additionally, barriering is included:
(1) Order payload-set before state-set during instantiation.
(2) Order state-read before payload-read when using the key.
Further separate barriering is necessary if RCU is being used to access the
payload content after reading the payload pointers.
Fixes: 146aa8b1453b ("KEYS: Merge the type-specific data with the payload data")
Cc: <stable at vger.kernel.org> [v4.4+]
Reported-by: Eric Biggers <ebiggers at google.com>
Signed-off-by: David Howells <dhowells at redhat.com>
diff --git a/include/linux/key.h b/include/linux/key.h
index e315e16b6ff8..eae10f302b44 100644
--- a/include/linux/key.h
+++ b/include/linux/key.h
@@ -138,6 +138,11 @@ struct key_restriction {
struct key_type *keytype;
};
+enum key_state {
+ KEY_IS_UNINSTANTIATED,
+ KEY_IS_INSTANTIATED,
+};
+
/*****************************************************************************/
/*
* authentication token / access credential / keyring
@@ -169,6 +174,7 @@ struct key {
* - may not match RCU dereferenced payload
* - payload should contain own length
*/
+ short state; /* Key state (+) or rejection error (-) */
#ifdef KEY_DEBUGGING
unsigned magic;
@@ -176,18 +182,16 @@ struct key {
#endif
unsigned long flags; /* status flags (change with bitops) */
-#define KEY_FLAG_INSTANTIATED 0 /* set if key has been instantiated */
-#define KEY_FLAG_DEAD 1 /* set if key type has been deleted */
-#define KEY_FLAG_REVOKED 2 /* set if key had been revoked */
-#define KEY_FLAG_IN_QUOTA 3 /* set if key consumes quota */
-#define KEY_FLAG_USER_CONSTRUCT 4 /* set if key is being constructed in userspace */
-#define KEY_FLAG_NEGATIVE 5 /* set if key is negative */
-#define KEY_FLAG_ROOT_CAN_CLEAR 6 /* set if key can be cleared by root without permission */
-#define KEY_FLAG_INVALIDATED 7 /* set if key has been invalidated */
-#define KEY_FLAG_BUILTIN 8 /* set if key is built in to the kernel */
-#define KEY_FLAG_ROOT_CAN_INVAL 9 /* set if key can be invalidated by root without permission */
-#define KEY_FLAG_KEEP 10 /* set if key should not be removed */
-#define KEY_FLAG_UID_KEYRING 11 /* set if key is a user or user session keyring */
+#define KEY_FLAG_DEAD 0 /* set if key type has been deleted */
+#define KEY_FLAG_REVOKED 1 /* set if key had been revoked */
+#define KEY_FLAG_IN_QUOTA 2 /* set if key consumes quota */
+#define KEY_FLAG_USER_CONSTRUCT 3 /* set if key is being constructed in userspace */
+#define KEY_FLAG_ROOT_CAN_CLEAR 4 /* set if key can be cleared by root without permission */
+#define KEY_FLAG_INVALIDATED 5 /* set if key has been invalidated */
+#define KEY_FLAG_BUILTIN 6 /* set if key is built in to the kernel */
+#define KEY_FLAG_ROOT_CAN_INVAL 7 /* set if key can be invalidated by root without permission */
+#define KEY_FLAG_KEEP 8 /* set if key should not be removed */
+#define KEY_FLAG_UID_KEYRING 9 /* set if key is a user or user session keyring */
/* the key type and key description string
* - the desc is used to match a key against search criteria
@@ -213,7 +217,6 @@ struct key {
struct list_head name_link;
struct assoc_array keys;
};
- int reject_error;
};
/* This is set on a keyring to restrict the addition of a link to a key
@@ -353,6 +356,12 @@ extern void key_set_timeout(struct key *, unsigned);
#define KEY_NEED_SETATTR 0x20 /* Require permission to change attributes */
#define KEY_NEED_ALL 0x3f /* All the above permissions */
+static inline short key_read_state(const struct key *key)
+{
+ /* Barrier versus mark_key_instantiated(). */
+ return smp_load_acquire(&key->state);
+}
+
/**
* key_is_instantiated - Determine if a key has been positively instantiated
* @key: The key to check.
@@ -362,8 +371,12 @@ extern void key_set_timeout(struct key *, unsigned);
*/
static inline bool key_is_instantiated(const struct key *key)
{
- return test_bit(KEY_FLAG_INSTANTIATED, &key->flags) &&
- !test_bit(KEY_FLAG_NEGATIVE, &key->flags);
+ return key_read_state(key) == KEY_IS_INSTANTIATED;
+}
+
+static inline bool key_is_negative(const struct key *key)
+{
+ return key_read_state(key) < 0;
}
#define dereference_key_rcu(KEY) \
diff --git a/security/keys/encrypted-keys/encrypted.c b/security/keys/encrypted-keys/encrypted.c
index 535db141f4da..d92cbf9687c3 100644
--- a/security/keys/encrypted-keys/encrypted.c
+++ b/security/keys/encrypted-keys/encrypted.c
@@ -854,7 +854,7 @@ static int encrypted_update(struct key *key, struct key_preparsed_payload *prep)
size_t datalen = prep->datalen;
int ret = 0;
- if (test_bit(KEY_FLAG_NEGATIVE, &key->flags))
+ if (key_is_negative(key))
return -ENOKEY;
if (datalen <= 0 || datalen > 32767 || !prep->data)
return -EINVAL;
diff --git a/security/keys/gc.c b/security/keys/gc.c
index 87cb260e4890..e7aeecbf7f19 100644
--- a/security/keys/gc.c
+++ b/security/keys/gc.c
@@ -129,14 +129,15 @@ static noinline void key_gc_unused_keys(struct list_head *keys)
while (!list_empty(keys)) {
struct key *key =
list_entry(keys->next, struct key, graveyard_link);
+ short state = key->state;
+
list_del(&key->graveyard_link);
kdebug("- %u", key->serial);
key_check(key);
/* Throw away the key data if the key is instantiated */
- if (test_bit(KEY_FLAG_INSTANTIATED, &key->flags) &&
- !test_bit(KEY_FLAG_NEGATIVE, &key->flags) &&
+ if (state == KEY_IS_INSTANTIATED &&
key->type->destroy)
key->type->destroy(key);
@@ -151,7 +152,7 @@ static noinline void key_gc_unused_keys(struct list_head *keys)
}
atomic_dec(&key->user->nkeys);
- if (test_bit(KEY_FLAG_INSTANTIATED, &key->flags))
+ if (state == KEY_IS_INSTANTIATED)
atomic_dec(&key->user->nikeys);
key_user_put(key->user);
diff --git a/security/keys/key.c b/security/keys/key.c
index eb914a838840..cdd8fea8ddc4 100644
--- a/security/keys/key.c
+++ b/security/keys/key.c
@@ -402,6 +402,18 @@ int key_payload_reserve(struct key *key, size_t datalen)
EXPORT_SYMBOL(key_payload_reserve);
/*
+ * Change the key state to being instantiated.
+ */
+static void mark_key_instantiated(struct key *key, int reject_error)
+{
+ /* Commit the payload before setting the state; barrier versus
+ * key_read_state().
+ */
+ smp_store_release(&key->state,
+ (reject_error < 0) ? reject_error : KEY_IS_INSTANTIATED);
+}
+
+/*
* Instantiate a key and link it into the target keyring atomically. Must be
* called with the target keyring's semaphore writelocked. The target key's
* semaphore need not be locked as instantiation is serialised by
@@ -424,14 +436,14 @@ static int __key_instantiate_and_link(struct key *key,
mutex_lock(&key_construction_mutex);
/* can't instantiate twice */
- if (!test_bit(KEY_FLAG_INSTANTIATED, &key->flags)) {
+ if (key->state == KEY_IS_UNINSTANTIATED) {
/* instantiate the key */
ret = key->type->instantiate(key, prep);
if (ret == 0) {
/* mark the key as being instantiated */
atomic_inc(&key->user->nikeys);
- set_bit(KEY_FLAG_INSTANTIATED, &key->flags);
+ mark_key_instantiated(key, 0);
if (test_and_clear_bit(KEY_FLAG_USER_CONSTRUCT, &key->flags))
awaken = 1;
@@ -577,13 +589,10 @@ int key_reject_and_link(struct key *key,
mutex_lock(&key_construction_mutex);
/* can't instantiate twice */
- if (!test_bit(KEY_FLAG_INSTANTIATED, &key->flags)) {
+ if (key->state == KEY_IS_UNINSTANTIATED) {
/* mark the key as being negatively instantiated */
atomic_inc(&key->user->nikeys);
- key->reject_error = -error;
- smp_wmb();
- set_bit(KEY_FLAG_NEGATIVE, &key->flags);
- set_bit(KEY_FLAG_INSTANTIATED, &key->flags);
+ mark_key_instantiated(key, -error);
now = current_kernel_time();
key->expiry = now.tv_sec + timeout;
key_schedule_gc(key->expiry + key_gc_delay);
@@ -752,8 +761,8 @@ static inline key_ref_t __key_update(key_ref_t key_ref,
ret = key->type->update(key, prep);
if (ret == 0)
- /* updating a negative key instantiates it */
- clear_bit(KEY_FLAG_NEGATIVE, &key->flags);
+ /* Updating a negative key positively instantiates it */
+ mark_key_instantiated(key, 0);
up_write(&key->sem);
@@ -986,8 +995,8 @@ int key_update(key_ref_t key_ref, const void *payload, size_t plen)
ret = key->type->update(key, &prep);
if (ret == 0)
- /* updating a negative key instantiates it */
- clear_bit(KEY_FLAG_NEGATIVE, &key->flags);
+ /* Updating a negative key positively instantiates it */
+ mark_key_instantiated(key, 0);
up_write(&key->sem);
diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index 365ff85d7e27..698309a8502e 100644
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -766,10 +766,9 @@ long keyctl_read_key(key_serial_t keyid, char __user *buffer, size_t buflen)
key = key_ref_to_ptr(key_ref);
- if (test_bit(KEY_FLAG_NEGATIVE, &key->flags)) {
- ret = -ENOKEY;
- goto error2;
- }
+ ret = key_read_state(key);
+ if (ret < 0)
+ goto error2; /* Negatively instantiated */
/* see if we can read it directly */
ret = key_permission(key_ref, KEY_NEED_READ);
@@ -901,7 +900,7 @@ long keyctl_chown_key(key_serial_t id, uid_t user, gid_t group)
atomic_dec(&key->user->nkeys);
atomic_inc(&newowner->nkeys);
- if (test_bit(KEY_FLAG_INSTANTIATED, &key->flags)) {
+ if (key->state == KEY_IS_INSTANTIATED) {
atomic_dec(&key->user->nikeys);
atomic_inc(&newowner->nikeys);
}
diff --git a/security/keys/keyring.c b/security/keys/keyring.c
index 4fa82a8a9c0e..816948abff89 100644
--- a/security/keys/keyring.c
+++ b/security/keys/keyring.c
@@ -553,7 +553,8 @@ static int keyring_search_iterator(const void *object, void *iterator_data)
{
struct keyring_search_context *ctx = iterator_data;
const struct key *key = keyring_ptr_to_key(object);
- unsigned long kflags = key->flags;
+ unsigned long kflags = READ_ONCE(key->flags);
+ short state = READ_ONCE(key->state);
kenter("{%d}", key->serial);
@@ -597,9 +598,8 @@ static int keyring_search_iterator(const void *object, void *iterator_data)
if (ctx->flags & KEYRING_SEARCH_DO_STATE_CHECK) {
/* we set a different error code if we pass a negative key */
- if (kflags & (1 << KEY_FLAG_NEGATIVE)) {
- smp_rmb();
- ctx->result = ERR_PTR(key->reject_error);
+ if (state < 0) {
+ ctx->result = ERR_PTR(state);
kleave(" = %d [neg]", ctx->skipped_ret);
goto skipped;
}
diff --git a/security/keys/proc.c b/security/keys/proc.c
index de834309d100..9510822c4d96 100644
--- a/security/keys/proc.c
+++ b/security/keys/proc.c
@@ -182,6 +182,7 @@ static int proc_keys_show(struct seq_file *m, void *v)
unsigned long timo;
key_ref_t key_ref, skey_ref;
char xbuf[16];
+ short state;
int rc;
struct keyring_search_context ctx = {
@@ -236,17 +237,19 @@ static int proc_keys_show(struct seq_file *m, void *v)
sprintf(xbuf, "%luw", timo / (60*60*24*7));
}
+ state = key_read_state(key);
+
#define showflag(KEY, LETTER, FLAG) \
(test_bit(FLAG, &(KEY)->flags) ? LETTER : '-')
seq_printf(m, "%08x %c%c%c%c%c%c%c %5d %4s %08x %5d %5d %-9.9s ",
key->serial,
- showflag(key, 'I', KEY_FLAG_INSTANTIATED),
+ state == KEY_IS_INSTANTIATED ? 'I' : '-',
showflag(key, 'R', KEY_FLAG_REVOKED),
showflag(key, 'D', KEY_FLAG_DEAD),
showflag(key, 'Q', KEY_FLAG_IN_QUOTA),
showflag(key, 'U', KEY_FLAG_USER_CONSTRUCT),
- showflag(key, 'N', KEY_FLAG_NEGATIVE),
+ state < 0 ? 'N' : '-',
showflag(key, 'i', KEY_FLAG_INVALIDATED),
refcount_read(&key->usage),
xbuf,
diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
index 293d3598153b..5a8b985d1d5f 100644
--- a/security/keys/process_keys.c
+++ b/security/keys/process_keys.c
@@ -729,8 +729,7 @@ key_ref_t lookup_user_key(key_serial_t id, unsigned long lflags,
}
ret = -EIO;
- if (!(lflags & KEY_LOOKUP_PARTIAL) &&
- !test_bit(KEY_FLAG_INSTANTIATED, &key->flags))
+ if (!(lflags & KEY_LOOKUP_PARTIAL) && !key_is_instantiated(key))
goto invalid_key;
/* check the permissions */
diff --git a/security/keys/request_key.c b/security/keys/request_key.c
index 63e63a42db3c..e8036cd0ad54 100644
--- a/security/keys/request_key.c
+++ b/security/keys/request_key.c
@@ -595,10 +595,9 @@ int wait_for_key_construction(struct key *key, bool intr)
intr ? TASK_INTERRUPTIBLE : TASK_UNINTERRUPTIBLE);
if (ret)
return -ERESTARTSYS;
- if (test_bit(KEY_FLAG_NEGATIVE, &key->flags)) {
- smp_rmb();
- return key->reject_error;
- }
+ ret = key_read_state(key);
+ if (ret < 0)
+ return ret;
return key_validate(key);
}
EXPORT_SYMBOL(wait_for_key_construction);
diff --git a/security/keys/trusted.c b/security/keys/trusted.c
index ddfaebf60fc8..bd85315cbfeb 100644
--- a/security/keys/trusted.c
+++ b/security/keys/trusted.c
@@ -1066,7 +1066,7 @@ static int trusted_update(struct key *key, struct key_preparsed_payload *prep)
char *datablob;
int ret = 0;
- if (test_bit(KEY_FLAG_NEGATIVE, &key->flags))
+ if (key_is_negative(key))
return -ENOKEY;
p = key->payload.data[0];
if (!p->migratable)
diff --git a/security/keys/user_defined.c b/security/keys/user_defined.c
index 3d8c68eba516..d993d030a644 100644
--- a/security/keys/user_defined.c
+++ b/security/keys/user_defined.c
@@ -114,7 +114,7 @@ int user_update(struct key *key, struct key_preparsed_payload *prep)
/* attach the new data, displacing the old */
key->expiry = prep->expiry;
- if (!test_bit(KEY_FLAG_NEGATIVE, &key->flags))
+ if (key_is_instantiated(key))
zap = dereference_key_locked(key);
rcu_assign_keypointer(key, prep->payload.data[0]);
prep->payload.data[0] = NULL;
--
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