[PATCH v3] KEYS: Protect pkey method checks against type removal

[Yuansheng Yang <0xbeac@gmail.com>]

keyctl_pkey_params_get() looks up the key, and keyctl_pkey_query(),
keyctl_pkey_e_d_s() and keyctl_pkey_verify() then dereference
key->type->asym_* outside key->sem.  A reference keeps the key alive, but
does not pin the module providing its type.

unregister_key_type() calls key_gc_keytype(), which retypes every
remaining key of the type to key_type_dead under key->sem for writing.
The pkey paths read key->type again after their checks, so a concurrent
module unload can turn the checked callback into the NULL key_type_dead
member and call through a NULL function pointer, e.g.:

  KEYCTL_PKEY_SIGN task              Module unload and key GC
  ------------------------           ------------------------
  keyctl_pkey_e_d_s()
    if (!key->type->asym_eds_op)     unregister_key_type()
                                       GC: down_write(&key->sem)
                                       GC: retype to key_type_dead
                                       GC: up_write(&key->sem)
                                     free_module()
    in = memdup_user(...)
    key->type->asym_eds_op(...)      -> NULL call

In-tree, turris-signing-key provides such methods.

Hold key->sem for reading around the method check and the matching call,
so that the type cannot be retired in between.  Each keyctl handler takes
and releases the lock itself; the semaphore is not passed out through the
parameter block.  User buffers are copied outside the critical section.

This is the same type-removal problem recently fixed for the read
(925c84e3cc21) and update (aef3a8a4f988) methods.

Fixes: 00d60fd3b932 ("KEYS: Provide keyctls to drive the new key type ops for asymmetric keys [ver #2]")
Cc: stable@vger.kernel.org
Signed-off-by: Yuansheng Yang <0xbeac@gmail.com>
---
Changes in v3:
- Keep the switch, allocations and existing error handling in place.
- Use one unlock point per method check/call pair.
- Set -EOPNOTSUPP inside the failed checks; add blank lines before labels.
- Use my real name in the author and sign-off fields.

v2: https://lore.kernel.org/all/20261104021414.50696-1-0xbeac@gmail.com/

 security/keys/keyctl_pkey.c | 47 ++++++++++++++++++++++++++++---------
 1 file changed, 36 insertions(+), 11 deletions(-)

diff --git a/security/keys/keyctl_pkey.c b/security/keys/keyctl_pkey.c
index 15b6bf6399..40475817dd 100644
--- a/security/keys/keyctl_pkey.c
+++ b/security/keys/keyctl_pkey.c
@@ -100,9 +100,6 @@ static int keyctl_pkey_params_get(key_serial_t id,
 		return PTR_ERR(key_ref);
 	params->key = key_ref_to_ptr(key_ref);
 
-	if (!params->key->type->asym_query)
-		return -EOPNOTSUPP;
-
 	return 0;
 }
 
@@ -129,7 +126,16 @@ static int keyctl_pkey_params_get_2(const struct keyctl_pkey_params __user *_par
 	if (ret < 0)
 		return ret;
 
+	down_read(&params->key->sem);
+	if (!params->key->type->asym_query) {
+		ret = -EOPNOTSUPP;
+		goto out_unlock;
+	}
+
 	ret = params->key->type->asym_query(params, &info);
+
+out_unlock:
+	up_read(&params->key->sem);
 	if (ret < 0)
 		return ret;
 
@@ -185,7 +191,16 @@ long keyctl_pkey_query(key_serial_t id,
 	if (ret < 0)
 		goto error;
 
+	down_read(&params.key->sem);
+	if (!params.key->type->asym_query) {
+		ret = -EOPNOTSUPP;
+		goto out_unlock;
+	}
+
 	ret = params.key->type->asym_query(&params, &res);
+
+out_unlock:
+	up_read(&params.key->sem);
 	if (ret < 0)
 		goto error;
 
@@ -230,10 +245,6 @@ long keyctl_pkey_e_d_s(int op,
 	if (ret < 0)
 		goto error_params;
 
-	ret = -EOPNOTSUPP;
-	if (!params.key->type->asym_eds_op)
-		goto error_params;
-
 	switch (op) {
 	case KEYCTL_PKEY_ENCRYPT:
 		params.op = kernel_pkey_encrypt;
@@ -260,7 +271,16 @@ long keyctl_pkey_e_d_s(int op,
 	if (!out)
 		goto error_in;
 
+	down_read(&params.key->sem);
+	if (!params.key->type->asym_eds_op) {
+		ret = -EOPNOTSUPP;
+		goto out_unlock;
+	}
+
 	ret = params.key->type->asym_eds_op(&params, in, out);
+
+out_unlock:
+	up_read(&params.key->sem);
 	if (ret < 0)
 		goto error_out;
 
@@ -307,10 +327,6 @@ long keyctl_pkey_verify(const struct keyctl_pkey_params __user *_params,
 	if (ret < 0)
 		goto error_params;
 
-	ret = -EOPNOTSUPP;
-	if (!params.key->type->asym_verify_signature)
-		goto error_params;
-
 	in = memdup_user(_in, params.in_len);
 	if (IS_ERR(in)) {
 		ret = PTR_ERR(in);
@@ -324,8 +340,17 @@ long keyctl_pkey_verify(const struct keyctl_pkey_params __user *_params,
 	}
 
 	params.op = kernel_pkey_verify;
+	down_read(&params.key->sem);
+	if (!params.key->type->asym_verify_signature) {
+		ret = -EOPNOTSUPP;
+		goto out_unlock;
+	}
+
 	ret = params.key->type->asym_verify_signature(&params, in, in2);
 
+out_unlock:
+	up_read(&params.key->sem);
+
 	kfree(in2);
 error_in:
 	kfree(in);
-- 
2.43.0