[PATCH v6] KEYS: Protect pkey method checks against type removal

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

keyctl_pkey_query(), keyctl_pkey_e_d_s() and keyctl_pkey_verify() check
that key->type has the ->asym_*() method they need and call it later,
without holding key->sem.  A reference keeps the key alive, but does not
pin the module that provides its type.

When that module is unloaded, unregister_key_type() waits for the key GC
to process every remaining key of the type.  For each such key, the GC
takes key->sem for writing, switches key->type to key_type_dead, whose
->asym_*() methods are NULL, and destroys the payload.  If this happens
between the check and the call, the keyctl calls a NULL method:

  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

If it happens while the method is running, the method is left running in
freed module text.  In-tree, turris-signing-key provides ->asym_query()
and ->asym_eds_op() and can be built as a module.

Every read of a ->asym_*() method pointer must therefore happen with
key->sem held for reading.  Take it around each method call and check the
method again under it, right before the call.  While key->sem is held,
the GC cannot switch the key to key_type_dead, so unregister_key_type(),
and with it the module unload, cannot complete: the type, its module and
the payload stay in place until the method returns.  If the type is
already gone when key->sem is taken, the check finds key_type_dead and
the keyctl fails with -EOPNOTSUPP, as it does for any key type without
the method.

The checks that keyctl_pkey_params_get(), keyctl_pkey_e_d_s() and
keyctl_pkey_verify() used to do before copying the input were themselves
unlocked reads of the method pointer.  Move them into the key->sem
critical section that keyctl_pkey_params_get_2() takes around
->asym_query(), after the buffer size validation, so that a key type
without the method still fails with -EOPNOTSUPP before memdup_user() and
with the same error precedence as before.  keyctl_pkey_params_get() no
longer looks at the type; its callers check ->asym_query() under key->sem
before they use it.

memdup_user() and copy_to_user() stay outside key->sem, as in
keyctl_read_key(), so no page fault is taken with key->sem held.  key->sem
is only taken for reading, so pkey operations and KEYCTL_READ on the same
key still run concurrently.  Writers such as KEYCTL_UPDATE,
KEYCTL_REVOKE and KEYCTL_SETPERM now wait for a pkey operation in
progress, as they already wait for ->read().  The in-tree ->asym_*()
methods do not take key->sem themselves.

Nothing else in the kernel calls these three methods, so the four call
sites below are the full coverage:

 - keyctl_pkey_query(): ->asym_query() for KEYCTL_PKEY_QUERY
 - keyctl_pkey_params_get_2(): ->asym_query() and the op-specific method
   check for KEYCTL_PKEY_ENCRYPT, _DECRYPT, _SIGN and _VERIFY
 - keyctl_pkey_e_d_s(): ->asym_eds_op() for KEYCTL_PKEY_ENCRYPT,
   _DECRYPT and _SIGN
 - keyctl_pkey_verify(): ->asym_verify_signature() for
   KEYCTL_PKEY_VERIFY

This problem was recently fixed for keyctl_read_key() by 925c84e3cc21
("keys: Protect the read method check against type removal") and for
key_update() by aef3a8a4f988 ("KEYS: Protect the update method check
against type removal").

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 v6:
- Address the Sashiko review of v5: keyctl_pkey_params_get(),
  keyctl_pkey_e_d_s() and keyctl_pkey_verify() still read a ->asym_*()
  method pointer without key->sem in their early checks.  Those checks
  are gone.  The op-specific check now happens in
  keyctl_pkey_params_get_2(), in the same key->sem critical section that
  calls ->asym_query(), and after the buffer size validation, so a key
  type without the method still fails with -EOPNOTSUPP before
  memdup_user() and the error precedence is unchanged.
  keyctl_pkey_params_get() no longer looks at the key type at all; both
  of its callers check ->asym_query() under key->sem before using it.
- Drop the LLM paragraph from the commit message (Jarkko).

Tested on x86_64 under QEMU/KVM, with KASAN, lockdep and
DEBUG_ATOMIC_SLEEP, on 7.4-rc1 with and without this patch.  The
test-only key type module is not for merging.

  race_sign, race_verify: the module is unloaded while the keyctl waits
  in memdup_user() after the type check.  Without the patch: NULL call
  from keyctl_pkey_e_d_s() / keyctl_pkey_verify().  With the patch: both
  return -EOPNOTSUPP and delete_module() has returned first.

  inop: the module's ->asym_eds_op() sleeps in module text while the
  module is unloaded.  Without the patch: return into freed module text.
  With the patch: delete_module() returns only after the method has
  returned (about 3.0 s vs 3.1 s).

  func_asym, func_fake: KEYCTL_PKEY_QUERY, _ENCRYPT, _DECRYPT, _SIGN and
  _VERIFY on a real RSA-2048 asymmetric key and on the test type; the
  CASE lines are identical with and without the patch.

  errno: a type with ->asym_query() but no ->asym_verify_signature()
  returns -EOPNOTSUPP before reading the input; a request with an
  oversized length still returns -EINVAL first, as before.  The CASE
  lines are identical with and without the patch.

In the forced races lockdep can report "bad unlock balance" for
key->sem.  Its lock class is the type's lock_class, which lockdep drops
when the module is freed, so any key->sem user on a key whose type was
unregistered can hit this, e.g. keyctl_read_key().  KASAN reports
nothing.  Not tested on Turris hardware.

v5: https://lore.kernel.org/all/20261108131417.77287-1-0xbeac@gmail.com/
v4: https://lore.kernel.org/all/20261105193632.34363-1-0xbeac@gmail.com/
v3: https://lore.kernel.org/all/9d47e202bd11857b3555e6b95694b3c3a4f2460b.1793852399.git.0xbeac@gmail.com/
v2: https://lore.kernel.org/all/20261104021414.50696-1-0xbeac@gmail.com/
v1: https://lore.kernel.org/all/20261103032940.51826-1-0xbeac@gmail.com/

 security/keys/keyctl_pkey.c | 103 ++++++++++++++++++++++++++----------
 1 file changed, 76 insertions(+), 27 deletions(-)

diff --git a/security/keys/keyctl_pkey.c b/security/keys/keyctl_pkey.c
index 15b6bf6399..5ac049b71a 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;
 }
 
@@ -129,45 +126,77 @@ 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_up_key_sem;
+	}
+
 	ret = params->key->type->asym_query(params, &info);
 	if (ret < 0)
-		return ret;
+		goto out_up_key_sem;
 
 	switch (op) {
 	case KEYCTL_PKEY_ENCRYPT:
 		if (uparams.in_len  > info.max_dec_size ||
-		    uparams.out_len > info.max_enc_size)
-			return -EINVAL;
+		    uparams.out_len > info.max_enc_size) {
+			ret = -EINVAL;
+			goto out_up_key_sem;
+		}
 
 		params->out_len = info.max_enc_size;
 		break;
 	case KEYCTL_PKEY_DECRYPT:
 		if (uparams.in_len  > info.max_enc_size ||
-		    uparams.out_len > info.max_dec_size)
-			return -EINVAL;
+		    uparams.out_len > info.max_dec_size) {
+			ret = -EINVAL;
+			goto out_up_key_sem;
+		}
 
 		params->out_len = info.max_dec_size;
 		break;
 	case KEYCTL_PKEY_SIGN:
 		if (uparams.in_len  > info.max_data_size ||
-		    uparams.out_len > info.max_sig_size)
-			return -EINVAL;
+		    uparams.out_len > info.max_sig_size) {
+			ret = -EINVAL;
+			goto out_up_key_sem;
+		}
 
 		params->out_len = info.max_sig_size;
 		break;
 	case KEYCTL_PKEY_VERIFY:
 		if (uparams.in_len  > info.max_data_size ||
-		    uparams.in2_len > info.max_sig_size)
-			return -EINVAL;
+		    uparams.in2_len > info.max_sig_size) {
+			ret = -EINVAL;
+			goto out_up_key_sem;
+		}
 
 		params->out_len = info.max_sig_size;
 		break;
 	default:
-		return -EOPNOTSUPP;
+		ret = -EOPNOTSUPP;
+		goto out_up_key_sem;
 	}
 
 	params->in_len  = uparams.in_len;
-	return 0;
+
+	switch (op) {
+	case KEYCTL_PKEY_VERIFY:
+		if (!params->key->type->asym_verify_signature)
+			ret = -EOPNOTSUPP;
+		break;
+	case KEYCTL_PKEY_ENCRYPT:
+	case KEYCTL_PKEY_DECRYPT:
+	case KEYCTL_PKEY_SIGN:
+		if (!params->key->type->asym_eds_op)
+			ret = -EOPNOTSUPP;
+		break;
+	}
+
+out_up_key_sem:
+	up_read(&params->key->sem);
+	return ret;
 }
 
 /*
@@ -185,7 +214,16 @@ 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_up_key_sem;
+	}
+
 	ret = params.key->type->asym_query(&params, &res);
+
+out_up_key_sem:
+	up_read(&params.key->sem);
 	if (ret < 0)
 		goto error;
 
@@ -230,10 +268,6 @@ 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 +294,16 @@ 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_up_key_sem;
+	}
+
 	ret = params.key->type->asym_eds_op(&params, in, out);
+
+out_up_key_sem:
+	up_read(&params.key->sem);
 	if (ret < 0)
 		goto error_out;
 
@@ -305,31 +348,37 @@ long keyctl_pkey_verify(const struct keyctl_pkey_params __user *_params,
 	ret = keyctl_pkey_params_get_2(_params, _info, KEYCTL_PKEY_VERIFY,
 				       &params);
 	if (ret < 0)
-		goto error_params;
-
-	ret = -EOPNOTSUPP;
-	if (!params.key->type->asym_verify_signature)
-		goto error_params;
+		goto out_free_params;
 
 	in = memdup_user(_in, params.in_len);
 	if (IS_ERR(in)) {
 		ret = PTR_ERR(in);
-		goto error_params;
+		goto out_free_params;
 	}
 
 	in2 = memdup_user(_in2, params.in2_len);
 	if (IS_ERR(in2)) {
 		ret = PTR_ERR(in2);
-		goto error_in;
+		goto out_free_in;
 	}
 
 	params.op = kernel_pkey_verify;
+	down_read(&params.key->sem);
+	if (!params.key->type->asym_verify_signature) {
+		ret = -EOPNOTSUPP;
+		goto out_up_key_sem;
+	}
+
 	ret = params.key->type->asym_verify_signature(&params, in, in2);
 
+out_up_key_sem:
+	up_read(&params.key->sem);
 	kfree(in2);
-error_in:
+
+out_free_in:
 	kfree(in);
-error_params:
+
+out_free_params:
 	keyctl_pkey_params_free(&params);
 	return ret;
 }
-- 
2.43.0