Re: [PATCH v3] fscrypt: add support for the encrypted key type

Eric Biggers
2018-01-26T00:37:56Z

On Thu, Jan 18, 2018 at 01:13:59PM +0000, André Draszik wrote:
> -static int validate_user_key(struct fscrypt_info *crypt_info,
> +static inline struct key *fscrypt_get_encrypted_key(const char *description)
> +{
> + if (IS_ENABLED(CONFIG_ENCRYPTED_KEYS))
> + return request_key(&key_type_encrypted, description, NULL);
> + return ERR_PTR(-ENOKEY);
> +}
> +
> +static int validate_keyring_key(struct fscrypt_info *crypt_info,
> struct fscrypt_context *ctx, u8 *raw_key,
> const char *prefix, int min_keysize)
> {
> char *description;
> struct key *keyring_key;
> - struct fscrypt_key *master_key;
> - const struct user_key_payload *ukp;
> + const u8 *master_key;
> + u32 master_key_len;
> int res;
>
> description = kasprintf(GFP_NOFS, "%s%*phN", prefix,
> @@ -83,39 +93,55 @@ static int validate_user_key(struct fscrypt_info *crypt_info,
> return -ENOMEM;
>
> keyring_key = request_key(&key_type_logon, description, NULL);
> + if (keyring_key == ERR_PTR(-ENOKEY))
> + keyring_key = fscrypt_get_encrypted_key(description);
> kfree(description);
> if (IS_ERR(keyring_key))
> return PTR_ERR(keyring_key);
> down_read(&keyring_key->sem);
>
> - if (keyring_key->type != &key_type_logon) {
> + if (keyring_key->type == &key_type_logon) {
> + const struct user_key_payload *ukp;
> + const struct fscrypt_key *fk;
> +
> + ukp = user_key_payload_locked(keyring_key);
> + if (!ukp) {
> + /* key was revoked before we acquired its semaphore */
> + res = -EKEYREVOKED;
> + goto out;
> + }
> + if (ukp->datalen != sizeof(struct fscrypt_key)) {
> + res = -EINVAL;
> + goto out;
> + }
> + fk = (struct fscrypt_key *)ukp->data;
> + master_key = fk->raw;
> + master_key_len = fk->size;
> + } else if (keyring_key->type == &key_type_encrypted) {
> + const struct encrypted_key_payload *ekp;
> +
> + ekp = keyring_key->payload.data[0];
> + master_key = ekp->decrypted_data;
> + master_key_len = ekp->decrypted_datalen;
> + } else {
> printk_once(KERN_WARNING
> - "%s: key type must be logon\n", __func__);
> + "%s: key type must be logon or encrypted\n",
> + __func__);
> res = -ENOKEY;
> goto out;
> }
> - ukp = user_key_payload_locked(keyring_key);
> - if (!ukp) {
> - /* key was revoked before we acquired its semaphore */
> - res = -EKEYREVOKED;
> - goto out;
> - }
> - if (ukp->datalen != sizeof(struct fscrypt_key)) {
> - res = -EINVAL;
> - goto out;
> - }
> - master_key = (struct fscrypt_key *)ukp->data;
> BUILD_BUG_ON(FS_AES_128_ECB_KEY_SIZE != FS_KEY_DERIVATION_NONCE_SIZE);
>
> - if (master_key->size < min_keysize || master_key->size > FS_MAX_KEY_SIZE
> - || master_key->size % AES_BLOCK_SIZE != 0) {
> + if (master_key_len < min_keysize || master_key_len > FS_MAX_KEY_SIZE
> + || master_key_len % AES_BLOCK_SIZE != 0) {
> printk_once(KERN_WARNING
> - "%s: key size incorrect: %d\n",
> - __func__, master_key->size);
> + "%s: key size incorrect: %u\n",
> + __func__, master_key_len);
> res = -ENOKEY;
> goto out;
> }
> - res = derive_key_aes(ctx->nonce, master_key, raw_key);
> + res = derive_key_aes(ctx->nonce, master_key, master_key_len, raw_key);
> +
> out:
> up_read(&keyring_key->sem);
> key_put(keyring_key);
> @@ -302,12 +328,12 @@ int fscrypt_get_encryption_info(struct inode *inode)
> if (!raw_key)
> goto out;
>
> - res = validate_user_key(crypt_info, &ctx, raw_key, FS_KEY_DESC_PREFIX,
> - keysize);
> + res = validate_keyring_key(crypt_info, &ctx, raw_key,
> + FS_KEY_DESC_PREFIX, keysize);
> if (res && inode->i_sb->s_cop->key_prefix) {
> - int res2 = validate_user_key(crypt_info, &ctx, raw_key,
> - inode->i_sb->s_cop->key_prefix,
> - keysize);
> + int res2 = validate_keyring_key(crypt_info, &ctx, raw_key,
> + inode->i_sb->s_cop->key_prefix,
> + keysize);
> if (res2) {
> if (res2 == -ENOKEY)
> res = -ENOKEY;
> --
> 2.15.1
>
> --
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The code changes here look fine, but unfortunately there is a lock ordering
problem exposed by using a key type that implements the key_type ->read()
method. The problem is that encrypted_read() can take a page fault during
copy_to_user() while holding the key semaphore, which then (with ext4) can
trigger the start of a jbd2 transaction. Meanwhile
fscrypt_get_encryption_info() can be called from within a jbd2 transaction via
fscrypt_inherit_context(), which results in a different locking order. We
probably will need to fix this by removing the call to
fscrypt_get_encryption_info() from fscrypt_inherit_context().

I've pasted the lockdep report I got below:

[ 432.705339]
[ 432.705681] ======================================================
[ 432.707015] WARNING: possible circular locking dependency detected
[ 432.708155] 4.15.0-rc8-next-20180119-00019-gab7ec46b6936 #31 Not tainted
[ 432.709365] ------------------------------------------------------
[ 432.710482] keyctl/877 is trying to acquire lock:
[ 432.711286] (&mm->mmap_sem){++++}, at: [<00000000e8bba1c7>] __might_fault+0xce/0x1a0
[ 432.712688]
[ 432.712688] but task is already holding lock:
[ 432.713684] (&type->lock_class#2){++++}, at: [<000000008ccfa156>] keyctl_read_key+0x174/0x240
[ 432.715176]
[ 432.715176] which lock already depends on the new lock.
[ 432.715176]
[ 432.716601]
[ 432.716601] the existing dependency chain (in reverse order) is:
[ 432.717901]
[ 432.717901] -> #3 (&type->lock_class#2){++++}:
[ 432.718924] lock_acquire+0xaa/0x170
[ 432.719632] down_read+0x37/0xa0
[ 432.720298] validate_keyring_key.isra.1+0x97/0x410
[ 432.721218] fscrypt_get_encryption_info+0x724/0x1200
[ 432.722233] fscrypt_inherit_context+0x2c1/0x330
[ 432.723067] __ext4_new_inode+0x34f5/0x44d0
[ 432.723830] ext4_create+0x195/0x4a0
[ 432.724499] lookup_open+0xbe2/0x1400
[ 432.725185] path_openat+0xb31/0x1e50
[ 432.725876] do_filp_open+0x175/0x250
[ 432.726572] do_sys_open+0x219/0x3f0
[ 432.727312] entry_SYSCALL_64_fastpath+0x1e/0x8b
[ 432.728153]
[ 432.728153] -> #2 (jbd2_handle){.+.+}:
[ 432.729008] lock_acquire+0xaa/0x170
[ 432.729680] start_this_handle+0x47b/0x1020
[ 432.730463] jbd2__journal_start+0x32e/0x5c0
[ 432.731276] __ext4_journal_start_sb+0xa8/0x190
[ 432.732183] ext4_truncate+0x4dd/0xd00
[ 432.732868] ext4_setattr+0xa62/0x1ac0
[ 432.733528] notify_change+0x672/0xdb0
[ 432.734198] do_truncate+0x111/0x1c0
[ 432.734831] path_openat+0xee6/0x1e50
[ 432.735476] do_filp_open+0x175/0x250
[ 432.736149] do_sys_open+0x219/0x3f0
[ 432.736785] entry_SYSCALL_64_fastpath+0x1e/0x8b
[ 432.737576]
[ 432.737576] -> #1 (&ei->i_mmap_sem){++++}:
[ 432.738508] lock_acquire+0xaa/0x170
[ 432.739148] down_read+0x37/0xa0
[ 432.739735] ext4_filemap_fault+0x7b/0xb0
[ 432.740446] __do_fault+0x7a/0x200
[ 432.741060] __handle_mm_fault+0x6e0/0x2040
[ 432.741801] handle_mm_fault+0x194/0x320
[ 432.742494] __do_page_fault+0x4e1/0xa70
[ 432.743184] page_fault+0x2c/0x60
[ 432.743784] __clear_user+0x3d/0x60
[ 432.744409] clear_user+0x76/0xa0
[ 432.745009] load_elf_binary+0x2bf6/0x2f10
[ 432.745753] search_binary_handler+0x136/0x450
[ 432.746522] do_execveat_common.isra.12+0x1241/0x19c0
[ 432.747339] do_execve+0x2c/0x40
[ 432.747881] try_to_run_init_process+0x12/0x40
[ 432.748611] kernel_init+0xf2/0x180
[ 432.749190] ret_from_fork+0x3a/0x50
[ 432.749785]
[ 432.749785] -> #0 (&mm->mmap_sem){++++}:
[ 432.750576] __lock_acquire+0x8d1/0x12d0
[ 432.751232] lock_acquire+0xaa/0x170
[ 432.751848] __might_fault+0x12b/0x1a0
[ 432.752478] _copy_to_user+0x27/0xc0
[ 432.753080] encrypted_read+0x54d/0x7b0
[ 432.753734] keyctl_read_key+0x1f2/0x240
[ 432.754396] SyS_keyctl+0x184/0x2a0
[ 432.754995] entry_SYSCALL_64_fastpath+0x1e/0x8b
[ 432.755778]
[ 432.755778] other info that might help us debug this:
[ 432.755778]
[ 432.756972] Chain exists of:
[ 432.756972] &mm->mmap_sem --> jbd2_handle --> &type->lock_class#2
[ 432.756972]
[ 432.758498] Possible unsafe locking scenario:
[ 432.758498]
[ 432.759302] CPU0 CPU1
[ 432.759919] ---- ----
[ 432.760536] lock(&type->lock_class#2);
[ 432.761100] lock(jbd2_handle);
[ 432.761926] lock(&type->lock_class#2);
[ 432.762836] lock(&mm->mmap_sem);
[ 432.763348]
[ 432.763348] *** DEADLOCK ***
[ 432.763348]
[ 432.764209] 1 lock held by keyctl/877:
[ 432.764765] #0: (&type->lock_class#2){++++}, at: [<000000008ccfa156>] keyctl_read_key+0x174/0x240
[ 432.766045]
[ 432.766045] stack backtrace:
[ 432.766671] CPU: 1 PID: 877 Comm: keyctl Not tainted 4.15.0-rc8-next-20180119-00019-gab7ec46b6936 #31
[ 432.768021] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
[ 432.769211] Call Trace:
[ 432.769559] dump_stack+0x8d/0xd5
[ 432.770041] print_circular_bug.isra.20+0x321/0x5e0
[ 432.770712] ? save_trace+0xd6/0x250
[ 432.771185] check_prev_add.constprop.27+0xa2a/0x1670
[ 432.771867] ? depot_save_stack+0x2d5/0x490
[ 432.772504] ? check_usage+0x13c0/0x13c0
[ 432.773047] ? trace_hardirqs_on_caller+0x3dc/0x550
[ 432.773712] ? _raw_spin_unlock_irqrestore+0x39/0x60
[ 432.774401] ? depot_save_stack+0x2d5/0x490
[ 432.774979] ? kasan_kmalloc+0x13a/0x170
[ 432.775522] ? validate_chain.isra.24+0x13ee/0x2410
[ 432.776187] validate_chain.isra.24+0x13ee/0x2410
[ 432.776831] ? check_prev_add.constprop.27+0x1670/0x1670
[ 432.777546] __lock_acquire+0x8d1/0x12d0
[ 432.778088] lock_acquire+0xaa/0x170
[ 432.778576] ? __might_fault+0xce/0x1a0
[ 432.779096] __might_fault+0x12b/0x1a0
[ 432.779608] ? __might_fault+0xce/0x1a0
[ 432.780029] _copy_to_user+0x27/0xc0
[ 432.780438] encrypted_read+0x54d/0x7b0
[ 432.780858] ? key_task_permission+0x16e/0x3b0
[ 432.781340] ? encrypted_instantiate+0x8b0/0x8b0
[ 432.781851] ? creds_are_invalid+0x9/0x50
[ 432.782287] ? lookup_user_key+0x19a/0xf50
[ 432.782736] ? __lock_acquire+0x8d1/0x12d0
[ 432.783182] ? key_validate+0xb0/0xb0
[ 432.783602] ? keyctl_read_key+0x174/0x240
[ 432.784058] keyctl_read_key+0x1f2/0x240
[ 432.784486] SyS_keyctl+0x184/0x2a0
[ 432.784875] entry_SYSCALL_64_fastpath+0x1e/0x8b
[ 432.785374] RIP: 0033:0x7f812b6c8259