fscrypt: Add missing superblock check in find_or_insert_direct_key()

The legacy 'fscrypt_direct_keys' table caches master keys that are used
by v1 encryption policies that have FSCRYPT_POLICY_FLAG_DIRECT_KEY.
It's just a global table for all filesystems (since the keys can be
provided by the legacy process-subscribed keyrings mechanism, which
makes it difficult to reuse super_block::s_master_keys).

The entries in it ('struct fscrypt_direct_key') do contain a super_block
pointer, though, for passing to fscrypt_destroy_inline_crypt_key() when
the last inode that references the key is evicted.

However, when finding the fscrypt_direct_key for an inode, we weren't
actually comparing the super_block pointer.  As a result, inodes with
different super_blocks could point to the same fscrypt_direct_key.  That
could extend the lifetime of a fscrypt_direct_key beyond the
super_block it points to, causing a use-after-free later.

Fix this by creating distinct fscrypt_direct_key structs for distinct
super_block structs.

Note that this problem doesn't exist in the v2 policy equivalent
("per-mode keys"), since the data structures there are per super_block.

Fixes: 22e9947a4b2b ("fscrypt: stop holding extra request_queue references")
Cc: stable@vger.kernel.org
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260717044303.425265-1-ebiggers%40kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20260719033120.122120-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

1 files changed, 7 insertions, 1 deletions

diff --git a/fs/crypto/keysetup_v1.c b/fs/crypto/keysetup_v1.c
index e6e527c73f16..7e3a58dc4b56 100644
--- a/fs/crypto/keysetup_v1.c
+++ b/fs/crypto/keysetup_v1.c
@@ -147,13 +147,19 @@ find_or_insert_direct_key(struct fscrypt_direct_key *to_insert,
 		if (memcmp(ci->ci_policy.v1.master_key_descriptor,
 			   dk->dk_descriptor, FSCRYPT_KEY_DESCRIPTOR_SIZE) != 0)
 			continue;
+		/* The sb is used at eviction time, so it must be the same. */
+		if (ci->ci_inode->i_sb != dk->dk_sb)
+			continue;
 		if (ci->ci_mode != dk->dk_mode)
 			continue;
 		if (!fscrypt_is_key_prepared(&dk->dk_key, ci))
 			continue;
 		if (crypto_memneq(raw_key, dk->dk_raw, ci->ci_mode->keysize))
 			continue;
-		/* using existing tfm with same (descriptor, mode, raw_key) */
+		/*
+		 * Use an existing prepared key with the same (descriptor, sb,
+		 * mode, inlinecrypt, raw_key) combination.
+		 */
 		refcount_inc(&dk->dk_refcount);
 		spin_unlock(&fscrypt_direct_keys_lock);
 		free_direct_key(to_insert);