[PATCH v3] blk-crypto: fix NULL deref and UAF in __blk_crypto_evict_key()

[Baokun Li <libaokun@linux.alibaba.com>]

Sashiko reported a possible NULL pointer dereference, triggered by the
following flow:

  dm-inlinecrypt ctor            # first use of the fallback
   ├─ blk_crypto_init_key()      # succeeds, ctx->key.size != 0
   ├─ blk_crypto_start_using_key()
   │   ├─ hw-wrapped key: -EOPNOTSUPP
   │   │    # profile never allocated (NULL)
   │   └─ raw key: fallback_start_using_mode()
   │      └─ blk_crypto_fallback_init() fails
   │         └─ kfree(profile)   # not NULLed
   ├─ goto bad
   └─ inlinecrypt_dtr()          # evicts whenever key.size is set
      └─ blk_crypto_evict_key()
         └─ blk_crypto_config_supported_natively() = false
            └─ blk_crypto_fallback_evict_key()
               └─ __blk_crypto_evict_key(blk_crypto_fallback_profile)
                  └─ profile->num_slots # BOOM!!!

This issue can be reproduced using the following commands:

  dd if=/dev/zero of=/tmp/backing bs=1M count=64
  LOOP=$(losetup -f --show /tmp/backing)
  SZ=$(blockdev --getsz $LOOP)
  KEY=$(head -c 64 /dev/urandom | od -An -tx1 | tr -d ' \n')
  dmsetup create ic --table "0 $SZ inlinecrypt aes-xts-plain64 \
    $KEY 0 $LOOP 0 1 keytype:hw-wrapped"

And the following oops is triggered:

  ==================================================================
  blk-crypto: loop0: no support for wrapped keys
  Oops: general protection fault, probably for non-canonical address
        0xdffffc000000000b: 0000 [#1] SMP KASAN PTI
  KASAN: null-ptr-deref in range [0x0000000000000058-0x000000000000005f]
  CPU: 5 UID: 0 PID: 901 Comm: dmsetup 7.3.0-rc3 #1 PREEMPT(lazy)
  RIP: 0010:__blk_crypto_evict_key+0x2e/0x3b0
  Call Trace:
   blk_crypto_evict_key+0x24c/0x320
   inlinecrypt_dtr+0xf6/0x160
   inlinecrypt_ctr+0x2d3/0xbd0
   dm_table_add_target+0x661/0xf40
   table_load+0x1c4/0x490
   ctl_ioctl+0x32f/0x5f0
   dm_ctl_ioctl+0xe/0x20
  ==================================================================

dm-inlinecrypt is the only caller that can evict a key that was never
started, as fscrypt frees such a key without evicting it.

Fix this by returning early from blk_crypto_fallback_evict_key() when
tfms_inited[] is not set for the key's mode.  tfms_inited[] is only set
after blk_crypto_fallback_init() has successfully initialized the
profile, so the crash above is no longer reachable.

The bug has been latent since commit c984ff1423ae ("blk-crypto:
dynamically allocate fallback profile") was merged in v6.5, and only
became actually triggerable when commit e7f57d2c47e2 ("dm-inlinecrypt:
add target for inline block device encryption") was merged in v7.2.

Fixes: c984ff1423ae ("blk-crypto: dynamically allocate fallback profile")
Cc: stable@vger.kernel.org
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://patch.msgid.link/sashiko-outbox-167341@kernel.org
Suggested-by: Eric Biggers <ebiggers@kernel.org>
Link: https://patch.msgid.link/20261014155131.GB322435@quark
Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
---
Changes since v2:
 * Skip fallback eviction if tfms_inited[] is unset
   (Suggested by Eric Biggers).
 * Drop v2 2/2; fix the weak ordering in fscrypt instead
   (Suggested by Eric Biggers).

Changes since v1:
 * Publish the profile only after successful init to avoid concurrent
   UAF (Reported by Sashiko).

v2: https://patch.msgid.link/20261014071846.3666398-1-libaokun@linux.alibaba.com
v1: https://patch.msgid.link/20261013095106.881914-1-libaokun@linux.alibaba.com

 block/blk-crypto-fallback.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/block/blk-crypto-fallback.c b/block/blk-crypto-fallback.c
index 2a8f40a65158..12e0b487e23a 100644
--- a/block/blk-crypto-fallback.c
+++ b/block/blk-crypto-fallback.c
@@ -524,6 +524,12 @@ bool blk_crypto_fallback_bio_prep(struct bio *bio)
 
 int blk_crypto_fallback_evict_key(const struct blk_crypto_key *key)
 {
+	/*
+	 * Pairs with smp_store_release() in
+	 * blk_crypto_fallback_start_using_mode()
+	 */
+	if (!smp_load_acquire(&tfms_inited[key->crypto_cfg.crypto_mode]))
+		return 0;
 	return __blk_crypto_evict_key(blk_crypto_fallback_profile, key);
 }
 

base-commit: 734bef548a96e8774d6eef5dfa2026c6edc4e80b
-- 
2.43.7