From: Eric Biggers ebiggers@google.com
Once all I/O using a blk_crypto_key has completed, filesystems can call blk_crypto_evict_key(). However, the block layer doesn't call blk_crypto_put_keyslot() until the request is being cleaned up, which happens after upper layers have been told (via bio_endio()) the I/O has completed. This causes a race condition where blk_crypto_evict_key() can see 'slot_refs > 0' without there being an actual bug.
This makes __blk_crypto_evict_key() hit the 'WARN_ON_ONCE(atomic_read(&slot->slot_refs) != 0)' and return without doing anything, eventually causing a use-after-free in blk_crypto_reprogram_all_keys(). (This is a very rare bug and has only been seen when per-file keys are being used with fscrypt.)
There are two options to fix this: either release the keyslot in blk_update_request() just before bio_endio() is called on the request's last bio, or just make __blk_crypto_evict_key() ignore slot_refs. Let's go with the latter solution for now, since it avoids adding overhead to the loop in blk_update_request(). (It does have the disadvantage that hypothetical bugs where a key is evicted while still in-use become harder to detect. But so far there haven't been any such bugs anyway.)
A related issue with __blk_crypto_evict_key() is that ->keyslot_evict failing would cause the same use-after-free as well. Fix this by always removing the key from the keyslot management structures.
Update the function documentation to properly document the semantics.
Fixes: 1b2628397058 ("block: Keyslot Manager for Inline Encryption") Cc: stable@vger.kernel.org Signed-off-by: Eric Biggers ebiggers@google.com --- block/blk-crypto-profile.c | 52 +++++++++++++++----------------------- block/blk-crypto.c | 24 +++++++++++------- 2 files changed, 36 insertions(+), 40 deletions(-)
diff --git a/block/blk-crypto-profile.c b/block/blk-crypto-profile.c index 0307fb0d95d3..29b4148cc50d 100644 --- a/block/blk-crypto-profile.c +++ b/block/blk-crypto-profile.c @@ -354,22 +354,11 @@ bool __blk_crypto_cfg_supported(struct blk_crypto_profile *profile, return true; }
-/** - * __blk_crypto_evict_key() - Evict a key from a device. - * @profile: the crypto profile of the device - * @key: the key to evict. It must not still be used in any I/O. - * - * If the device has keyslots, this finds the keyslot (if any) that contains the - * specified key and calls the driver's keyslot_evict function to evict it. - * - * Otherwise, this just calls the driver's keyslot_evict function if it is - * implemented, passing just the key (without any particular keyslot). This - * allows layered devices to evict the key from their underlying devices. - * - * Context: Process context. Takes and releases profile->lock. - * Return: 0 on success or if there's no keyslot with the specified key, -EBUSY - * if the keyslot is still in use, or another -errno value on other - * error. +/* + * This is an internal function that evicts a key from an inline encryption + * device that can be either a real device or the blk-crypto-fallback "device". + * It is used only for blk_crypto_evict_key(). For details on what this does, + * see the documentation for blk_crypto_evict_key(). */ int __blk_crypto_evict_key(struct blk_crypto_profile *profile, const struct blk_crypto_key *key) @@ -389,22 +378,23 @@ int __blk_crypto_evict_key(struct blk_crypto_profile *profile,
blk_crypto_hw_enter(profile); slot = blk_crypto_find_keyslot(profile, key); - if (!slot) - goto out_unlock; - - if (WARN_ON_ONCE(atomic_read(&slot->slot_refs) != 0)) { - err = -EBUSY; - goto out_unlock; + if (slot) { + /* + * Note: it is a bug if the key is still in use by I/O here. + * But 'slot_refs > 0' can't be used to detect such bugs here, + * since the keyslot isn't released until after upper layers + * have already been told the I/O is complete. + */ + err = profile->ll_ops.keyslot_evict( + profile, key, blk_crypto_keyslot_index(slot)); + /* + * Even on ->keyslot_evict failure, we must remove the + * blk_crypto_key from the keyslot management structures, since + * the caller is allowed to free it regardless. + */ + hlist_del(&slot->hash_node); + slot->key = NULL; } - err = profile->ll_ops.keyslot_evict(profile, key, - blk_crypto_keyslot_index(slot)); - if (err) - goto out_unlock; - - hlist_del(&slot->hash_node); - slot->key = NULL; - err = 0; -out_unlock: blk_crypto_hw_exit(profile); return err; } diff --git a/block/blk-crypto.c b/block/blk-crypto.c index 45378586151f..3dcbe578beb2 100644 --- a/block/blk-crypto.c +++ b/block/blk-crypto.c @@ -399,17 +399,23 @@ int blk_crypto_start_using_key(struct block_device *bdev, }
/** - * blk_crypto_evict_key() - Evict a key from any inline encryption hardware - * it may have been programmed into - * @bdev: The block_device who's associated inline encryption hardware this key - * might have been programmed into - * @key: The key to evict + * blk_crypto_evict_key() - Evict a blk_crypto_key from a block_device + * @bdev: a block_device on which I/O using the key may have been done + * @key: the key to evict * - * Upper layers (filesystems) must call this function to ensure that a key is - * evicted from any hardware that it might have been programmed into. The key - * must not be in use by any in-flight IO when this function is called. + * For a given block_device, this function removes the given blk_crypto_key from + * the keyslot management structures and evicts it from any underlying hardware + * or fallback keyslot(s) it may have been programmed into. * - * Return: 0 on success or if the key wasn't in any keyslot; -errno on error. + * Upper layers must call this before freeing the blk_crypto_key. It must be + * called for every block_device the key may have been used on. The key must no + * longer be in use by any I/O when this function is called. + * + * Context: May sleep. + * Return: 0 on success or if the key wasn't in any keyslot; -errno if the key + * failed to be evicted from a hardware keyslot. Even in the -errno + * case, the key is removed from the keyslot management structures and + * the caller is allowed (and expected) to free the blk_crypto_key. */ int blk_crypto_evict_key(struct block_device *bdev, const struct blk_crypto_key *key)
base-commit: 489fa31ea873282b41046d412ec741f93946fc2d
Hey Eric,
On Sun, Feb 26, 2023 at 12:43 PM Eric Biggers ebiggers@kernel.org wrote:
From: Eric Biggers ebiggers@google.com
Once all I/O using a blk_crypto_key has completed, filesystems can call blk_crypto_evict_key(). However, the block layer doesn't call blk_crypto_put_keyslot() until the request is being cleaned up, which happens after upper layers have been told (via bio_endio()) the I/O has completed. This causes a race condition where blk_crypto_evict_key() can see 'slot_refs > 0' without there being an actual bug.
This makes __blk_crypto_evict_key() hit the 'WARN_ON_ONCE(atomic_read(&slot->slot_refs) != 0)' and return without doing anything, eventually causing a use-after-free in blk_crypto_reprogram_all_keys(). (This is a very rare bug and has only been seen when per-file keys are being used with fscrypt.)
There are two options to fix this: either release the keyslot in blk_update_request() just before bio_endio() is called on the request's last bio, or just make __blk_crypto_evict_key() ignore slot_refs. Let's go with the latter solution for now, since it avoids adding overhead to the loop in blk_update_request(). (It does have the disadvantage that hypothetical bugs where a key is evicted while still in-use become harder to detect. But so far there haven't been any such bugs anyway.)
I disagree with the proposal to ignore the race condition in blk_crypto_evict_key(). As you said, ignoring the error could lead to undetected bugs in the future. Instead, I think we should focus on fixing the function ordering so that blk_crypto_put_keyslot() is called before blk_crypto_evict_key().
I think the overhead is a necessary trade-off to ensure correctness.
Thanks, Huck
A related issue with __blk_crypto_evict_key() is that ->keyslot_evict failing would cause the same use-after-free as well. Fix this by always removing the key from the keyslot management structures.
Update the function documentation to properly document the semantics.
Fixes: 1b2628397058 ("block: Keyslot Manager for Inline Encryption") Cc: stable@vger.kernel.org Signed-off-by: Eric Biggers ebiggers@google.com
block/blk-crypto-profile.c | 52 +++++++++++++++----------------------- block/blk-crypto.c | 24 +++++++++++------- 2 files changed, 36 insertions(+), 40 deletions(-)
diff --git a/block/blk-crypto-profile.c b/block/blk-crypto-profile.c index 0307fb0d95d3..29b4148cc50d 100644 --- a/block/blk-crypto-profile.c +++ b/block/blk-crypto-profile.c @@ -354,22 +354,11 @@ bool __blk_crypto_cfg_supported(struct blk_crypto_profile *profile, return true; }
-/**
- __blk_crypto_evict_key() - Evict a key from a device.
- @profile: the crypto profile of the device
- @key: the key to evict. It must not still be used in any I/O.
- If the device has keyslots, this finds the keyslot (if any) that contains the
- specified key and calls the driver's keyslot_evict function to evict it.
- Otherwise, this just calls the driver's keyslot_evict function if it is
- implemented, passing just the key (without any particular keyslot). This
- allows layered devices to evict the key from their underlying devices.
- Context: Process context. Takes and releases profile->lock.
- Return: 0 on success or if there's no keyslot with the specified key, -EBUSY
if the keyslot is still in use, or another -errno value on other
error.+/*
- This is an internal function that evicts a key from an inline encryption
- device that can be either a real device or the blk-crypto-fallback "device".
- It is used only for blk_crypto_evict_key(). For details on what this does,
*/
- see the documentation for blk_crypto_evict_key().
int __blk_crypto_evict_key(struct blk_crypto_profile *profile, const struct blk_crypto_key *key) @@ -389,22 +378,23 @@ int __blk_crypto_evict_key(struct blk_crypto_profile *profile,
blk_crypto_hw_enter(profile); slot = blk_crypto_find_keyslot(profile, key);
if (!slot)goto out_unlock;if (WARN_ON_ONCE(atomic_read(&slot->slot_refs) != 0)) {err = -EBUSY;goto out_unlock;
if (slot) {/** Note: it is a bug if the key is still in use by I/O here.* But 'slot_refs > 0' can't be used to detect such bugs here,* since the keyslot isn't released until after upper layers* have already been told the I/O is complete.*/err = profile->ll_ops.keyslot_evict(profile, key, blk_crypto_keyslot_index(slot));/** Even on ->keyslot_evict failure, we must remove the* blk_crypto_key from the keyslot management structures, since* the caller is allowed to free it regardless.*/hlist_del(&slot->hash_node);slot->key = NULL; }
err = profile->ll_ops.keyslot_evict(profile, key,blk_crypto_keyslot_index(slot));if (err)goto out_unlock;hlist_del(&slot->hash_node);slot->key = NULL;err = 0;-out_unlock: blk_crypto_hw_exit(profile); return err; } diff --git a/block/blk-crypto.c b/block/blk-crypto.c index 45378586151f..3dcbe578beb2 100644 --- a/block/blk-crypto.c +++ b/block/blk-crypto.c @@ -399,17 +399,23 @@ int blk_crypto_start_using_key(struct block_device *bdev, }
/**
- blk_crypto_evict_key() - Evict a key from any inline encryption hardware
it may have been programmed into
- @bdev: The block_device who's associated inline encryption hardware this key
might have been programmed into
- @key: The key to evict
- blk_crypto_evict_key() - Evict a blk_crypto_key from a block_device
- @bdev: a block_device on which I/O using the key may have been done
- @key: the key to evict
- Upper layers (filesystems) must call this function to ensure that a key is
- evicted from any hardware that it might have been programmed into. The key
- must not be in use by any in-flight IO when this function is called.
- For a given block_device, this function removes the given blk_crypto_key from
- the keyslot management structures and evicts it from any underlying hardware
- or fallback keyslot(s) it may have been programmed into.
- Return: 0 on success or if the key wasn't in any keyslot; -errno on error.
- Upper layers must call this before freeing the blk_crypto_key. It must be
- called for every block_device the key may have been used on. The key must no
- longer be in use by any I/O when this function is called.
- Context: May sleep.
- Return: 0 on success or if the key wasn't in any keyslot; -errno if the key
failed to be evicted from a hardware keyslot. Even in the -errno
case, the key is removed from the keyslot management structures and*/
the caller is allowed (and expected) to free the blk_crypto_key.int blk_crypto_evict_key(struct block_device *bdev, const struct blk_crypto_key *key)
base-commit: 489fa31ea873282b41046d412ec741f93946fc2d
2.39.2
On Thu, Mar 02, 2023 at 02:28:00PM -0800, Nathan Huckleberry wrote:
Hey Eric,
On Sun, Feb 26, 2023 at 12:43 PM Eric Biggers ebiggers@kernel.org wrote:
From: Eric Biggers ebiggers@google.com
Once all I/O using a blk_crypto_key has completed, filesystems can call blk_crypto_evict_key(). However, the block layer doesn't call blk_crypto_put_keyslot() until the request is being cleaned up, which happens after upper layers have been told (via bio_endio()) the I/O has completed. This causes a race condition where blk_crypto_evict_key() can see 'slot_refs > 0' without there being an actual bug.
This makes __blk_crypto_evict_key() hit the 'WARN_ON_ONCE(atomic_read(&slot->slot_refs) != 0)' and return without doing anything, eventually causing a use-after-free in blk_crypto_reprogram_all_keys(). (This is a very rare bug and has only been seen when per-file keys are being used with fscrypt.)
There are two options to fix this: either release the keyslot in blk_update_request() just before bio_endio() is called on the request's last bio, or just make __blk_crypto_evict_key() ignore slot_refs. Let's go with the latter solution for now, since it avoids adding overhead to the loop in blk_update_request(). (It does have the disadvantage that hypothetical bugs where a key is evicted while still in-use become harder to detect. But so far there haven't been any such bugs anyway.)
I disagree with the proposal to ignore the race condition in blk_crypto_evict_key(). As you said, ignoring the error could lead to undetected bugs in the future. Instead, I think we should focus on fixing the function ordering so that blk_crypto_put_keyslot() is called before blk_crypto_evict_key().
I think the overhead is a necessary trade-off to ensure correctness.
Thanks,
Sure, I'm concerned about pushback on adding something to blk_update_request() that's not critical, but I'll send out that patch for consideration too.
- Eric
linux-stable-mirror@lists.linaro.org
-
Eric Biggers -
Nathan Huckleberry