| Message ID | 20230226203816.207449-1-ebiggers@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | blk-crypto: make blk_crypto_evict_key() always try to evict | expand |
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
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)