On 10/20/22 19:56, Eric Biggers wrote:
On Thu, Oct 20, 2022 at 06:55:04PM -0400, Sweet Tea Dorminy wrote:On 10/20/22 17:45, Eric Biggers wrote:On Thu, Oct 20, 2022 at 12:58:23PM -0400, Sweet Tea Dorminy wrote:Some filesystems need to encrypt data based on extents, rather than on inodes, due to features incompatible with inode-based encryption. For instance, btrfs can have multiple inodes referencing a single block of data, and moves logical data blocks to different physical locations on disk in the background; these two features mean traditional inode-based file contents encryption will not work for btrfs. This change introduces fscrypt_extent_context objects, in analogy to existing context objects based on inodes. For a filesystem which opts to use extent-based encryption, a new hook provides a new fscrypt_extent_context, generated in close analogy to the IVs generated with existing policies. During file content encryption/decryption, the existing fscrypt_context object provides key information, while the new fscrypt_extent_context provides IV information. For filename encryption, the existing IV generation methods are still used, since filenames are not stored in extents. Signed-off-by: Sweet Tea Dorminy <sweettea-kernel@xxxxxxxxxx> --- fs/crypto/crypto.c | 20 ++++++++-- fs/crypto/fscrypt_private.h | 25 +++++++++++- fs/crypto/inline_crypt.c | 28 ++++++++++--- fs/crypto/policy.c | 79 +++++++++++++++++++++++++++++++++++++ include/linux/fscrypt.h | 47 ++++++++++++++++++++++ 5 files changed, 189 insertions(+), 10 deletions(-) diff --git a/fs/crypto/crypto.c b/fs/crypto/crypto.c index 7fe5979fbea2..08b495dc5c0c 100644 --- a/fs/crypto/crypto.c +++ b/fs/crypto/crypto.c @@ -81,8 +81,22 @@ void fscrypt_generate_iv(union fscrypt_iv *iv, u64 lblk_num, const struct fscrypt_info *ci) { u8 flags = fscrypt_policy_flags(&ci->ci_policy); + struct inode *inode = ci->ci_inode; + const struct fscrypt_operations *s_cop = inode->i_sb->s_cop; - memset(iv, 0, ci->ci_mode->ivsize); + memset(iv, 0, sizeof(*iv)); + if (s_cop->get_extent_context && lblk_num != U64_MAX) { + size_t extent_offset; + union fscrypt_extent_context ctx; + int ret; + + ret = fscrypt_get_extent_context(inode, lblk_num, &ctx, + &extent_offset, NULL); + WARN_ON_ONCE(ret); + memcpy(iv->raw, ctx.v1.iv.raw, sizeof(*iv)); + iv->lblk_num += cpu_to_le64(extent_offset); + return; + }Please read through my review comment https://lore.kernel.org/linux-fscrypt/Yx6MnaUqUTdjCmX+@quark/ again, as it doesn't seem that you've addressed it. - EricI probably didn't understand it correctly. I think there were three points in it: 1) reconsider per-extent keys 2) make IV generation work for non-directkey policies as similarly as possible to how they work in inode-based filesystems 3) never use 'file-based' except in contrast to dm-crypt and other block-layer encryption. For point 2, I changed the initial extent context generation to match up with fscrypt_generate_iv() (and probably didn't call that out enough in the description). (Looking at it again, I could literally call fscrypt_generate_iv() to generate the initial extent context; I didn't realize that before). Then adding lblk_num to the existing lblk_num in the iv from the start of the extent should be the same as the iv->lblk_num setting in the inode-based case: for lblk 12, for instance, the same IV should result from inode-based with lblk 12, as with extent-based with an initial lblk_num of 9 and an extent_offset of 3. For shared extents, they'll be different, but for singly-referenced extents, the IVs should be exactly the same in theory. I'm not sure whether I misunderstood the points or didn't address them fully, I apologize. Would you be up for elaborating where I missed, either by email or by videochat whenever works for you?It seems you misunderstood point (2). See what I said below: So if you do want to implement the DIRECT_KEY method, the natural thing to do would be to store a 16-byte nonce along with each extent, and use the DIRECT_KEY IV generation method as-is. It seems that you've done it a bit differently; you store a 32-byte nonce and generate the IV as 'nonce + lblk_num', instead of 'nonce || lblk_num'. I think that's a mistake -- it should be exactly the same. If the issue is that the 'nonce || lblk_num' method doesn't allow for AES-XTS support, we could extend DIRECT_KEY to do 'nonce + lblk_num' *if* the algorithm has a 16-byte IV size and thus has to tolerate some chance of IV reuse. Note that this change would be unrelated to extent-based encryption, and could be applied regardless of it. So: 1.) Provided that you've decided against per-extent keys, and are not trying to support UFS and eMMC inline encryption hardware, then you should *only* support DIRECT_KEY -- not other settings that don't make sense. 2.) There should be a preparatory patch that makes DIRECT_KEY be allowed when the IV length is 16 bytes, using the method 'nonce + lblk_num' -- assuming that you need AES-XTS support and aren't planning on supporting Adiantum or AES-HCTR2 only. (The small chance for IV reuse that it results in is not ideal, but it's probably tolerable. Maybe the nonce should also be hashed with a secret key, like what IV_INO_LBLK_32 does with the inode number; I'll have to think about it.) If you plan to just support AES-HCTR2 instead of AES-XTS, then you'd need a patch to allow AES-HCTR2 for contents encryption, as currently it is only allowed for filenames. 3.) Each extent context should contain a 16-byte random nonce, that is newly generated just for that extent -- not copied from anywhere. 4.) IVs should be generated using the DIRECT_KEY method. That is, 'nonce || lblk_num' if the IV length allows it, otherwise 'nonce + lblk_num' as mentioned in (2). For inode-based encryption, nonce means the inode's nonce, and lblk_num means the index of the block in the inode. For extent-based encryption, nonce will mean the extent's nonce, and lblk_num will mean the index of the block in the extent. - Eric
Awesome, thank you for the elaboration. I'll give it a shot tonight and will send out v4 as soon as it's ready.
-Sweet Tea
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