[PATCH v2 5/5] fscrypt: fix race where ->lookup() marks plaintext dentry as ciphertext

Theodore Ts'o tytso at mit.edu
Wed Apr 17 07:24:35 PDT 2019


On Wed, Mar 20, 2019 at 11:39:13AM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers at google.com>
> 
> ->lookup() in an encrypted directory begins as follows:
> 
> 1. fscrypt_prepare_lookup():
>     a. Try to load the directory's encryption key.
>     b. If the key is unavailable, mark the dentry as a ciphertext name
>        via d_flags.
> 2. fscrypt_setup_filename():
>     a. Try to load the directory's encryption key.
>     b. If the key is available, encrypt the name (treated as a plaintext
>        name) to get the on-disk name.  Otherwise decode the name
>        (treated as a ciphertext name) to get the on-disk name.
> 
> But if the key is concurrently added, it may be found at (2a) but not at
> (1a).  In this case, the dentry will be wrongly marked as a ciphertext
> name even though it was actually treated as plaintext.
> 
> This will cause the dentry to be wrongly invalidated on the next lookup,
> potentially causing problems.  For example, if the racy ->lookup() was
> part of sys_mount(), then the new mount will be detached when anything
> tries to access it.  This is despite the mountpoint having a plaintext
> path, which should remain valid now that the key was added.
> 
> Of course, this is only possible if there's a userspace race.  Still,
> the additional kernel-side race is confusing and unexpected.
> 
> Close the kernel-side race by changing fscrypt_prepare_lookup() to also
> set the on-disk filename (step 2b), consistent with the d_flags update.
> 
> Fixes: 28b4c263961c ("ext4 crypto: revalidate dentry after adding or removing the key")
> Signed-off-by: Eric Biggers <ebiggers at google.com>

Looks good, applied.

					- Ted



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