Re: [dm-devel] [PATCH v2 3/4] dm: add support for passing through inline crypto support

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Oct 27, 2020 at 11:58:47PM +0000, Satya Tangirala wrote:
> > > +/**
> > > + * blk_ksm_update_capabilities() - Update the restrictions of a KSM to those of
> > > + *				   another KSM
> > > + * @target_ksm: The KSM whose restrictions to update.
> > > + * @reference_ksm: The KSM to whose restrictions this function will update
> > > + *		   @target_ksm's restrictions to,
> > > + */
> > > +void blk_ksm_update_capabilities(struct blk_keyslot_manager *target_ksm,
> > > +				 struct blk_keyslot_manager *reference_ksm)
> > > +{
> > > +	memcpy(target_ksm->crypto_modes_supported,
> > > +	       reference_ksm->crypto_modes_supported,
> > > +	       sizeof(target_ksm->crypto_modes_supported));
> > > +
> > > +	target_ksm->max_dun_bytes_supported =
> > > +				reference_ksm->max_dun_bytes_supported;
> > > +}
> > > +EXPORT_SYMBOL_GPL(blk_ksm_update_capabilities);
> > 
> > Wouldn't it be easier to replace the original blk_keyslot_manager, rather than
> > modify it?  Then blk_ksm_update_capabilities() wouldn't be needed.
> > 
> I didn't want to replace the original blk_keyslot_manager because it's
> possible that e.g. fscrypt is checking for crypto capabilities support
> via blk_ksm_crypto_cfg_supported() when DM wants to replace the
> blk_keyslot_manager. DM would have to free the memory used by the
> blk_keyslot_manager, but blk_ksm_crypto_cfg_supported() might still
> be trying to access that memory. I did it this way to avoid having to
> add refcounts or something else to the blk_keyslot_manager...(And I
> didn't bother adding any synchronization code since the capabilities
> only ever expand, and never contract).

Are you sure that's possible?  That would imply that there is no synchronization
between limits/capabilities in the request_queue being changed and the
request_queue being used.  That's already buggy.  Maybe it's the sort of thing
that is gotten away with in practice, in which case avoiding a free() would
indeed be a good idea, but it's worth explicitly clarifying whether all this
code is indeed racy by design...

> > > diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c
> > > index cd0478d44058..2b3efa9f9fae 100644
> > > --- a/drivers/md/dm-ioctl.c
> > > +++ b/drivers/md/dm-ioctl.c
> > > @@ -1358,6 +1358,10 @@ static int table_load(struct file *filp, struct dm_ioctl *param, size_t param_si
> > >  		goto err_unlock_md_type;
> > >  	}
> > >  
> > > +	r = dm_verify_inline_encryption(md, t);
> > > +	if (r)
> > > +		goto err_unlock_md_type;
> > > +
> > >  	if (dm_get_md_type(md) == DM_TYPE_NONE) {
> > >  		/* Initial table load: acquire type of table. */
> > >  		dm_set_md_type(md, dm_table_get_type(t));
> > > @@ -2114,6 +2118,10 @@ int __init dm_early_create(struct dm_ioctl *dmi,
> > >  	if (r)
> > >  		goto err_destroy_table;
> > >  
> > > +	r = dm_verify_inline_encryption(md, t);
> > > +	if (r)
> > > +		goto err_destroy_table;
> > > +
> > >  	md->type = dm_table_get_type(t);
> > >  	/* setup md->queue to reflect md's type (may block) */
> > >  	r = dm_setup_md_queue(md, t);
> > 
> > Both table_load() and dm_early_create() call dm_setup_md_queue().  Wouldn't it
> > be simpler to handle inline encryption in dm_setup_md_queue(), instead of doing
> > it in both table_load() and dm_early_create()?
> > 
> table_load() only calls dm_setup_md_queue() on initial table load (when
> the md_type is DM_TYPE_NONE), so I can't call
> dm_verify_inline_encryption() in only dm_setup_md_queue(), because
> dm_verify_inline_encryption() needs to run on every table load.

Where do all the other limitations and capabilities of the request_queue get
updated on non-initial table loads, then?

> > > +/**
> > > + * dm_verify_inline_encryption() - Verifies that the current keyslot manager of
> > > + *				   the mapped_device can be replaced by the
> > > + *				   keyslot manager of a given dm_table.
> > > + * @md: The mapped_device
> > > + * @t: The dm_table
> > > + *
> > > + * In particular, this function checks that the keyslot manager that will be
> > > + * constructed for the dm_table will support a superset of the capabilities that
> > > + * the current keyslot manager of the mapped_device supports.
> > > + *
> > > + * Return: 0 if the table's keyslot_manager can replace the current keyslot
> > > + *	   manager of the mapped_device. Negative value otherwise.
> > > + */
> > > +int dm_verify_inline_encryption(struct mapped_device *md, struct dm_table *t)
> > > +{
> > > +	struct blk_keyslot_manager *ksm = dm_init_inline_encryption(md, t);
> > > +
> > > +	if (IS_ERR(ksm))
> > > +		return PTR_ERR(ksm);
> > > +	blk_ksm_destroy(ksm);
> > > +
> > > +	return 0;
> > > +}
> > 
> > This function seems redundant with dm_init_inline_encryption().  Wouldn't it be
> > simpler to do:
> > 
> > - dm_setup_md_queue() and dm_swap_table() call dm_init_inline_encryption() after
> >   dm_calculate_queue_limits().
> > 
> > - ksm gets passed to dm_table_set_restrictions(), which calls
> >   dm_update_keyslot_manager() (maybe rename to dm_update_inline_encryption()?)
> >   to actually set q->ksm.
> > 
> > That way, the crypto capabilities would be handled similarly to how the
> > queue_limits are already handled.
> > 
> If we call it from dm_swap_table(), we could have it pass the returned
> ksm to __bind(), either as a new argument, or by adding the ksm to the
> queue_limits (I'll have to check if that's ok/a good idea in the first
> place), and __bind() could send the argument to
> dm_table_set_restrictions()
> 
> But the real issue is, I think we should check whether a new table is
> valid (from the ksm capabilities support perspective) at the time that
> table is loaded (as opposed to only checking it when DM attempts to swap
> it in, which might be a lot later, when the user resumes the device) - so
> I can't only call it from dm_setup_md_queue(), and I'd have to call it
> from table_load() anyway. And the returned ksm that table_load() obtains
> from dm_init_inline_encryption() can't really be used - because
> 1) the ksm constructed at dm_swap_table() might actually support more
> capabilities than the ksm constructed in table_load(), because
> underlying devices might get resumed, and have new tables swapped in,
> and might support more capabilities than before
> 2) a subsequent dm_swap_table() call could fail for whatever reason, and
> we'll need to revert to the current ksm.
> 
> What I'm doing right now is simply freeing the ksm returned by
> dm_init_inline_encryption() whenever it's called from table_load()
> (and I'm trying to make that process a little nicer by wrapping it in a
> function called dm_verify_inline_encryption()) - so if we're going to
> have to call dm_init_inline_encryption() and then freeing the returned
> ksm in table_load(), I think it might be better to continue to have
> dm_verify_inline_encryption(), unless you'd prefer just open coding the
> function directly.

I don't understand why this needs to be so complicated.  Doesn't the dm layer
have the same problem for all the other queue limits and capabilities?  What
makes inline encryption different?

- Eric

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