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SubjectRe: [PATCH] Add an option to dm-verity to validate hashes at most once
Hi Patrik,

On Tue, Mar 06, 2018 at 03:14:56PM -0800, Patrik Torstensson wrote:
> Add an option to dm-verity to validate hashes at most once
> to allow platforms that is CPU/memory contraint to be
> protected by dm-verity against offline attacks.
>
> The option introduces a bitset that is used to check if
> a block has been validated before or not. A block can
> be validated more than once as there is no thread protection
> for the bitset.
>
> This patch has been developed and tested on entry-level
> Android Go devices.
>
> Signed-off-by: Patrik Torstensson <totte@google.com>
> ---
> drivers/md/dm-verity-target.c | 58 +++++++++++++++++++++++++++++++++--
> drivers/md/dm-verity.h | 1 +
> 2 files changed, 56 insertions(+), 3 deletions(-)

The new option needs to be documented in Documentation/device-mapper/verity.txt,
including a description of what the option does as well as how it affects the
security properties of dm-verity. There should also be a mention of why the
option applies to data blocks but not hash blocks, assuming that's intentional.

verity_status() also needs to be updated to show the new option, otherwise it
will not be visible via the DM_TABLE_STATUS ioctl ('dmsetup table' on the
command line).

Also the minor version number in the struct target_type needs to be incremented,
so that userspace can determine whether the option is supported.

>
> for (b = 0; b < io->n_blocks; b++) {
> int r;
> + sector_t cur_block = io->block + b;
> struct ahash_request *req = verity_io_hash_req(v, io);
>
> + if (v->validated_blocks &&
> + likely(test_bit(cur_block, v->validated_blocks))) {
> + verity_bv_skip_block(v, io, &io->iter);
> + continue;
> + }
> +
> r = verity_hash_for_block(v, io, io->block + b,

Can you replace 'io->block + b' with 'cur_block' here as well?

Thanks,

- Eric

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