[csrng] Make reseed counters always readable, allow locking down internal state access per instance #23539
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vogelpi
requested review from
marnovandermaas,
h-filali,
johannheyszl and
andreaskurth
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a team and
marnovandermaas
moidx
approved these changes
Jun 6, 2024
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Thanks @vogelpi. This LGTM. |
Previously, the reseed counters were only readable if the otp_en_csrng_sw_app_read fuses were enabled and if the CTRL.READ_INT_STATE field was enabled, i.e., if the full internal state was readable. However, the reseed counter should be readable even if the internal state is not readable because this allows firmware to track the age of the different CSRNG contexts. The reseed counter itself is not sensitive information as it doesn't allow inferring the values of Key and V. Signed-off-by: Pirmin Vogel <vogelpi@lowrisc.org>
Previously, the design featured a single read enable switch plus an OTP fuse controlling read access for all three CTR_DRBG instances. This is non-ideal as firmware may want to perform known-answer testing of individual instances after enabling and then lock down instances individually. This commit thus adds a per-instance internal state read enable on top of the previously implemented mechanisms. After enabling, the internal state of all instances can be read and firmware can disable read access on a per-instance basis. Once firmware is done with the known-answer testing, it can lock the per-instance internal state read enable settings until the next reset. This resolves lowRISC#21141. Signed-off-by: Pirmin Vogel <vogelpi@lowrisc.org>
Signed-off-by: Pirmin Vogel <vogelpi@lowrisc.org>
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Thanks for your review @moidx . As discussed in the triage meeting yesterday it would be better to allow locking the read enable settings until the next reset in ROM_EXT . I've now modified the PR accordingly. |
moidx
approved these changes
Jun 7, 2024
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All tests except for |
This was referenced Jun 25, 2024
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As discussed here: #21141 (comment)