Hi Jeffrey,
On Fri, Nov 18, 2022 at 11:02 PM Jeffrey Kardatzke via OP-TEE op-tee@lists.trustedfirmware.org wrote:
Hello,
New to this list, I'm working on integrating Op-Tee into ChromeOS ARM devices for the purpose of Widevine DRM.
Does anybody know if Op-Tee creates mappings in the MMU that go beyond the registered TZ memory region and the static non-secure shared memory region?
TL;DR
Does anyone know if there would be problems associated with invoking register_ddr()[1] with a size larger than the physical RAM size? The reason I want to do this is because we have multiple different SKUs for the same reference board, and for ChromeOS we build per-board images and not per-SKU images. We could use a DT to do this, but that's running into other issues...so I'm trying to determine if this easy fix will solve the problem w/out creating new ones. (it does work fine when I test it)
register_ddr() defines the ranges that OP-TEE accepts as non-secure shared memory. I you add memory that doesn't exist then OP-TEE can be tricked into mapping memory that doesn't exist. When using such mappings you may get unexpected external aborts. However, as long as normal world doesn't use those addresses you will not notice any difference.
If you could find a way of probing the amount of available memory at boot you could use that instead of a static registration a bit like how we read from DT.
I understand that an exploited normal world could send a PA pointing to a region outside of the physical bounds for dynamic shared memory, and then Op-Tee would try to map that and undesirable things may happen (but I don't think they would lead to secure world exploits since the memory will be mapped as non-secure, so it shouldn't be any worse that the normal world trying to access physical memory out of bounds).
Secure world panic() seems a likely worst-case scenario unless it triggers an unexpected bug in secure world.
One of the main concerns is around whether Op-Tee is creating mappings in the MMU that reference the whole nonsecure DDR size for some other reason. Then the CPU may do speculative execution and try to access the physical memory at that invalid mapping and create problems, which could then lead to failure when someone isn't trying to exploit the system.
Does anybody know if Op-Tee creates mappings in the MMU that go beyond the registered TZ memory region and the static non-secure shared memory region?
That's only done if such addresses are supplied by normal world.
Cheers, Jens
[1] https://github.com/OP-TEE/optee_os/blob/master/core/arch/arm/plat-mediatek/m...
Thanks,
Jeffrey Kardatzke jkardatzke@google.com Google, Inc.