49 lines
1.6 KiB
Markdown
49 lines
1.6 KiB
Markdown
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# Memory clearing
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The main memory on computer platforms in high security environments contains
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sensible data. On unexpected reboot the data might persist and could be
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read by a malicious application in the bootflow or userspace.
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In order to prevent leaking information from pre-reset, the boot firmware can
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clear the main system memory on boot, wiping all information.
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A common API indicates if the main memory has to be cleared. That could be
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on user request or by a Trusted Execution Environment indicating that secrets
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are in memory.
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As every platform has different bring-up mechanisms and memory-layouts, every
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The device must indicate support for memory clearing as part of the boot
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process.
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## Requirements
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1. The platform must clear all platform memory (DRAM) if requested
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2. Code that is placed in DRAM might be skipped (as workaround)
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3. Stack that is placed in DRAM might be skipped (as workaround)
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4. All DRAM is cleared with zeros
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## Implementation
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A platform that supports memory clearing selects Kconfig
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``PLATFORM_HAS_DRAM_CLEAR`` and calls
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```C
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bool security_clear_dram_request(void);
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```
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to detect if memory should be cleared.
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The memory is cleared in ramstage as part of `DEV_INIT` stage. It's possible to
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clear it earlier on some platforms, but on x86 MTRRs needs to be programmed
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first, which happens in `DEV_INIT`.
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Without MTRRs (and caches enabled) clearing memory takes multiple seconds.
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## Exceptions
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As some platforms place code and stack in DRAM (FSP1.0), the regions can be
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skipped.
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## Architecture specific implementations
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* [x86 PAE](../arch/x86/pae.md)
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