30 lines
917 B
Markdown
30 lines
917 B
Markdown
# x86 System Management Mode
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## Introduction
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The code running in System Management Mode (SMM) provides runtime services
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to applications running in [ring0]. It has a higher privilege level than
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[ring0] and resides in the SMRAM region which cannot be accessed from [ring0].
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SMM can be entered by issuing System Management Interrupts (SMIs).
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## Secure data exchange
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In order to not leak SMM internals or accidentally overwrite parts of SMM,
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[ring0] provided data (pointers, offsets, sizes, ...) must be checked before
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using them in SMM.
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There exist two methods to verify data:
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```C
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/* Returns true if the region overlaps with the SMM */
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bool smm_region_overlaps_handler(struct region *r);
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```
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```C
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/* Returns true if the memory pointed to overlaps with SMM reserved memory. */
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static inline bool smm_points_to_smram(const void *ptr, const size_t len);
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```
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[ring0]: https://en.wikipedia.org/wiki/Protection_ring
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