Memory Protection Keys
References:
Overview:
Up to 16 protection domains with 4-bit tag on each page:
- Four (previously unused) bits in each page-table entry can be used to assign one of sixteen “key” values to any given page.
- A new 32-bit processor register with two bits for each key value.
- Setting “write disable” bit for a given key will block all attempts to write a page with that key value;
- Setting “access disable” bit will block reads;
- MPK feature thus allows a process to partition its memory into a maximum of sixteen regions and to selectively disable or enable access to any of those regions.
- Note: tags on virtual memory.
APIs to use the hardware feature:
mprotect() with four new flags:
Benefits:
- Originally, changing the protections on a region of memory can require individually changing the page-table entries for thousands (or more) pages.
- Now with MPK, once the protections keys are set, a region of memory can be enabled or disabled with a single register write. For any application that frequently changes the protections on regions of its address space, the performance improvement will be large.
- Usage scenario 1: handling sensitive cryptographic data.
- A network-facing daemon could use a cryptographic key to encrypt data to be sent over the wire, then disable access to the memory holding the key (and the plain-text data) before writing the data out. At that point, there is no way that the daemon can leak the key or the plain text over the wire;
- Protecting sensitive data in this way might also make applications a bit more resistant to attack.
- Usage scenario 2: protect regions of data from being corrupted by “stray” write operations.
- An in-memory database could prevent writes to the actual data most of the time, enabling them only briefly when an actual change needs to be made.
- In this way, database corruption due to bugs could be fended off, at least some of the time.
- Being able to turn off unexpected writes (quickly in large scale) could be essential useful when the underlying memory is a persistent memory device;
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