Set NX on direct map PML4Es.
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Authored by markj on Sep 22 2019, 7:51 PM.

Details

Reviewers

alc
kib

Commits

rS352624: Set NX on some non-leaf direct map page table entries.

Summary

For a while now we've been using the direct map without execute
permissions, and we have the ability to dynamically create executable
mappings in the kernel map. To help ensure that the direct map is never
used for code execution, mark the direct map PML4Es as NX as well. No
functional change intended.

Diff Detail

Repository

rS FreeBSD src repository - subversion

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Unit

Tests Not Applicable

Event Timeline

markj created this revision.Sep 22 2019, 7:51 PM

Harbormaster completed remote builds in B26629: Diff 62437.Sep 22 2019, 7:51 PM

markj added reviewers: alc, kib.Sep 22 2019, 7:53 PM

kib accepted this revision.Sep 22 2019, 8:21 PM

This revision is now accepted and ready to land.Sep 22 2019, 8:21 PM

alc added inline comments.Sep 22 2019, 8:58 PM

sys/amd64/amd64/pmap.c
1545–1559 ↗	(On Diff #62437)	Doesn't setting pg_nx on the direct map's PML4 entries conflict with what this block is trying to do?

markj added inline comments.Sep 22 2019, 9:23 PM

sys/amd64/amd64/pmap.c
1545–1559 ↗	(On Diff #62437)	I think the omission of pg_nx in the PDPEs is an oversight, and doesn't have any effect since it is set in the PDEs (which also do not set PG_RW). The intent of this block is to ensure that the kernel text is not writeable via the direct map, not to permit execution. So I believe the answer to your question is no.

alc added inline comments.Sep 22 2019, 10:09 PM

sys/amd64/amd64/pmap.c
1545–1559 ↗	(On Diff #62437)	I would argue that the phrase "appropriate permissions" in the above comment is simply too vague. I shouldn't have to infer what "appropriate" means. :-)