tree 1a01fdc9a4fcc97dc1e969367da1ca01191d1dd7
parent 8bcc565750e2446017a4d0a3323b4b50ba42359a
author Venkatesh Srinivas <venkateshs@google.com> 1561008296 +0000
committer CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org> 1561008296 +0000

[kernel][x86] user_copy: Move CLAC/STAC to immediately around REP MOVSB

Where available, Zircon enables SMAP; this means accesses to non-
supervisor pages in supervisor mode need to be bracketed by CLAC/
STAC instructions (to disable that checking).

Move the CLAC and STAC instruction pair to immediately before/after
REP MOVSB, with no other intervening memory accesses.

The old code would perform one supervisor access inside the CLAC/STAC
unprotected region, to the |fault_return| address. If a caller managed
to trick this function into being called with a user |fault_return|,
the kernel would write unexpectedly to user mode addresses.

No functional change, cinches the belt and suspenders a little tighter.

Change-Id: Iacc09fdc71c17486874cc4a0ecf1fe0e1fbdfcba