CVS commit: src/sys/arch/amd64/amd64

["Maxime Villard" <maxv%netbsd.org@localhost>]

Module Name:    src
Committed By:   maxv
Date:           Thu Mar 23 17:25:51 UTC 2017

Modified Files:
        src/sys/arch/amd64/amd64: locore.S machdep.c trap.c

Log Message:
Remove this call gate on amd64, it is useless and vulnerable.

Call gates do not modify %rflags, so interrupts are not disabled when
entering the gate. There is a small window where we are in kernel mode and
with a userland %gs, and if an interrupt happens here we will rejump into
the kernel but not switch to the kernel TLS.

Userland can simply perform a gate call in a loop, and hope that at some
point an interrupt will be received in this window - which necessarily will
be the case. With a specially-crafted %gs it is certainly enough to
escalate privileges.


To generate a diff of this commit:
cvs rdiff -u -r1.121 -r1.122 src/sys/arch/amd64/amd64/locore.S
cvs rdiff -u -r1.253 -r1.254 src/sys/arch/amd64/amd64/machdep.c
cvs rdiff -u -r1.94 -r1.95 src/sys/arch/amd64/amd64/trap.c

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.