CVS commit: src/sys

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

Module Name:    src
Committed By:   maxv
Date:           Sun Mar 24 13:15:43 UTC 2019

Modified Files:
        src/sys/arch/amd64/amd64: machdep.c
        src/sys/compat/linux/arch/amd64: linux_machdep.c

Log Message:
Fix a tiny race in setregs and linux_setregs. Between the moment we set
pcb_flags to zero, and the moment cpu_segregs64_zero resets pcb_gs, we may
be preempted.

If this happens, and if the calling LWP was a 32bit thread, when switching
back to that LWP, the context switcher sees that PCB_COMPAT32 is not set in
pcb_flags and tries to perform a 64bit context switch; but pcb_gs contains
a 32bit GDT descriptor, and not a 64bit GS.base value. The wrmsr therefore
faults because the value is non-canonical, and this fault is fatal.

Rearrange the code so that the update of pcb_flags and pcb_gs/pcb_fs is non
interruptible. This fixes the problem, tested with a reproducer (which
therefore doesn't work anymore).

Likely fixes PR/53993.


To generate a diff of this commit:
cvs rdiff -u -r1.327 -r1.328 src/sys/arch/amd64/amd64/machdep.c
cvs rdiff -u -r1.56 -r1.57 src/sys/compat/linux/arch/amd64/linux_machdep.c

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