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PR/53993 CVS commit: src/sys

The following reply was made to PR port-amd64/53993; it has been noted by GNATS.

From: "Maxime Villard" <>
Subject: PR/53993 CVS commit: src/sys
Date: Sun, 24 Mar 2019 13:15:43 +0000

 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.

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