minidumps are unsafe on amd64

Hi,

Kernel minidumps on amd64 SMP can write beyond the bounds
of the configured dump device causing (as in our case) the
file system data following swap partition to be overwritten
with the dump contents.

The problem is that while we're in the process of dumping
mapped physical pages via a bitmap (in minidump_machdep.c),
other CPUs continue to work and may modify page mappings of
processes.  This in turn causes the modifications to
pv_entries, which in turn modifies the bitmap of pages to
dump.  As the result, we can dump more pages than we've
calculated, and since dumps are written to the end of the
dump device, we may end up overwriting it.

The attached patch mitigates the problem, but the real solution
seems to be to disable interrupts (there's an XXX about this
in kern_shutdown.c before calling doadump()), and stopping
other CPUs, so we don't modify page tables while we're dumping.

This only affects 7.x/8.x amd64 SMP systems configured with
minidump.  i386 systems aren't affected.


Cheers,
-- 
Ruslan Ermilov
ru_at_FreeBSD.org
FreeBSD committer