Date: Wed, 2 Dec 1998 20:26:51 +1100 From: Peter Jeremy <peter.jeremy@auss2.alcatel.com.au> To: mike@smith.net.au Cc: current@FreeBSD.ORG Subject: Re: sio breakage Message-ID: <98Dec2.202617est.40351@border.alcanet.com.au>
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Mike Smith <mike@smith.net.au> writes:
>> man 4 io.
>
>Having run the test program I used to check this as root, I see that it
>is indeed possible (although it should not be).
As stated in io(4), FreeBSD allows a process to execute I/O instructions
by changing IOPL to allow a ring-3 (user) task to perform I/O.
For reasons known best to Intel(*), CLI, STI and changing the IF using
POPF are controlled by IOPL, rather than being ring-0 instructions
(like the other privileged instructions).
An alternative approach would be to leave IOPL at 0 and use the I/O
Permission bitmap in the TSS. This allows I/O without allowing
CLI/STI. The disadvantage is that either you have a per-process TSS
(which is expensive in memory), or the TSS needs to be re-written on a
context switch (which is expensive in time). BSDI (at least BSDI 1.1)
used this approach - but the bitmap was system-wide, so enabling an
I/O port for any process enabled it for all processes (which isn't
particularly secure). [And at least on 486's, I/O instructions are
much slower is the IOPB is used instead of IOPL - even slower than
accessing the ISA bus :-(].
(*) The need to atomically issue multiple I/O instructions could be
seen to be a justification for this.
>There is no reference to manipulating interrupt state here.
I agree that io(4) should mention this.
>I'm still somewhat at a loss as to how it's meant to be possible for the
>page fault you posit to be handled in the case where PSL_I is cleared in
>all cases (eg. disk drivers with no interrupt timeout handlers).
There are occasional CLI/STI pairs in the kernel. Presumably an STI
gets executed somewhere along the line...
Peter
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