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Re: [Xen-devel] [PATCH v3] x86emul/fuzz: add a state sanity checking function
- To: Jan Beulich <JBeulich@xxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: George Dunlap <george.dunlap@xxxxxxxxxx>
- Date: Mon, 27 May 2019 11:51:04 +0100
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- Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- Delivery-date: Mon, 27 May 2019 10:51:16 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 4/2/19 2:01 PM, Jan Beulich wrote:
> This is to accompany sanitize_input(). Just like for initial state we
> want to have state between two emulated insns sane, at least as far as
> assumptions in the main emulator go. Do minimal checking after segment
> register, CR, and MSR writes, and roll back to the old value in case of
> failure (raising #GP(0) at the same time).
>
> In the particular case observed, a CR0 write clearing CR0.PE was
> followed by a VEX-encoded insn, which the decoder accepts based on
> guest address size, restricting things just outside of the 64-bit case
> (real and virtual modes don't allow VEX-encoded insns). Subsequently
> _get_fpu() would then assert that CR0.PE must be set (and EFLAGS.VM
> clear) when trying to invoke YMM, ZMM, or OPMASK state.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Reviewed-by: George Dunlap <george.dunlap@xxxxxxxxxx>
That said, I wonder if there's a way to avoid the duplication between
sanitize_input() and check_state(). Another option would be to rework
sanitize_input() (perhaps as sanizite_state()):
* Accept a parameter saying whether to do optional changes (like
CANONICALIZE_MAYBE)
* Return a boolean saying whether any state was in fact sanitized.
Then the current callers of check_state() could instead call
sanitize_state(), and throw an exception if it returns 1. (Or some
variation thereof.)
Just a thought; I'm OK with checking this in as it is.
-George
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