Re: [PATCH] KVM: x86: fix RSM into 64-bit protected mode, round 2

["Radim Krčmář" <rkrcmar@redhat.com>]

2015-10-23 23:43+0200, Laszlo Ersek:
> Commit b10d92a54dac ("KVM: x86: fix RSM into 64-bit protected mode")
> reordered the rsm_load_seg_64() and rsm_enter_protected_mode() calls,
> relative to each other. The argument that said commit made was correct,
> however putting rsm_enter_protected_mode() first whole-sale violated the
> following (correct) invariant from em_rsm():
> 
>          * Get back to real mode, to prepare a safe state in which to load
>          * CR0/CR3/CR4/EFER.  Also this will ensure that addresses passed
>          * to read_std/write_std are not virtual.

Nice catch.

> Namely, rsm_enter_protected_mode() may re-enable paging, *after* which
> 
>   rsm_load_seg_64()
>     GET_SMSTATE()
>       read_std()
> 
> will try to interpret the (smbase + offset) address as a virtual one. This
> will result in unexpected page faults being injected to the guest in
> response to the RSM instruction.

I think this is a good time to introduce the read_phys helper, which we
wanted to avoid with that assumption.

> Split rsm_load_seg_64() in two parts:
> 
> - The first part, rsm_stash_seg_64(), shall call GET_SMSTATE() while in
>   real mode, and save the relevant state off SMRAM into an array local to
>   rsm_load_state_64().
> 
> - The second part, rsm_load_seg_64(), shall occur after entering protected
>   mode, but the segment details shall come from the local array, not the
>   guest's SMRAM.
> 
> Fixes: b10d92a54dac25a6152f1aa1ffc95c12908035ce
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Cc: Jordan Justen <jordan.l.justen@intel.com>
> Cc: Michael Kinney <michael.d.kinney@intel.com>
> Cc: stable@vger.kernel.org
> Signed-off-by: Laszlo Ersek <lersek@redhat.com>
> ---

The code would be cleaner if we had a different approach, but this works
too and is safer for stable. In case you prefer to leave the rewrite for
a future victim,

Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>