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    SubjectRe: [patch V2 08/16] x86/ioperm: Add bitmap sequence number
    On Tue, 12 Nov 2019, Andy Lutomirski wrote:
    > On Mon, Nov 11, 2019 at 2:35 PM Thomas Gleixner <tglx@linutronix.de> wrote:
    > > /*
    > > + * The bitmap pointer and the sequence number of the last active
    > > + * bitmap. last_bitmap cannot be dereferenced. It's solely for
    > > + * comparison.
    > > + */
    > > + struct io_bitmap *last_bitmap;
    > > + u64 last_sequence;
    > > +
    > > + /*
    > > * Store the dirty size of the last io bitmap offender. The next
    > > * one will have to do the cleanup as the switch out to a non io
    > > * bitmap user will just set x86_tss.io_bitmap_base to a value
    >
    > Why is all this stuff in the TSS? Would it make more sense in a
    > percpu variable tss_state? By putting it in the TSS, you're putting
    > it in cpu_entry_area, which is at least a bit odd if not an actual
    > security problem.
    >
    > And why do you need a last_bitmap pointer? I thin that comparing just
    > last_sequence should be enough. Keeping last_bitmap around at all is
    > asking for trouble, since it might point to freed memory.

    The bitmap pointer is pointless as I said in an earlier reply to Peter. It
    will go away. The sequence number and the dirty size are surely not a
    problem leakage wise, but yes, we could put it into a per cpu variable as
    well. Not sure whether it buys much.

    > > - memcpy(tss->io_bitmap_bytes, iobm->bitmap_bytes,
    > > - max(tss->io_bitmap_prev_max, iobm->io_bitmap_max));
    > > + if (tss->last_bitmap != iobm ||
    > > + tss->last_sequence != iobm->sequence)
    >
    > As above, I think this could just be if (tss->last_sequence !=
    > iobm->sequence) or even if (this_cpu_read(cpu_tss_state.iobm_sequence)
    > != iobm->sequence).

    Already fixed as per Peter's comments.

    Thanks,

    tglx

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