Re: [patch] x86: fix ESP corruption CPU bug

[Stas Sergeev]

Hi.

Pavel Machek wrote:
> > +	andl $(VM_MASK | (4 << 8) | 3), %eax
> > +	cmpl $((4 << 8) | 3), %eax
> > +	je ldt_ss			# returning to user-space with LDT SS
> All common linux apps use same %ss, no? Perhaps it would be more
> efficient to just check if %ss == 0x7b, and proceed directly to
> restore_nocheck if not?
Such an optimization will cost three more
instructions, one of which is a "taken"
jump. It seems like the "taken" jump on
a fast path is not good, while now it is
only 5 instructions with a not-taken jump.
I am not sure here, but I think the current
solution is better (depends on how bad the
"taken" jump is, and how bad it is to have
the three extra insns for that optimization
purpose).

> Or perhaps we could only enable this code
> after application loads custom ldt?
The good thing here is that the code
actually does what you say, i.e. it jumps
to ldt_ss only when the app has loaded
the custom ldt and loaded that selector
to %ss. The way it is implemented, is
probably different from what you mean,
I assume you mean the new per-thread flag?
But I don't see how it can be more optimal,
i.e. you propose to check whether or not
the app altered the ldt (which can just be
the old glibc I think), while the current
solution is to also check whether it was
loaded to %ss (so the glibc case is avoided,
IIRC glibc used to load %gs with LDT selector).
I.e. since right now we jump to ldt_ss only
when the %ss is loaded with an LDT selector,
I think the extra checks are not needed.

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