Re: [PATCH] xen/arm64: Zero the top 32 bits of gp registers on entry...

[Julien Grall <julien@xxxxxxx>]

On 14/12/2021 09:17, Michal Orzel wrote:
> Hi Julien, Jan
Hi,

> On 08.12.2021 10:55, Julien Grall wrote:
> > Hi,
> >
> > On 08/12/2021 07:20, Jan Beulich wrote:
> > > On 07.12.2021 20:11, Julien Grall wrote:
> > > > On 07/12/2021 08:37, Michal Orzel wrote:
> > > > > Hi Julien,
> > > > Hi,
> > > >
> > > > > On 06.12.2021 16:29, Julien Grall wrote:
> > > > > > Hi,
> > > > > >
> > > > > > On 06/12/2021 14:20, Michal Orzel wrote:
> > > > > > > to hypervisor when switching to AArch32 state.
> > > > > I will change to "from AArch32 state".
> > > > > > > According to section D1.20.2 of Arm Arm(DDI 0487A.j):
> > > > > > > "If the general-purpose register was accessible from AArch32 state the
> > > > > > > upper 32 bits either become zero, or hold the value that the same
> > > > > > > architectural register held before any AArch32 execution.
> > > > > > > The choice between these two options is IMPLEMENTATIONDEFINED"
> > > > > > Typo: Missing space between IMPLEMENTATION and DEFINED.
> > > > > Ok.
> > > > > > > Currently Xen does not ensure that the top 32 bits are zeroed and this
> > > > > > > needs to be fixed.
> > > > > > Can you outline why this is a problem and why we need to protect? IIRC, the 
> > > > > > main concern is Xen may misinterpret what the guest requested but we are not 
> > > > > > concerned about Xen using wrong value.
> > > > > I would say:
> > > > > "
> > > > > The reason why this is a problem is that there are places in Xen where we 
> > > > > assume that top 32bits are zero for AArch32 guests.
> > > > > If they are not, this can lead to misinterpretation of Xen regarding what the 
> > > > > guest requested.
> > > > > For example hypercalls returning an error encoded in a signed long like 
> > > > > do_sched_op, do_hmv_op, do_memory_op would return -ENOSYS
> > > > > if the command passed as the first argument was clobbered,
> > > > > "
> > > > > > > Fix this bug by zeroing the upper 32 bits of these registers on an
> > > > > > > entry to hypervisor when switching to AArch32 state.
> > > > > > >
> > > > > > > Set default value of parameter compat of macro entry to 0 (AArch64 mode
> > > > > > > as we are on 64-bit hypervisor) to avoid checking if parameter is blank
> > > > > > > when not passed.
> > > > > > Which error do you see otherwise? Is it a compilation error?
> > > > > Yes, this is a compilation error. The errors appear at each line when "entry" is called 
> > > > > without passing value for "compat".
> > > > > So basically in all the places where entry is called with hyp=1.
> > > > > When taking the current patch and removing default value for compat you will 
> > > > > get:
> > > > > ```
> > > > > entry.S:254: Error: ".endif" without ".if"
> > > > > entry.S:258: Error: symbol `.if' is already defined
> > > > > entry.S:258: Error: ".endif" without ".if"
> > > > > entry.S:262: Error: symbol `.if' is already defined
> > > > > entry.S:262: Error: ".endif" without ".if"
> > > > > entry.S:266: Error: symbol `.if' is already defined
> > > > > entry.S:266: Error: ".endif" without ".if"
> > > > > entry.S:278: Error: symbol `.if' is already defined
> > > > > entry.S:278: Error: ".endif" without ".if"
> > > > > entry.S:292: Error: symbol `.if' is already defined
> > > > > entry.S:292: Error: ".endif" without ".if"
> > > > > entry.S:317: Error: symbol `.if' is already defined
> > > > > entry.S:317: Error: ".endif" without ".if"
> > > > > ```
> > > > Thanks for input. I am concerned with your suggested approach (or using
> > > > .if 0\compat as suggested by Jan) because they allow the caller to not
> > > > properly specify compat when hyp=0. The risk here is we may generate the
> > > > wrong entry.
> > > >
> > > > compat should need to be specified when hyp=1 as we will always run in
> > > > aarch64 mode. So could we protect this code with hyp=0?
> > > Since my suggestion was only to avoid the need for specifying a default
> > > for the parameter (which you didn't seem to be happy about), it would
> > > then merely extend to
> > >
> > > .if !0\hyp && 0\compat
> > Isn't it effectively the same as setting a default value?
> >
> > The reason we seem to get away is because other part of the macro (e.g. 
> > entry_guest) will need compat to be valid.
> >
> > But that seems pretty fragile to me. So I would prefer if the new code it added 
> > within a macro that is only called when hyp==0.
> So you would like to have a macro that is called if hyp=0 (which means compat 
> had to be passed) and inside this macro additional check if compat is 1?
Yes. This is the only way I could think to avoid making 'compat'optional.

> > Cheers,
> >
> > > or something along those lines.
> > >
> > > Jan
> So when it comes to zeroing the top 32bits by pushing zero to higher halves of 
> stack slots I would do in a loop:
> stp wzr, wzr, [sp #8 * 0]
> stp wzr, wzr, [sp #8 * 1]
> ...
I don't think you can use stp here because this would store two 32-bit 
values consecutively. Instead, you would need to use ldr to store one 
32-bit value at the time.
Cheers,

--
Julien Grall