linux-kernel - Re: [PATCH v2 0/5] x86/fpu: don't abuse x86_task_fpu(PF_USER_WORKER) in .regset

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Message-ID: <78d2f583a0d3008c7d0e2b0e6b70a1b3258cc659.camel@intel.com>
Date: Wed, 3 Sep 2025 15:46:55 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "oleg@...hat.com" <oleg@...hat.com>
CC: "debug@...osinc.com" <debug@...osinc.com>, "mingo@...nel.org"
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	<sohil.mehta@...el.com>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v2 0/5] x86/fpu: don't abuse x86_task_fpu(PF_USER_WORKER)
 in .regset_get() paths

On Wed, 2025-09-03 at 11:54 +0200, Oleg Nesterov wrote:
> > Hmm, I actually do see a potential concrete issue...
> > 
> > fpu_clone() will wipe out the FPU state for PF_USER_WORKER, which means if
> > xsaves decides to use the init optimization for CET, "get_xsave_addr(xsave,
> > XFEATURE_CET_USER)" could return NULL and trigger a warning.
> 
> Even if get_xsave_addr() returns a valid pointer, what is the point to try to
> report cetregs->user_ssp which doesn't match the reality?
> Again, update_fpu_shstk() was not called, ->user_ssp can't be correct.

I think it would be better to have less special cases in the FPU. I'm not sure
what you mean by "correct". As above, it gets zeroed in fpu_clone(). I guess you
want it to be something else.

If we are talking pure correctness, and not functional issues, I'm questioning
whether it actually should be zeroed for user workers. The behavior would be
leave shadow stack enabled, but don't allocate a shadow stack. But all this adds
complexity cost, which seems there isn't appetite for.

I do see the potential to hit a warning from userspace. That seems like
something that could be fixed and does have a clear purpose.