linux-kernel - Re: [RFC PATCH v2 1/1] x86/pkeys: update PKRU to enable pkey 0 before XSAVE

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Date: Fri, 22 Mar 2024 08:40:56 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Aruna Ramakrishna <aruna.ramakrishna@...cle.com>,
 linux-kernel@...r.kernel.org
Cc: x86@...nel.org, dave.hansen@...ux.intel.com, tglx@...utronix.de,
 matthias.neugschwandtner@...cle.com, andrew.brownsword@...cle.com
Subject: Re: [RFC PATCH v2 1/1] x86/pkeys: update PKRU to enable pkey 0 before
 XSAVE

On 3/21/24 14:56, Aruna Ramakrishna wrote:
> +/*
> + * Ensure that the both the current stack and the alternate signal
> + * stack is writeable. The alternate stack must be accessible by the
> + * init PKRU value.
> + */
> +static inline u32 sig_prepare_pkru(void)
> +{
> +	u32 current_pkru = read_pkru();
> +	u32 init_pkru_snapshot = pkru_get_init_value();
> +
> +	write_pkru(current_pkru & init_pkru_snapshot);
> +	return current_pkru;
> +}

That comment is quite misleading.  This code has *ZERO* knowledge of the
permissions on either the current or alternate stack.  It _assumes_ that
the current PKRU permissions allow writes to the current stack and
_assumes_ that the init PKRU value can write to the alternative stack.

Those aren't bad assumptions, but they _are_ assumptions and need to be
clearly called out as such.

The '&' operation looks rather random and needs an explanation.  What is
that logically trying to do?  It's trying to clear bits in the old
(pre-signal) PKRU value so that it gains write access to the alt stack.
Please say that.

Which leads me to ask: Why bother with the '&'?  It would be simpler to,
for instance, just wrpkru(0).  What is being written to the old stack at
this point?

I also dislike something being called 'current_pkru' when it's clearly
the old value by the time it is returned.

> +static inline void sig_restore_pkru(u32 pkru)
> +{
> +	write_pkru(pkru);
> +}

This seems like unnecessary abstraction.