[PATCH v4 17/29] arm64: implement PKEYS support

[Joey Gouly]

Hi Szabolcs,

On Fri, May 31, 2024 at 03:57:07PM +0100, Szabolcs Nagy wrote:
> The 05/03/2024 14:01, Joey Gouly wrote:
> > Implement the PKEYS interface, using the Permission Overlay Extension.
> ...
> > +#ifdef CONFIG_ARCH_HAS_PKEYS
> > +int arch_set_user_pkey_access(struct task_struct *tsk, int pkey, unsigned long init_val)
> > +{
> > +	u64 new_por = POE_RXW;
> > +	u64 old_por;
> > +	u64 pkey_shift;
> > +
> > +	if (!arch_pkeys_enabled())
> > +		return -ENOSPC;
> > +
> > +	/*
> > +	 * This code should only be called with valid 'pkey'
> > +	 * values originating from in-kernel users.  Complain
> > +	 * if a bad value is observed.
> > +	 */
> > +	if (WARN_ON_ONCE(pkey >= arch_max_pkey()))
> > +		return -EINVAL;
> > +
> > +	/* Set the bits we need in POR:  */
> > +	if (init_val & PKEY_DISABLE_ACCESS)
> > +		new_por = POE_X;
> > +	else if (init_val & PKEY_DISABLE_WRITE)
> > +		new_por = POE_RX;
> > +
> 
> given that the architecture allows r,w,x permissions to be
> set independently, should we have a 'PKEY_DISABLE_EXEC' or
> similar api flag?
> 
> (on other targets it can be some invalid value that fails)

I didn't think about the best way to do that yet. PowerPC has a PKEY_DISABLE_EXECUTE.

We could either make that generic, and X86 has to error if it sees that bit, or
we add a arch-specific PKEY_DISABLE_EXECUTE like PowerPC.

A user can still set it by interacting with the register directly, but I guess
we want something for the glibc interface..

Dave, any thoughts here?

> 
> > +	/* Shift the bits in to the correct place in POR for pkey: */
> > +	pkey_shift = pkey * POR_BITS_PER_PKEY;
> > +	new_por <<= pkey_shift;
> > +
> > +	/* Get old POR and mask off any old bits in place: */
> > +	old_por = read_sysreg_s(SYS_POR_EL0);
> > +	old_por &= ~(POE_MASK << pkey_shift);
> > +
> > +	/* Write old part along with new part: */
> > +	write_sysreg_s(old_por | new_por, SYS_POR_EL0);
> > +
> > +	return 0;
> > +}
> > +#endif

Thanks,
Joey