linux-kernel - RE: [PATCH 09/11] x86: remove address space overrides using set

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Date:   Mon, 17 Aug 2020 08:23:11 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christoph Hellwig' <hch@....de>,
        Al Viro <viro@...iv.linux.org.uk>,
        Michael Ellerman <mpe@...erman.id.au>,
        "x86@...nel.org" <x86@...nel.org>
CC:     Kees Cook <keescook@...omium.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: RE: [PATCH 09/11] x86: remove address space overrides using set_fs()

From: Christoph Hellwig
> Sent: 17 August 2020 08:32
>
> Stop providing the possibility to override the address space using
> set_fs() now that there is no need for that any more.  To properly
> handle the TASK_SIZE_MAX checking for 4 vs 5-level page tables on
> x86 a new alternative is introduced, which just like the one in
> entry_64.S has to use the hardcoded virtual address bits to escape
> the fact that TASK_SIZE_MAX isn't actually a constant when 5-level
> page tables are enabled.
....
> @@ -93,7 +69,7 @@ static inline bool pagefault_disabled(void);
>  #define access_ok(addr, size)					\
>  ({									\
>  	WARN_ON_IN_IRQ();						\
> -	likely(!__range_not_ok(addr, size, user_addr_max()));		\
> +	likely(!__range_not_ok(addr, size, TASK_SIZE_MAX));		\
>  })

Can't that always compare against a constant even when 5-levl
page tables are enabled on x86-64?

On x86-64 it can (probably) reduce to (addr | (addr + size)) < 0.

	David

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