Re: [patch V3 07/12] uaccess: Provide scoped masked user access regions

From: Thomas Gleixner <tglx@linutronix.de>
To: David Laight <david.laight.linux@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
	"Christophe Leroy" <christophe.leroy@csgroup.eu>,
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	"Linus Torvalds" <torvalds@linux-foundation.org>,
	"kernel test robot" <lkp@intel.com>,
	"Russell King" <linux@armlinux.org.uk>,
	linux-arm-kernel@lists.infradead.org, x86@kernel.org,
	"Madhavan Srinivasan" <maddy@linux.ibm.com>,
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	linux-fsdevel@vger.kernel.org
Subject: Re: [patch V3 07/12] uaccess: Provide scoped masked user access regions
Date: Tue, 21 Oct 2025 16:42:22 +0200	[thread overview]
Message-ID: <874irsz581.ffs@tglx> (raw)
In-Reply-To: <877bwoz5sp.ffs@tglx>

On Tue, Oct 21 2025 at 16:29, Thomas Gleixner wrote:
> On Mon, Oct 20 2025 at 19:28, David Laight wrote:
>> There is no requirement to do the accesses in strict memory order
>> (or to access the lowest address first).
>> The only constraint is that gaps must be significantly less than 4k.
>
> The requirement is that the access is not spilling over into the kernel
> address space, which means:
>
>        USR_PTR_MAX <= address < (1U << 63)
>
> USR_PTR_MAX on x86 is either
>             (1U << 47) - PAGE_SIZE (4-level page tables)
>          or (1U << 57) - PAGE_SIZE (5-level page tables)
>
> Which means at least ~8 EiB of unmapped space in both cases.
>
> The access order does not matter at all.

I just noticed that LAM reduces that gap to one page, but then the
kernel has a 8EiB gap right at the kernel/user boundary, which means
even in the LAM case an access with less than 8EiB offset from
USR_PTR_MAX is guaranteed to fault and not to be able to speculatively
access actual kernel memory.

Thanks,

        tglx