linux-kernel - RE: [PATCH v3 2/6] x86/uaccess: Avoid barrier_nospec() in 64-bit __get

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Message-ID: <2d7744d7ce504b288c3f1356f27910ec@AcuMS.aculab.com>
Date: Sun, 24 Nov 2024 16:11:04 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Linus Torvalds' <torvalds@...ux-foundation.org>, Josh Poimboeuf
	<jpoimboe@...nel.org>
CC: "x86@...nel.org" <x86@...nel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>, Peter Zijlstra <peterz@...radead.org>, "Pawan
 Gupta" <pawan.kumar.gupta@...ux.intel.com>, Waiman Long <longman@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>, Ingo Molnar <mingo@...hat.com>,
	Michael Ellerman <mpe@...erman.id.au>, "linuxppc-dev@...ts.ozlabs.org"
	<linuxppc-dev@...ts.ozlabs.org>, Andrew Cooper <andrew.cooper3@...rix.com>,
	Mark Rutland <mark.rutland@....com>, "Kirill A . Shutemov"
	<kirill@...temov.name>
Subject: RE: [PATCH v3 2/6] x86/uaccess: Avoid barrier_nospec() in 64-bit
 __get_user()

From: Linus Torvalds
> Sent: 22 November 2024 19:35
> 
> On Fri, 22 Nov 2024 at 11:13, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > I've only compiled it so far, about to actually boot into it.
> 
> Looks fine. Sent out a proper patch with commit message etc at
> 
>    https://lore.kernel.org/all/20241122193305.7316-1-torvalds@linux-foundation.org/
> 
> because it looks good to me. Comments?

+static __always_inline int futex_read_inatomic(u32 *dest, u32 __user *from)
+{
+	u32 val;
+
+	if (can_do_masked_user_access())
+		from = masked_user_access_begin(from);
+	else if (!user_read_access_begin(from, sizeof(*from)))
+		return -EFAULT;
+	unsafe_get_user(val, from, Efault);
+	user_access_end();
+	*dest = val;
+	return 0;
+Efault:
+	user_access_end();
+	return -EFAULT;
+}
+
+static inline int futex_get_value_locked(u32 *dest, u32 __user *from)
+{
+	int ret;
+
+	pagefault_disable();
+	ret = futex_read_inatomic(dest, from);
+	pagefault_enable();
+
+	return ret;
+}

Is there an 'unsafe_get_user_nofault()' that uses a trap handler
that won't fault in a page?
That would save the inc/dec done by pagefault_en/disable().

I'd also have thought that the trap handler for unsafe_get_user()
would jump to the Efault label having already done user_access_end().
But maybe it doesn't work out that way?

	David

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