linux-kernel - Re: [PATCH v3] x86/uaccess: Use pointer masking to limit uaccess speculation

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Date:   Mon, 14 Sep 2020 21:21:56 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, X86 ML <x86@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Will Deacon <will@...nel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andy Lutomirski <luto@...nel.org>,
        Christoph Hellwig <hch@....de>,
        David Laight <David.Laight@...lab.com>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v3] x86/uaccess: Use pointer masking to limit uaccess
 speculation

On Mon, Sep 14, 2020 at 11:48:55AM -0700, Dan Williams wrote:
> > Err, stupid question: can this macro then be folded into access_ok() so
> > that you don't have to touch so many places and the check can happen
> > automatically?
> 
> I think that ends up with more changes because it changes the flow of
> access_ok() from returning a boolean to returning a modified user
> address that can be used in the speculative path.

I mean something like the totally untested, only to show intent hunk
below? (It is late here so I could very well be missing an aspect):

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 2bffba2a1b23..c94e1589682c 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -7,6 +7,7 @@
 #include <linux/compiler.h>
 #include <linux/kasan-checks.h>
 #include <linux/string.h>
+#include <linux/nospec.h>
 #include <asm/asm.h>
 #include <asm/page.h>
 #include <asm/smap.h>
@@ -92,8 +93,15 @@ static inline bool pagefault_disabled(void);
  */
 #define access_ok(addr, size)					\
 ({									\
+	bool range;							\
+	typeof(addr) a = addr, b;					\
+									\
 	WARN_ON_IN_IRQ();						\
-	likely(!__range_not_ok(addr, size, user_addr_max()));		\
+									\
+	range = __range_not_ok(addr, size, user_addr_max());		\
+	b = (typeof(addr)) array_index_nospec((__force unsigned long)addr, TASK_SIZE_MAX); \
+									\
+	likely(!range && a == b);					\
 })
 
 /*

-- 
Regards/Gruss,
    Boris.

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