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Date:   Wed, 2 Jun 2021 17:11:57 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     Al Viro <viro@zeniv.linux.org.uk>, x86@kernel.org,
        linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Will Deacon <will@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Waiman Long <longman@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Andy Lutomirski <luto@kernel.org>,
        Christoph Hellwig <hch@lst.de>,
        David Laight <David.Laight@aculab.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCH v4 3/4] x86/uaccess: Use pointer masking to limit uaccess
 speculation
Message-ID: <YLe73YcUeXBaLwHr@google.com>
References: <cover.1620186182.git.jpoimboe@redhat.com>
 <5ba93cdbf35ab40264a9265fc24575a9b2f813b3.1620186182.git.jpoimboe@redhat.com>
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On Tue, May 04, 2021, Josh Poimboeuf wrote:
> The x86 uaccess code uses barrier_nospec() in various places to prevent
> speculative dereferencing of user-controlled pointers (which might be
> combined with further gadgets or CPU bugs to leak data).
> 
> There are some issues with the current implementation:
> 
> - The barrier_nospec() in copy_from_user() was inadvertently removed
>   with: 4b842e4e25b1 ("x86: get rid of small constant size cases in
>   raw_copy_{to,from}_user()")

Mostly out of curiosity, wasn't copy_{from,to}_user() flawed even before that
patch?  Non-constant sizes would go straight to copy_user_generic(), and even if
string ops are used and strings are magically not vulnerable, small sizes would
skip to normal loads/stores in _copy_short_string when using
copy_user_enhanced_fast_string().