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[209.85.161.41]) by smtp.gmail.com with ESMTPSA id k2-v6sm5085529ywa.93.2018.08.25.21.43.11 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 25 Aug 2018 21:43:11 -0700 (PDT) Received: by mail-yw1-f41.google.com with SMTP id n21-v6so4392735ywh.5 for ; Sat, 25 Aug 2018 21:43:11 -0700 (PDT) X-Received: by 2002:a0d:db10:: with SMTP id d16-v6mr4520518ywe.124.1535258590773; Sat, 25 Aug 2018 21:43:10 -0700 (PDT) MIME-Version: 1.0 Received: by 2002:a25:2c11:0:0:0:0:0 with HTTP; Sat, 25 Aug 2018 21:43:09 -0700 (PDT) In-Reply-To: References: <20180822153012.173508681@infradead.org> <20180822154046.823850812@infradead.org> <20180822155527.GF24124@hirez.programming.kicks-ass.net> <20180823134525.5f12b0d3@roar.ozlabs.ibm.com> <776104d4c8e4fc680004d69e3a4c2594b638b6d1.camel@au1.ibm.com> <20180823133958.GA1496@brain-police> <20180824084717.GK24124@hirez.programming.kicks-ass.net> <20180824180438.GS24124@hirez.programming.kicks-ass.net> <56A9902F-44BE-4520-A17C-26650FCC3A11@gmail.com> <9A38D3F4-2F75-401D-8B4D-83A844C9061B@gmail.com> <8E0D8C66-6F21-4890-8984-B6B3082D4CC5@gmail.com> <20180826112341.f77a528763e297cbc36058fa@kernel.org> From: Kees Cook Date: Sat, 25 Aug 2018 21:43:09 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: TLB flushes on fixmap changes To: Andy Lutomirski Cc: Masami Hiramatsu , Nadav Amit , Linus Torvalds , Paolo Bonzini , Jiri Kosina , Peter Zijlstra , Will Deacon , Benjamin Herrenschmidt , Nick Piggin , "the arch/x86 maintainers" , Borislav Petkov , Rik van Riel , Jann Horn , Adin Scannell , Dave Hansen , Linux Kernel Mailing List , linux-mm , David Miller , Martin Schwidefsky , Michael Ellerman Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Aug 25, 2018 at 9:21 PM, Andy Lutomirski wrote: > On Sat, Aug 25, 2018 at 7:23 PM, Masami Hiramatsu wrote: >> On Fri, 24 Aug 2018 21:23:26 -0700 >> Andy Lutomirski wrote: >>> Couldn't text_poke() use kmap_atomic()? Or, even better, just change CR3? >> >> No, since kmap_atomic() is only for x86_32 and highmem support kernel. >> In x86-64, it seems that returns just a page address. That is not >> good for text_poke, since it needs to make a writable alias for RO >> code page. Hmm, maybe, can we mimic copy_oldmem_page(), it uses ioremap_cache? >> > > I just re-read text_poke(). It's, um, horrible. Not only is the > implementation overcomplicated and probably buggy, but it's SLOOOOOW. > It's totally the wrong API -- poking one instruction at a time > basically can't be efficient on x86. The API should either poke lots > of instructions at once or should be text_poke_begin(); ...; > text_poke_end();. > > Anyway, the attached patch seems to boot. Linus, Kees, etc: is this > too scary of an approach? With the patch applied, text_poke() is a > fantastic exploit target. On the other hand, even without the patch > applied, text_poke() is every bit as juicy. I tried to convince Ingo to use this method for doing "write rarely" and he soundly rejected it. :) I've always liked this because AFAICT, it's local to the CPU. I had proposed it in https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=kspp/write-rarely&id=9ab0cb2618ebbc51f830ceaa06b7d2182fe1a52d With that, text_poke() mostly becomes: rare_write_begin() memcpy(addr, opcode, len); rare_write_end() As for juiciness, if an attacker already has execution control, they can do more interesting things than text_poke(). But regardless, yes, it's always made me uncomfortable. :) -Kees -- Kees Cook Pixel Security