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([2601:647:4580:b719:b5ce:2cca:790b:b1c4]) by smtp.gmail.com with ESMTPSA id h4-v6sm12985784pgp.13.2018.08.25.22.54.09 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 25 Aug 2018 22:54:10 -0700 (PDT) Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 11.5 \(3445.9.1\)) Subject: Re: TLB flushes on fixmap changes From: Nadav Amit In-Reply-To: Date: Sat, 25 Aug 2018 22:53:47 -0700 Cc: Masami Hiramatsu , 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-Transfer-Encoding: quoted-printable Message-Id: 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> To: Kees Cook , Andy Lutomirski X-Mailer: Apple Mail (2.3445.9.1) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org at 9:43 PM, Kees Cook wrote: > 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? >>>=20 >>> 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? >>=20 >> 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();. >>=20 >> 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. >=20 > 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=3D= kspp/write-rarely&id=3D9ab0cb2618ebbc51f830ceaa06b7d2182fe1a52d >=20 > With that, text_poke() mostly becomes: >=20 > rare_write_begin() > memcpy(addr, opcode, len); > rare_write_end() >=20 > 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. :) I think that the key to harden the security of text_poke() against its = use as a gadget in a ROP/JOP attack is to add a check/assertion for the old (expected) value, such as: rare_write_begin() if (*addr =3D=3D prev_opcode) memcpy(addr, opcode, len); rare_write_end()