First installment of "When Varnish met CHERI"

doc/sphinx/phk/cheri1.rst

+.. _phk_cheri_1:
+
+How Varnish met CHERI 1/N
+=========================
+
+I should warn you up front that Robert Watson has been my friend
+for a couple of decades, so I am by no means a neutral observer.
+
+But Robert is one of the smartest people I know, and when he first
+explained his ideas for hardware capabilities, I sold all my shares
+in sliced bread the very next morning.
+
+Roberts ideas grew to become CHERI, and if you have not heard about
+it yet, you should read up on it, now:
+
+https://www.cl.cam.ac.uk/research/security/ctsrd/cheri/
+
+The core idea in CHERI is that pointers are not integers, whicb
+means that you cannot randomly make up or modify pointers to point
+at random things any more, whatever your intentions might be.
+
+From a security point of view, this circumscribes several large
+classes of commonly used attack-vectors, and Microsoft Research
+found that CHERI stopped a full 43% of all the vulnerabilities they
+saw in 2019:
+
+https://msrc-blog.microsoft.com/2022/01/20/an_armful_of_cheris/
+
+Yes, we can pause while you sell all your shares in sliced bread.
+
+I have been eagerly waiting to see how my own Varnish HTTP Cache
+Software would fare under CHERI, because one of my goals with the
+Varnish software, was to turn the quality dial up to 11 to see if
+it made any real-life difference.
+
+Robert has graciously lent me a shell-account on one of his shiny
+new MORELLO machines, which rock an ARM64 prototype CPU with CHERI
+capabilites.
+
+In this sequence of rants I will sing the saga of How Varnish meets
+CHERI, as it happens.
+
+My hope is that it will inspire and help other software projects
+to take the CHERI-plunge, and to help convince ARM that "MORELLO"
+should become a permanent part of the ARM architecture.
+
+A very thin layer of Varnish
+----------------------------
+
+For those of you not familiar with Varnish, you will need to know:
+
+* Varnish is an afterburner cache to HTTP webservers, written in
+  C and it clocks in around 100KLOC.
+
+* Around 20% of all webtraffic in the world pass through a Varnish instance.
+
+* You configure Varnish with a domain-specific programming
+  language called VCL, which is translated to C source code,
+  compiled to a shared library, dlopen(3)'ed and executed.
+
+* Varnish runs as two processes, a "manager" and a "child".
+  The child process does not ``exec(2)`` after the ``fork(2)``.
+
+* The source code is written in a very paranoid style, around 10%
+  of all lines are asserts, and there are other layers of paranoia on
+  top of that, for instance we always check that ``close(2)`` returns zero.
+
+* We have 900+ test cases, which exercise 90%+ of our source lines.
+
+* In 16 years, we have had just a single "Oh Shit!" security issue.
+
+I still hate Autocrap tools
+---------------------------
+
+Autocrap is a hack on a hack on hack which really ruins software
+portability, but it is the "industry standard" so we use it also
+for Varnish, no matter how much I hate it.
+
+https://dl.acm.org/doi/abs/10.1145/2346916.2349257
+
+Because a lot of software does not work in CHERI mode, there are two
+kinds of packages for CheriBSD:  Regular and Cheri.
+
+https://ctsrd-cheri.github.io/cheribsd-getting-started/packages/index.html
+
+Autocrap does not grok that some packages end up in /usr/local and
+some in /usr/local64, so the first thing I had to do, was to explain
+this::
+
+	export LIBTOOLIZE=/usr/local64/bin/libtoolize
+	export PCRE2_LIBS=`/usr/local/bin/pcre2-config --libs8`
+	export PCRE2_CFLAGS=`/usr/local/bin/pcre2-config --cflags`
+	${SRCDIR}/configure \
+		[the usual options]
+
+Things you just can't do with CHERI
+-----------------------------------
+
+A long long time ago, I wrote a "persistent storage" module for
+Varnish, and in order to not rewrite too much of the rest of
+the source code, the architecture ended up files ``mmap(2)``'ed to
+a consistent address, containing data structures with pointers.
+
+The advent of memory space layout randomization as a bandaid for
+buffer-overflows (dont get me started!), made that architecture
+unworkable, and for close to a decade this "stevedore" has been
+named ``deprecated_persistent``.
+
+We still keep it around, because it helps us test the internal APIs,
+but it is obviously not going to work with CHERI so::
+	
+	${SRCDIR}/configure \
+		--without-persistent-storage \
+		[the usual options]
+
+Dont panic (quite as detailed)
+------------------------------
+
+Varnish has a built in panic-handler which dumps a lot of
+valuable information, so people dont need to send us 1TB
+coredumps.
+
+Part of the information dumped is a backtrace produced with
+``libunwind``, which is not available in a CHERI version (yet),
+so::
+
+	${SRCDIR}/configure \
+		--without-unwind \
+		[the usual options]
+
+[u]intptr_t is or isn't ?
+-------------------------
+
+The two typedefs ``uintptr_t`` and ``intptr_t`` are big enough to
+hold a pointer so that you can write "portable" code which
+does the kind the kind of integer-pointer-mis-math which
+CHERI prevents you from doing.
+
+In theory we should not have any ``[u]intptr_t`` in Varnish,
+since one of our quality policies is to never convert an integer
+to a pointer.
+
+But there are a couple of places where we have used them
+because for "private" struct members, instead of unions.
+
+Those become the first stumbling block::
+
+   vsm.c:601:15: error: shift count >= width of type
+        vd->serial = VSM_PRIV_LOW(vd->serial + 1);
+                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The confusing message is because In CHERI, ``[u]intptr_t``, like pointers,
+are 16 bytes wide but in this case the integer-view is used as a bit-map.
+
+For now, I change them to ``uint64_t``, and put them on the TODO list.
+
+One of them is printed as part of the panic output::
+
+    VSB_printf(vsb, "priv2 = %zd,\n", vfe->priv2);
+
+But that doesn't work with the wider type, so::
+
+    VSB_printf(vsb, "priv2 = %jd,\n", (intmax_t)vfe->priv2);
+
+And with that Varnish compiles under CHERI, which we can check with::
+
+    % file bin/varnishd/varnishd
+    bin/varnishd/varnishd: […] CheriABI […]
+
+First test-run
+--------------
+
+Just to see how bad it is, we run the main test-scripts::
+
+    % cd bin/varnishtest
+    % ./varnistest -i -k -q tests/*.vtc
+    […]
+    38 tests failed, 33 tests skipped, 754 tests passed
+
+That's not half bad…
+
+/phk

doc/sphinx/phk/index.rst

@@ -13,6 +13,7 @@ You may or may not want to know what Poul-Henning thinks.
 .. toctree::
 	:maxdepth: 1
 
+	cheri1.rst
 	routine.rst
 	503aroundtheworld.rst
 	legacy.rst