linux-kernel - Re: [PATCH v22 00/28] riscv control-flow integrity for usermode

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Message-ID: <aQptcHayBOY1sw1J@debug.ba.rivosinc.com>
Date: Tue, 4 Nov 2025 13:17:36 -0800
From: Deepak Gupta <debug@...osinc.com>
To: Joel Stanley <joel@....id.au>
Cc: Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, Arnd Bergmann <arnd@...db.de>,
	linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
	linux-arch@...r.kernel.org, linux-kselftest@...r.kernel.org,
	alistair.francis@....com, jim.shu@...ive.com, kito.cheng@...ive.com,
	charlie@...osinc.com, atishp@...osinc.com, evan@...osinc.com,
	cleger@...osinc.com, alexghiti@...osinc.com,
	Zong Li <zong.li@...ive.com>,
	Michael Ellerman <mpe@...storrent.com>
Subject: Re: [PATCH v22 00/28] riscv control-flow integrity for usermode

Hi Joel,

Thanks a lot for testing it out.

Comments inline.

On Tue, Nov 04, 2025 at 05:34:11PM +1030, Joel Stanley wrote:
>Hello Deepak,
>
>On Fri, 24 Oct 2025 at 03:31, Deepak Gupta via B4 Relay
><devnull+debug.rivosinc.com@...nel.org> wrote:
>>
>> v22: fixing build error due to -march=zicfiss being picked in gcc-13 and above
>> but not actually doing any codegen or recognizing instruction for zicfiss.
>> Change in v22 makes dependence on `-fcf-protection=full` compiler flag to
>> ensure that toolchain has support and then only CONFIG_RISCV_USER_CFI will be
>> visible in menuconfig.
>
>Following our discussion at the riscv summit I spent some time with
>this patch set with the goal of giving a test run on emulation. I only
>got as far as qemu, as I couldn't get the selftests passing there.
>
>I had trouble running the podman container so I built a toolchain
>using the riscv-gnu-toolchain branch (cfi-dev, d19f3009f6c2) you
>pointed to.
>
>The opensbi branch was a bit old and wouldn't build with GCC 15, so I
>tried to rebase and noticed the patches were already upstream. Have
>you tested using v1.7 (or newer) there? Is there something I missed,
>do we need more patches on upstream opensbi?
>
>I booted it in qemu 10.1.2 with the zicfi extensions both on and off.
>
>qemu-system-riscv64 -M virt,aia=aplic-imsic,aia-guests=5 \
>  -cpu rv64,zicfilp=true,zicfiss=true,zimop=true,zcmop=true
>  -smp 8 -nographic -bios fw_dynamic.elf
>  -m 1024M -kernel arch/riscv/boot/Image \
>  -initrd selftests/selftests.cpio \
>  -append 'init=mini-init command="cfitests"'
>
>My results:
>
>no zicfi, no z*mop (crash, as expected):
>-------------------------------------------------
>
>Running command: cfitests
>system_opcode_insn: Invalid opcode for CSR read/write instruction[
>0.462709] cfitests[85]: unhandled signal 4 code 0x1 at
>0x0000000000011c44 in cfitests[1c44,10000+6d000]
>[    0.463141] CPU: 4 UID: 0 PID: 85 Comm: cfitests Not tainted
>6.18.0-rc3-tt-defconfig-jms-00090-g6e2297f1edbc #93 NONE
>[    0.463338] Hardware name: riscv-virtio,qemu (DT)
>[    0.463573] epc : 0000000000011c44 ra : 00000000000104e0 sp :
>00007fffebd0ddb0
>...
>[    0.465177] status: 0000000200004020 badaddr: 00000000ce104073
>cause: 0000000000000002
>[    0.465410] Code: 0893 05d0 4501 0073 0000 b7f5 4501 b7f9 0017 0000
>(4073) ce10
>
>no zicfi, z*mop (failed to start, as expected):
>-----------------------------------------------------------
>
>Running command: cfitests
>TAP version 13
># Starting risc-v CFI tests
>Bail out! Get landing pad status failed with -22
>
>zicfi, z*mop (failed to start, unexpected):
>-------------------------------------------------------
>Running command: cfitests
>TAP version 13
># Starting risc-v CFI tests
>Bail out! Get landing pad status failed with -22
>
>I went digging to see why the zicfi enabled kernel failed. The
>userspace binary was built with CFI:
>
>$ riscv64-unknown-linux-gnu-readelf -n selftests/cfitests
>
>Displaying notes found in: .note.gnu.property
>  Owner                Data size     Description
>  GNU                  0x00000010    NT_GNU_PROPERTY_TYPE_0
>      Properties: RISC-V AND feature: CFI_LP_UNLABELED, CFI_SS
>
>I then tested your opensbi tree with some hacks to get it built with a
>newer compiler. This produced different results, which was unexpected:
>

Opensbi doesn't need to be built with new compiler. All it needs do are
reflect MPELP bit back to S-mode (if its taking a trap and then reflecting
back to S-mode) and ofcourse have SSE support. Both of these are upstream
in opensbi.

I'll test with upstream opensbi, test and report back.

>Running command: cfitests
>TAP version 13
># Starting risc-v CFI tests
>Bail out! Landing pad is not enabled, should be enabled via glibc
># Totals: pass:0 fail:0 xfail:0 xpass:0 skip:0 error:0
>
>The selftest binary and the little toy init that starts it are both
>statically linked and built against the toolchain's glibc, so I would
>expect this to work.
>
>$ riscv64-unknown-linux-gnu-readelf -n sifive-cfi-build/sysroot/usr/lib/libc.a
>
>File: sifive-cfi-build/sysroot/usr/lib/libc.a(init-first.o)
>
>Displaying notes found in: .note.gnu.property
>  Owner                Data size        Description
>  GNU                  0x00000010       NT_GNU_PROPERTY_TYPE_0
>      Properties: RISC-V AND feature: CFI_LP_UNLABELED, CFI_SS
>
>The kernel seems to have detected that CFI is available and is built with it:
>
>$ grep CFI .config
>CONFIG_RISCV_USER_CFI=y
>CONFIG_ARCH_SUPPORTS_CFI=y
>
>I did notice the func-sig-dev gcc branch is a few commits ahead of
>what the sifive riscv-gnu-toolchain points to.

Most likely PRCTL_* is not updated in cfi-dev branch? I'll take a look.

>
>I had to context switch to some other tasks at this point. I wanted to
>do some more digging to work out what was wrong, but I haven't found
>time, so here are my notes in the hope that they are useful. I'll let
>you know if I discover anything further.

Thanks once again.
>
>Cheers,
>
>Joel
>
>
>> How to test this series
>> =======================
>>
>> Toolchain
>> ---------
>> $ git clone git@...hub.com:sifive/riscv-gnu-toolchain.git -b cfi-dev
>> $ riscv-gnu-toolchain/configure --prefix=<path-to-where-to-build> --with-arch=rv64gc_zicfilp_zicfiss --enable-linux --disable-gdb  --with-extra-multilib-test="rv64gc_zicfilp_zicfiss-lp64d:-static"
>> $ make -j$(nproc)
>>
>> Qemu
>> ----
>> Get the lastest qemu
>> $ cd qemu
>> $ mkdir build
>> $ cd build
>> $ ../configure --target-list=riscv64-softmmu
>> $ make -j$(nproc)
>>
>> Opensbi
>> -------
>> $ git clone git@...hub.com:deepak0414/opensbi.git -b v6_cfi_spec_split_opensbi
>> $ make CROSS_COMPILE=<your riscv toolchain> -j$(nproc) PLATFORM=generic
>>
>> Linux
>> -----
>> Running defconfig is fine. CFI is enabled by default if the toolchain
>> supports it.
>>
>> $ make ARCH=riscv CROSS_COMPILE=<path-to-cfi-riscv-gnu-toolchain>/build/bin/riscv64-unknown-linux-gnu- -j$(nproc) defconfig
>> $ make ARCH=riscv CROSS_COMPILE=<path-to-cfi-riscv-gnu-toolchain>/build/bin/riscv64-unknown-linux-gnu- -j$(nproc)
>>
>> Running
>> -------
>>
>> Modify your qemu command to have:
>> -bios <path-to-cfi-opensbi>/build/platform/generic/firmware/fw_dynamic.bin
>> -cpu rv64,zicfilp=true,zicfiss=true,zimop=true,zcmop=true