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

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Message-ID: <aQv8zMHs0WnKlNwj@debug.ba.rivosinc.com>
Date: Wed, 5 Nov 2025 17:41:32 -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>, Jesse Huang <jesse.huang@...ive.com>,
	Michael Ellerman <mpe@...storrent.com>
Subject: Re: [PATCH v22 00/28] riscv control-flow integrity for usermode

On Wed, Nov 05, 2025 at 01:16:19PM -0800, Deepak Gupta wrote:
>On Wed, Nov 05, 2025 at 12:24:41PM -0800, Deepak Gupta wrote:
>>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:
>>>
>>>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
>
>Ok so I think atleast I have narrowed down the issue. It seems like
>something is going on with how libc will initialize cfi in case of
>static compile.
>I enabled logging whenever prctl set/get are called and my rootfs
>binaries which are all dynamically linked are setting cfi prctls
>correctly. Except when I run statically compiled `cfitest`. It bails
>on "Shadow stack is not enabled" (unlike in your case where it bails
>saying landing pad is not enabled).

I think I have figured out why its happening on my end.
In case of, riscv-gnu-toolchain's "cfi-dev" branch's glibc submodule
is not setting rest of the parameters for prctl to `0` (for static compile)
https://github.com/sifive/riscv-glibc/blob/20b27db7d9aa8f25419b0cbaf62004e53713315e/sysdeps/riscv/libc-start.h#L73

Kernel will reject the prctl for cfi set/get, if any of the not required
parameter is set to `0`.

Although latest glibc patches from Jesse on libc-alpha do that correctly
https://public-inbox.org/libc-alpha/20251027042634.2665717-8-jesse.huang@sifive.com/T/#u

>
>Shortlog
>"""
>Welcome to Buildroot
>buildroot login: root
>[   51.361564] login[129]: arch_set_shadow_stack_status set successfully, shadow stack base 7fffbb977000size 800000
>[   51.362708] login[129]: arch_set_indir_br_lp_status set successfully
>[   51.363805] login[129]: arch_get_indir_br_lp_status called
>[   51.364296] login[129]: arch_get_shadow_stack_status called
>[   51.448918] bash[129]: arch_set_shadow_stack_status set successfully, shadow stack base 7fff9b516000size 800000
>[   51.450030] bash[129]: arch_set_indir_br_lp_status set successfully
>[   51.450568] bash[129]: arch_get_indir_br_lp_status called
>[   51.451047] bash[129]: arch_get_shadow_stack_status called
>[   51.552710] id[130]: arch_set_shadow_stack_status set successfully, shadow stack base 7fff7fa2a000size 800000
>[   51.553798] id[130]: arch_set_indir_br_lp_status set successfully
>[   51.554306] id[130]: arch_get_indir_br_lp_status called
>[   51.554757] id[130]: arch_get_shadow_stack_status called
># mount -t 9p -o trans=virtio,version=9p2000.L host0 /mnt
>[   65.484939] mount[131]: arch_set_shadow_stack_status set successfully, shadow stack base 7fffa71a0000size 800000
>[   65.487002] mount[131]: arch_set_indir_br_lp_status set successfully
>[   65.488482] mount[131]: arch_get_indir_br_lp_status called
>[   65.489346] mount[131]: arch_get_shadow_stack_status called
># /mnt/cfitests
>[   74.235075] cfitests[132]: arch_set_indir_br_lp_status set successfully
>[   74.249763] cfitests[132]: arch_get_indir_br_lp_status called
>[   74.250823] cfitests[132]: arch_get_shadow_stack_status called
>TAP version 13
># Starting risc-v tests
>[   74.374286] cfitests[132]: arch_get_indir_br_lp_status called
>[   74.375497] cfitests[132]: arch_get_shadow_stack_status called
>Bail out! Shadow stack is not enabled, should be enabled via glibc
># Totals: pass:0 fail:0 xfail:0 xpass:0 skip:0 error:0
>#
>"""
>
>Full log (with cfi prctls get/set) of boot at pastebin below
>https://pastebin.com/JbX2Yv5W
>
>CCed: Jesse Huang who is leading glibc patches. Can help us figure out
>why statically compiled binaries are failing to set cfi prctls appropriately.
>
>Paul,
>
>Given that it's not an issue in kernel changes, we should still be going ahead
>with it for 6.19
>
>>>
>>>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.
>>>
>>>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.
>>
>>
>>I have it working on my end with latest upstream opensbi (no hacks, same
>>compiler)
>>
>>"""
>>$ git log
>>commit 38a6106b1099646f25657bba53cefb80886721a7 (HEAD -> master, origin/master, origin/HEAD)
>>Author: Benoît Monin <benoit.monin@...tlin.com>
>>Date:   Mon Oct 27 14:12:17 2025 +0100
>>
>>   lib: utils/ipi: mswi: add MIPS P8700 compatible
>>....
>>"""
>>
>>I am surprised that change of compiler on opensbi changed errorcode for userspace
>>in your setup. That's quite bizarre.
>>
>>Output from cfitests (with toolchain that's on docker. I didn't compile from
>>cfi-dev branch).
>>
>># /mnt/cfitests
>>TAP version 13
>># Starting risc-v tests
>># Landing pad and shadow stack are enabled for binary
>># cfi_ptrace_test, ptrace test succeeded
>># Executing RISC-V shadow stack self tests
>>1..5
>># Exercising shadow stack fork test
>># Parent pid 133 and child pid 135
>># dummy calls for sspush and sspopchk in context of parent
>># Spewing out shadow stack ptr: 7fffbf4a9fb8
>> This is to ensure shadow stack is indeed enabled and working
>># Waiting on child to finish
>># dummy calls for sspush and sspopchk in context of child
>># Spewing out shadow stack ptr: 7fffbf4a9fb8
>> This is to ensure shadow stack is indeed enabled and working
>>ok 1 shstk fork test
>># Exercising shadow stack map test
>>ok 2 map shadow stack syscall
>># Exercising shadow stack gup tests
>>ok 3 shadow stack gup tests
>># Exercising shadow stack signal test
>>ok 4 shadow stack signal tests
>># Exercising shadow stack protection test (WPT)
>>ok 5 memory protections of shadow stack memory
>># Totals: pass:5 fail:0 xfail:0 xpass:0 skip:0 error:0
>>#
>>
>>Is there a place where I can grab your kselftest `cfitests` binary?
>>
>>Only difference I can see is that `cfitests` in my case is not statically
>>compiled
>>
>>"""
>>$ riscv64-unknown-linux-gnu-readelf -d /scratch/debug/sources/spectacles/cfitests | grep NEEDED
>>0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
>>
>>"""
>>
>>>
>>>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