Re: [PATCH 0/5] riscv: SCS support

Sami Tolvanen
2023-08-14T18:58:08Z

On Mon, Aug 14, 2023 at 11:33 AM Kees Cook <keescook@chromium.org> wrote:
>
> On Mon, Aug 14, 2023 at 10:59:28AM -0700, Nathan Chancellor wrote:
> > Hi Sami,
> >
> > On Fri, Aug 11, 2023 at 11:35:57PM +0000, Sami Tolvanen wrote:
> > > Hi folks,
> > >
> > > This series adds Shadow Call Stack (SCS) support for RISC-V. SCS
> > > uses compiler instrumentation to store return addresses in a
> > > separate shadow stack to protect them against accidental or
> > > malicious overwrites. More information about SCS can be found
> > > here:
> > >
> > > https://clang.llvm.org/docs/ShadowCallStack.html
> > >
> > > Patch 1 is from Deepak, and it simplifies VMAP_STACK overflow
> > > handling by adding support for accessing per-CPU variables
> > > directly in assembly. The patch is included in this series to
> > > make IRQ stack switching cleaner with SCS, and I've simply
> > > rebased it. Patch 2 uses this functionality to clean up the stack
> > > switching by moving duplicate code into a single function. On
> > > RISC-V, the compiler uses the gp register for storing the current
> > > shadow call stack pointer, which is incompatible with global
> > > pointer relaxation. Patch 3 moves global pointer loading into a
> > > macro that can be easily disabled with SCS. Patch 4 implements
> > > SCS register loading and switching, and allows the feature to be
> > > enabled, and patch 5 adds separate per-CPU IRQ shadow call stacks
> > > when CONFIG_IRQ_STACKS is enabled.
> > >
> > > Note that this series requires Clang 17. Earlier Clang versions
> > > support SCS on RISC-V, but use the x18 register instead of gp,
> > > which isn't ideal. gcc has SCS support for arm64, but I'm not
> > > aware of plans to support RISC-V. Once the Zicfiss extension is
> > > ratified, it's probably preferable to use hardware-backed shadow
> > > stacks instead of SCS on hardware that supports the extension,
> > > and we may want to consider implementing CONFIG_DYNAMIC_SCS to
> > > patch between the implementation at runtime (similarly to the
> > > arm64 implementation, which switches to SCS when hardware PAC
> > > support isn't available).
> >
> > I took this series for a spin on top of 6.5-rc6 with both LLVM 18 (built
> > within the past couple of days) and LLVM 17.0.0-rc2 but it seems that
> > the CFI_BACKWARDS LKDTM test does not pass with
> > CONFIG_SHADOW_CALL_STACK=y.
> >
> > [ 73.324652] lkdtm: Performing direct entry CFI_BACKWARD
> > [ 73.324900] lkdtm: Attempting unchecked stack return address redirection ...
> > [ 73.325178] lkdtm: Eek: return address mismatch! 0000000000000002 != ffffffff80614982
> > [ 73.325478] lkdtm: FAIL: stack return address manipulation failed!
> >
> > Does the test need to be adjusted or is there some other issue?
>
> Does it pass without the series? I tried to write it to be
> arch-agnostic, but I never tested it on RISC-V. It's very possible that
> test needs adjusting for the architecture. Besides the label horrors,
> the use of __builtin_frame_address may not work there either...

Looks like __builtin_frame_address behaves differently on RISC-V.
After staring at the disassembly a bit, using
__builtin_frame_address(0) - 1 instead of + 1 seems to yield correct
results. WIth that change, here's the test without SCS:

# echo CFI_BACKWARD > /sys/kernel/debug/provoke-crash/DIRECT
[ 16.272028] lkdtm: Performing direct entry CFI_BACKWARD
[ 16.272368] lkdtm: Attempting unchecked stack return address redirection ...
[ 16.272671] lkdtm: ok: redirected stack return address.
[ 16.272885] lkdtm: Attempting checked stack return address redirection ...
[ 16.273384] lkdtm: FAIL: stack return address was redirected!
[ 16.273755] lkdtm: This is probably expected, since this kernel
(6.5.0-rc5-00005-g5a1201f89265-dirty riscv64) was built *without*
CONFIG_ARM64_PTR_AUTH_KERNEL=y nor CONFIG_SHADOW_CALL_STACK=y

And with SCS:

# echo CFI_BACKWARD > /sys/kernel/debug/provoke-crash/DIRECT
[ 17.429551] lkdtm: Performing direct entry CFI_BACKWARD
[ 17.430184] lkdtm: Attempting unchecked stack return address redirection ...
[ 17.431402] lkdtm: ok: redirected stack return address.
[ 17.432031] lkdtm: Attempting checked stack return address redirection ...
[ 17.432861] lkdtm: ok: control flow unchanged.

Sami