Messages in this thread | | | Date | Mon, 14 Aug 2023 10:59:28 -0700 | From | Nathan Chancellor <> | Subject | Re: [PATCH 0/5] riscv: SCS support |
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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?
Cheers, Nathan
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