From 6c7674b5b7ae513cecae22aa9dcdcf533862cf5c Mon Sep 17 00:00:00 2001 From: Zong Li Date: Mon, 27 Apr 2026 19:41:05 -0700 Subject: [PATCH] riscv: cfi: reduce shadow stack size limit from 4GB to 2GB Follow the ARM64 GCS (Guarded Control Stack) implementation approach by reducing the shadow stack size allocation from min(RLIMIT_STACK, 4GB) to min(RLIMIT_STACK/2, 2GB). See commit 506496bcbb42 ("arm64/gcs: Ensure that new threads have a GCS") Rationale: 1. Shadow stacks only store return addresses (8 bytes per entry), not local variables, function parameters, or saved registers. A 2GB shadow stack is far more than sufficient for any practical application, even with extremely deep recursion. Using half the size maintains adequate margin while being more resource-efficient. 2. On memory-constrained systems (e.g., platforms with only 4GB of physical memory, which is a common configuration), allocating 4GB of virtual address space for shadow stack per process/thread can lead to virtual memory allocation failures when the overcommit mode is set to OVERCOMMIT_GUESS or OVERCOMMIT_NEVER: Error: "__vm_enough_memory: not enough memory for the allocation" This reduces virtual address space consumption by 50% while maintaining more than adequate space for return address storage. Signed-off-by: Zong Li Link: https://patch.msgid.link/20260428024105.645162-1-zong.li@sifive.com [pjw@kernel.org: clean up patch description] Signed-off-by: Paul Walmsley --- arch/riscv/kernel/usercfi.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/arch/riscv/kernel/usercfi.c b/arch/riscv/kernel/usercfi.c index 6eaa0d94fdfe..cbfb4e495e9f 100644 --- a/arch/riscv/kernel/usercfi.c +++ b/arch/riscv/kernel/usercfi.c @@ -109,15 +109,16 @@ void set_indir_lp_lock(struct task_struct *task, bool lock) task->thread_info.user_cfi_state.ufcfi_locked = lock; } /* - * If size is 0, then to be compatible with regular stack we want it to be as big as - * regular stack. Else PAGE_ALIGN it and return back + * The shadow stack only stores the return address and not any variables + * this should be more than sufficient for most applications. + * Else PAGE_ALIGN it and return back */ static unsigned long calc_shstk_size(unsigned long size) { if (size) return PAGE_ALIGN(size); - return PAGE_ALIGN(min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G)); + return PAGE_ALIGN(min(rlimit(RLIMIT_STACK) / 2, SZ_2G)); } /*