[PATCH v3] riscv: cif: reduce shadow stack size limit from 4GB to 2GB

[Zong Li]

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 while being more resource-efficient margin

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.

Additionally, add max(PAGE_SIZE, size) constraint, which covers the
case where RLIMIT_STACK is smaller than PAGE_SIZE.

Signed-off-by: Zong Li <zong.li at sifive.com>
---

Changed in v2:
- Add max() in case RLIMIT_STACK is smaller than PAGE_SIZE. Suggested by
  Paul Walmsley and Sashiko

Changed in v1:
- Use min() instead of min_t(). Suggested by David Laight

 arch/riscv/kernel/usercfi.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/kernel/usercfi.c b/arch/riscv/kernel/usercfi.c
index 6eaa0d94fdfe..0f75e8f5d0ec 100644
--- a/arch/riscv/kernel/usercfi.c
+++ b/arch/riscv/kernel/usercfi.c
@@ -109,15 +109,17 @@ 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
+ * 2G should be more than sufficient for most applications.
  */
 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));
+	size = PAGE_ALIGN(min(rlimit(RLIMIT_STACK) / 2, SZ_2G));
+
+	return max(PAGE_SIZE, size);
 }
 
 /*
-- 
2.43.7