Re: [Bug report] hash_name() may cross page boundary and trigger sleep in RCU context

["Russell King (Oracle)" <linux@armlinux.org.uk>]

On Fri, Nov 28, 2025 at 09:06:50AM -0800, Linus Torvalds wrote:
> I don't think it's necessarily all that big of a deal. Yeah, this is
> old code, and yeah, it could probably be cleaned up a bit, but at the
> same time, "old and crusty" also means "fairly well tested". This
> whole fault on a kernel address is a fairly unusual case, and as
> mentioned, I *think* the above fix is sufficient.

We have another issue in the code - which has the branch predictor
hardening for spectre issues, which can be called with interrupts
enabled, causing a kernel warning - obviously not good.

There's another issue which PREEMPT_RT has picked up on - which is
that delivering signals via __do_user_fault() with interrupts disabled
causes spinlocks (which can sleep on PREEMPT_RT) to warn.

What I'm thinking is to address both of these by handling kernel space
page faults (which will be permission or PTE-not-present) separately
(not even build tested):

diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
index 2bc828a1940c..972bce697c6c 100644
--- a/arch/arm/mm/fault.c
+++ b/arch/arm/mm/fault.c
@@ -175,7 +175,8 @@ __do_kernel_fault(struct mm_struct *mm, unsigned long addr, unsigned int fsr,
 
 /*
  * Something tried to access memory that isn't in our memory map..
- * User mode accesses just cause a SIGSEGV
+ * User mode accesses just cause a SIGSEGV. Ensure interrupts are enabled
+ * here, which is safe as the fault being handled is from userspace.
  */
 static void
 __do_user_fault(unsigned long addr, unsigned int fsr, unsigned int sig,
@@ -183,8 +184,7 @@ __do_user_fault(unsigned long addr, unsigned int fsr, unsigned int sig,
 {
 	struct task_struct *tsk = current;
 
-	if (addr > TASK_SIZE)
-		harden_branch_predictor();
+	local_irq_enable();
 
 #ifdef CONFIG_DEBUG_USER
 	if (((user_debug & UDBG_SEGV) && (sig == SIGSEGV)) ||
@@ -259,6 +259,38 @@ static inline bool ttbr0_usermode_access_allowed(struct pt_regs *regs)
 }
 #endif
 
+static int __kprobes
+do_kernel_address_page_fault(unsigned long addr, unsigned int fsr,
+			     struct pt_regs *regs)
+{
+	if (user_mode(regs)) {
+		/*
+		 * Fault from user mode for a kernel space address. User mode
+		 * should not be faulting in kernel space, which includes the
+		 * vector/khelper page. Handle the Spectre issues while
+		 * interrupts are still disabled, then send a SIGSEGV. Note
+		 * that __do_user_fault() will enable interrupts.
+		 */
+		harden_branch_predictor();
+		__do_user_fault(addr, fsr, SIGSEGV, SEGV_MAPERR, regs);
+	} else {
+		/*
+		 * Fault from kernel mode. Enable interrupts if they were
+		 * enabled in the parent context. Section (upper page table)
+		 * translation faults are handled via do_translation_fault(),
+		 * so we will only get here for a non-present kernel space
+		 * PTE or kernel space permission fault. Both of these should
+		 * not happen.
+		 */
+		if (interrupts_enabled(regs))
+			local_irq_enable();
+
+		__do_kernel_fault(mm, addr, fsr, regs);
+	}
+
+	return 0;
+}
+
 static int __kprobes
 do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 {
@@ -272,6 +304,8 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 	if (kprobe_page_fault(regs, fsr))
 		return 0;
 
+	if (addr >= TASK_SIZE)
+		return do_kernel_address_page_fault(addr, fsr, regs);
 
 	/* Enable interrupts if they were enabled in the parent context. */
 	if (interrupts_enabled(regs))

... and I think there was a bug in the branch predictor handling -
addr == TASK_SIZE should have been included.

Does this look sensible?

-- 
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