Patch "arm64: avoid returning from bad_mode" has been added to the 3.18-stable tree

This is a note to let you know that I've just added the patch titled

arm64: avoid returning from bad_mode

to the 3.18-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
arm64-avoid-returning-from-bad_mode.patch
and it can be found in the queue-3.18 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Mark Rutland <[email protected]>
Date: Wed, 18 Jan 2017 17:23:41 +0000
Subject: arm64: avoid returning from bad_mode

From: Mark Rutland <[email protected]>

commit 7d9e8f71b989230bc613d121ca38507d34ada849 upstream.

Generally, taking an unexpected exception should be a fatal event, and bad_mode is intended to cater for this. However, it should be possible to contain unexpected synchronous exceptions from EL0 without bringing the kernel down, by sending a SIGILL to the task.

We tried to apply this approach in commit 9955ac47f4ba1c95 ("arm64:
don't kill the kernel on a bad esr from el0"), by sending a signal for any bad_mode call resulting from an EL0 exception.

However, this also applies to other unexpected exceptions, such as SError and FIQ. The entry paths for these exceptions branch to bad_mode without configuring the link register, and have no kernel_exit. Thus, if we take one of these exceptions from EL0, bad_mode will eventually return to the original user link register value.

This patch fixes this by introducing a new bad_el0_sync handler to cater for the recoverable case, and restoring bad_mode to its original state, whereby it calls panic() and never returns. The recoverable case branches to bad_el0_sync with a bl, and returns to userspace via the usual ret_to_user mechanism.

---
 arch/arm64/kernel/entry.S |    4 ++--
 arch/arm64/kernel/traps.c |   28 ++++++++++++++++++++++++----
 2 files changed, 26 insertions(+), 6 deletions(-)

@@ -551,8 +551,8 @@ el0_inv:
 	mov	x0, sp
 	mov	x1, #BAD_SYNC
 	mrs	x2, esr_el1
-	adr	lr, ret_to_user
-	b	bad_mode
+	bl	bad_el0_sync
+	b	ret_to_user
 ENDPROC(el0_sync)
 
 	.align	6

@@ -308,16 +308,33 @@ asmlinkage long do_ni_syscall(struct pt_
 }
 
 /*
- * bad_mode handles the impossible case in the exception vector.
+ * bad_mode handles the impossible case in the exception vector. This is always
+ * fatal.
  */
 asmlinkage void bad_mode(struct pt_regs *regs, int reason, unsigned int esr)
 {
-	siginfo_t info;
-	void __user *pc = (void __user *)instruction_pointer(regs);
 	console_verbose();
 
 	pr_crit("Bad mode in %s handler detected, code 0x%08x\n",
 		handler[reason], esr);
+
+	die("Oops - bad mode", regs, 0);
+	local_irq_disable();
+	panic("bad mode");
+}
+
+/*
+ * bad_el0_sync handles unexpected, but potentially recoverable synchronous
+ * exceptions taken from EL0. Unlike bad_mode, this returns.
+ */
+asmlinkage void bad_el0_sync(struct pt_regs *regs, int reason, unsigned int esr)
+{
+	siginfo_t info;
+	void __user *pc = (void __user *)instruction_pointer(regs);
+	console_verbose();
+
+	pr_crit("Bad EL0 synchronous exception detected on CPU%d, code 0x%08x\n",
+		smp_processor_id(), esr);
 	__show_regs(regs);
 
 	info.si_signo = SIGILL;

@@ -325,7 +342,10 @@ asmlinkage void bad_mode(struct pt_regs
 	info.si_code  = ILL_ILLOPC;
 	info.si_addr  = pc;
 
-	arm64_notify_die("Oops - bad mode", regs, &info, 0);
+	current->thread.fault_address = 0;
+	current->thread.fault_code = 0;
+
+	force_sig_info(info.si_signo, &info, current);
 }
 
 void __pte_error(const char *file, int line, unsigned long val)

Patches currently in stable-queue which might be from [email protected] are

queue-3.18/kvm-arm-arm64-fix-locking-for-kvm_free_stage2_pgd.patch queue-3.18/arm64-avoid-returning-from-bad_mode.patch