[PATCH] arm64: avoid returning from bad_mode

[PATCH] arm64: avoid returning from bad_mode

[Mark Rutland]

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.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Fixes: 9955ac47f4ba1c95 ("arm64: don't kill the kernel on a bad esr from el0")
Reported-by: Mark Salter <msalter@redhat.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: stable at vger.kernel.org
---
 arch/arm64/kernel/entry.S |  2 +-
 arch/arm64/kernel/traps.c | 28 ++++++++++++++++++++++++----
 2 files changed, 25 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 923841f..43512d4 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -683,7 +683,7 @@ el0_inv:
 	mov	x0, sp
 	mov	x1, #BAD_SYNC
 	mov	x2, x25
-	bl	bad_mode
+	bl	bad_el0_sync
 	b	ret_to_user
 ENDPROC(el0_sync)
 
diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
index 5b830be..659b2e6 100644
--- a/arch/arm64/kernel/traps.c
+++ b/arch/arm64/kernel/traps.c
@@ -604,17 +604,34 @@ const char *esr_get_class_string(u32 esr)
 }
 
 /*
- * 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 on CPU%d, code 0x%08x -- %s\n",
 		handler[reason], smp_processor_id(), esr,
 		esr_get_class_string(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 -- %s\n",
+		smp_processor_id(), esr, esr_get_class_string(esr));
 	__show_regs(regs);
 
 	info.si_signo = SIGILL;
@@ -622,7 +639,10 @@ asmlinkage void bad_mode(struct pt_regs *regs, int reason, unsigned int esr)
 	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)
-- 
1.9.1

[PATCH] arm64: avoid returning from bad_mode

[Catalin Marinas]

On Wed, Jan 18, 2017 at 05:23:41PM +0000, Mark Rutland wrote:
> 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.
> 
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Fixes: 9955ac47f4ba1c95 ("arm64: don't kill the kernel on a bad esr from el0")
> Reported-by: Mark Salter <msalter@redhat.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: stable at vger.kernel.org

Thanks. I'll queue this for 4.10.

-- 
Catalin