[PATCH 3.18 16/47] arm64: avoid returning from bad_mode

Greg Kroah-Hartman
2017-04-28T08:35:01Z

From: Mark Rutland <mark.rutland@arm.com>

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.

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: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -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
--- a/arch/arm64/kernel/traps.c
+++ b/arch/arm64/kernel/traps.c
@@ -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)