[PATCHv3] arm64: Handle el1 synchronous instruction aborts cleanly

[Laura Abbott]

Executing from a non-executable area gives an ugly message:

lkdtm: Performing direct entry EXEC_RODATA
lkdtm: attempting ok execution at ffff0000084c0e08
lkdtm: attempting bad execution at ffff000008880700
Bad mode in Synchronous Abort handler detected on CPU2, code 0x8400000e -- IABT (current EL)
CPU: 2 PID: 998 Comm: sh Not tainted 4.7.0-rc2+ #13
Hardware name: linux,dummy-virt (DT)
task: ffff800077e35780 ti: ffff800077970000 task.ti: ffff800077970000
PC is at lkdtm_rodata_do_nothing+0x0/0x8
LR is at execute_location+0x74/0x88

The 'IABT (current EL)' indicates the error but it's a bit cryptic
without knowledge of the ARM ARM. There is also no indication of the
specific address which triggered the fault. The increase in kernel
page permissions makes hitting this case more likely as well.
Handling the case in the vectors gives a much more familiar looking
error message:

lkdtm: Performing direct entry EXEC_RODATA
lkdtm: attempting ok execution at ffff0000084c0840
lkdtm: attempting bad execution at ffff000008880680
Unable to handle kernel paging request at virtual address ffff000008880680
pgd = ffff8000089b2000
[ffff000008880680] *pgd=00000000489b4003, *pud=0000000048904003, *pmd=0000000000000000
Internal error: Oops: 8400000e [#1] PREEMPT SMP
Modules linked in:
CPU: 1 PID: 997 Comm: sh Not tainted 4.7.0-rc1+ #24
Hardware name: linux,dummy-virt (DT)
task: ffff800077f9f080 ti: ffff800008a1c000 task.ti: ffff800008a1c000
PC is at lkdtm_rodata_do_nothing+0x0/0x8
LR is at execute_location+0x74/0x88

Signed-off-by: Laura Abbott <labbott at redhat.com>
---
v3: Fixup permission in do_page_fault to detect the kernel iabort, don't run
fixup handlers on kernel instruction aborts.

Dropped the Acked-by since the addition of checks is pretty significant.
---
 arch/arm64/kernel/entry.S | 18 ++++++++++++++++++
 arch/arm64/mm/fault.c     | 11 +++++++++--
 2 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 12e8d2b..54e93d12 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -336,6 +336,8 @@ el1_sync:
 	lsr	x24, x1, #ESR_ELx_EC_SHIFT	// exception class
 	cmp	x24, #ESR_ELx_EC_DABT_CUR	// data abort in EL1
 	b.eq	el1_da
+	cmp	x24, #ESR_ELx_EC_IABT_CUR	// instruction abort in EL1
+	b.eq	el1_ia
 	cmp	x24, #ESR_ELx_EC_SYS64		// configurable trap
 	b.eq	el1_undef
 	cmp	x24, #ESR_ELx_EC_SP_ALIGN	// stack alignment exception
@@ -347,6 +349,22 @@ el1_sync:
 	cmp	x24, #ESR_ELx_EC_BREAKPT_CUR	// debug exception in EL1
 	b.ge	el1_dbg
 	b	el1_inv
+el1_ia:
+	/*
+	 * Instruction abort handling
+	 */
+	mrs	x0, far_el1
+	enable_dbg
+	// re-enable interrupts if they were enabled in the aborted context
+	tbnz	x23, #7, 1f			// PSR_I_BIT
+	enable_irq
+1:
+	mov	x2, sp				// struct pt_regs
+	bl	do_mem_abort
+
+	// disable interrupts before pulling preserved data off the stack
+	disable_irq
+	kernel_exit 1
 el1_da:
 	/*
 	 * Data abort handling
diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 013e2cb..e25b0891 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -131,6 +131,11 @@ int ptep_set_access_flags(struct vm_area_struct *vma,
 }
 #endif
 
+static bool is_el1_instruction_abort(unsigned int esr)
+{
+	return ESR_ELx_EC(esr) == ESR_ELx_EC_IABT_CUR;
+}
+
 /*
  * The kernel tried to access some page that wasn't present.
  */
@@ -139,8 +144,9 @@ static void __do_kernel_fault(struct mm_struct *mm, unsigned long addr,
 {
 	/*
 	 * Are we prepared to handle this kernel fault?
+	 * We are almost certainly not prepared to handle instruction faults.
 	 */
-	if (fixup_exception(regs))
+	if (!is_el1_instruction_abort(esr) && fixup_exception(regs))
 		return;
 
 	/*
@@ -247,7 +253,8 @@ static inline int permission_fault(unsigned int esr)
 	unsigned int ec       = (esr & ESR_ELx_EC_MASK) >> ESR_ELx_EC_SHIFT;
 	unsigned int fsc_type = esr & ESR_ELx_FSC_TYPE;
 
-	return (ec == ESR_ELx_EC_DABT_CUR && fsc_type == ESR_ELx_FSC_PERM);
+	return (ec == ESR_ELx_EC_DABT_CUR && fsc_type == ESR_ELx_FSC_PERM) ||
+	       (ec == ESR_ELx_EC_IABT_CUR && fsc_type == ESR_ELx_FSC_PERM);
 }
 
 static int __kprobes do_page_fault(unsigned long addr, unsigned int esr,
-- 
2.7.4