ARM: 8036/1: Enable IRQs before attempting to read user space in __und_usr

The Undef abort handler in the kernel reads the undefined instruction
from user space. If the page table was modified from another CPU, the
user access could fail and do_page_fault() will be executed with
interrupts disabled. This can potentially deadlock on ARM11MPCore or on
Cortex-A15 with erratum 798181 workaround enabled (both implying IPI for
TLB maintenance with page table lock held).

This patch enables the IRQs in __und_usr before attempting to read the
instruction from user space.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Arun KS <getarunks@gmail.com>
Cc: Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ryan Mallon <rmallon@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

1 files changed, 7 insertions, 4 deletions

diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
index 1879e8dd2acc..5fc897cf409b 100644
--- a/arch/arm/kernel/entry-armv.S
+++ b/arch/arm/kernel/entry-armv.S
@@ -413,6 +413,11 @@ __und_usr:
         @
         adr     r9, BSYM(ret_from_exception)
 
+        @ IRQs must be enabled before attempting to read the instruction from
+        @ user space since that could cause a page/translation fault if the
+        @ page table was modified by another CPU.
+        enable_irq
+
         tst     r3, #PSR_T_BIT                  @ Thumb mode?
         bne     __und_usr_thumb
         sub     r4, r2, #4                      @ ARM instr at LR - 4
@@ -517,7 +522,7 @@ ENDPROC(__und_usr)
  *  r9  = normal "successful" return address
  *  r10 = this threads thread_info structure
  *  lr  = unrecognised instruction return address
- * IRQs disabled, FIQs enabled.
+ * IRQs enabled, FIQs enabled.
  */
         @
         @ Fall-through from Thumb-2 __und_usr
@@ -624,7 +629,6 @@ call_fpe:
 #endif
 
 do_fpe:
-        enable_irq
         ldr     r4, .LCfp
         add     r10, r10, #TI_FPSTATE           @ r10 = workspace
         ldr     pc, [r4]                        @ Call FP module USR entry point
@@ -652,8 +656,7 @@ __und_usr_fault_32:
         b       1f
 __und_usr_fault_16:
         mov     r1, #2
-1:      enable_irq
-        mov     r0, sp
+1:      mov     r0, sp
         adr     lr, BSYM(ret_from_exception)
         b       __und_fault
 ENDPROC(__und_usr_fault_32)