Russell King - ARM Linux admin <linux@xxxxxxxxxxxxxxx> writes:
> On Fri, Aug 30, 2019 at 09:31:17PM +0800, Jing Xiangfeng wrote:
>> The function do_alignment can handle misaligned address for user and
>> kernel space. If it is a userspace access, do_alignment may fail on
>> a low-memory situation, because page faults are disabled in
>> probe_kernel_address.
>>
>> Fix this by using __copy_from_user stead of probe_kernel_address.
>>
>> Fixes: b255188 ("ARM: fix scheduling while atomic warning in alignment handling code")
>> Signed-off-by: Jing Xiangfeng <jingxiangfeng@xxxxxxxxxx>
>
> NAK.
>
> The "scheduling while atomic warning in alignment handling code" is
> caused by fixing up the page fault while trying to handle the
> mis-alignment fault generated from an instruction in atomic context.
>
> Your patch re-introduces that bug.
And the patch that fixed scheduling while atomic apparently introduced a
regression. Admittedly a regression that took 6 years to track down but
still.
So it looks like the code needs to do something like:
diff --git a/arch/arm/mm/alignment.c b/arch/arm/mm/alignment.c
index 04b36436cbc0..5e2b8623851e 100644
--- a/arch/arm/mm/alignment.c
+++ b/arch/arm/mm/alignment.c
@@ -784,6 +784,9 @@ do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
instrptr = instruction_pointer(regs);
+ if (user_mode(regs))
+ goto user;
+
if (thumb_mode(regs)) {
u16 *ptr = (u16 *)(instrptr & ~1);
fault = probe_kernel_address(ptr, tinstr);
@@ -933,6 +936,34 @@ do_alignment(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
return 1;
user:
+ if (thumb_mode(regs)) {
+ u16 *ptr = (u16 *)(instrptr & ~1);
+ fault = get_user(tinstr, ptr);
+ tinstr = __mem_to_opcode_thumb16(tinstr);
+ if (!fault) {
+ if (cpu_architecture() >= CPU_ARCH_ARMv7 &&
+ IS_T32(tinstr)) {
+ /* Thumb-2 32-bit */
+ u16 tinst2 = 0;
+ fault = get_user(ptr + 1, tinst2);
+ tinst2 = __mem_to_opcode_thumb16(tinst2);
+ instr = __opcode_thumb32_compose(tinstr, tinst2);
+ thumb2_32b = 1;
+ } else {
+ isize = 2;
+ instr = thumb2arm(tinstr);
+ }
+ }
+ } else {
+ fault = get_user(instr, (u32*)instrptr);
+ instr = __mem_to_opcode_arm(instr);
+ }
+
+ if (fault) {
+ type = TYPE_FAULT;
+ goto bad_or_fault;
+ }
+
ai_user += 1;
if (ai_usermode & UM_WARN)
Eric
