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SubjectRe: [PATCH] arm: fix page faults in do_alignment
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On 2019/8/30 21:35, Russell King - ARM Linux admin wrote:
> 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@huawei.com>
>
> 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.

__might_sleep is called in the function __get_user which lead to that bug.
And that bug is triggered in a kernel space. Page fault can not be generated.
Right?

> Your patch re-introduces that bug.
>
>> ---
>> arch/arm/mm/alignment.c | 16 +++++++++++++---
>> 1 file changed, 13 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/arm/mm/alignment.c b/arch/arm/mm/alignment.c
>> index 04b3643..2ccabd3 100644
>> --- a/arch/arm/mm/alignment.c
>> +++ b/arch/arm/mm/alignment.c
>> @@ -774,6 +774,7 @@ static ssize_t alignment_proc_write(struct file *file, const char __user *buffer
>> unsigned long instr = 0, instrptr;
>> int (*handler)(unsigned long addr, unsigned long instr, struct pt_regs *regs);
>> unsigned int type;
>> + mm_segment_t fs;
>> unsigned int fault;
>> u16 tinstr = 0;
>> int isize = 4;
>> @@ -784,16 +785,22 @@ static ssize_t alignment_proc_write(struct file *file, const char __user *buffer
>>
>> instrptr = instruction_pointer(regs);
>>
>> + fs = get_fs();
>> + set_fs(KERNEL_DS);
>> if (thumb_mode(regs)) {
>> u16 *ptr = (u16 *)(instrptr & ~1);
>> - fault = probe_kernel_address(ptr, tinstr);
>> + fault = __copy_from_user(tinstr,
>> + (__force const void __user *)ptr,
>> + sizeof(tinstr));
>> 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 = probe_kernel_address(ptr + 1, tinst2);
>> + fault = __copy_from_user(tinst2,
>> + (__force const void __user *)(ptr+1),
>> + sizeof(tinst2));
>> tinst2 = __mem_to_opcode_thumb16(tinst2);
>> instr = __opcode_thumb32_compose(tinstr, tinst2);
>> thumb2_32b = 1;
>> @@ -803,10 +810,13 @@ static ssize_t alignment_proc_write(struct file *file, const char __user *buffer
>> }
>> }
>> } else {
>> - fault = probe_kernel_address((void *)instrptr, instr);
>> + fault = __copy_from_user(instr,
>> + (__force const void __user *)instrptr,
>> + sizeof(instr));
>> instr = __mem_to_opcode_arm(instr);
>> }
>>
>> + set_fs(fs);
>> if (fault) {
>> type = TYPE_FAULT;
>> goto bad_or_fault;
>> --
>> 1.8.3.1
>>
>>
>


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