linux-kernel - Re: [v3 PATCH] arm64: mm: force write fault for atomic RMW instructions

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Date: Wed, 5 Jun 2024 10:31:51 -0700
From: Yang Shi <yang@...amperecomputing.com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: will@...nel.org, anshuman.khandual@....com, scott@...amperecomputing.com,
 cl@...two.org, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org
Subject: Re: [v3 PATCH] arm64: mm: force write fault for atomic RMW
 instructions

>> @@ -557,6 +587,11 @@ static int __kprobes do_page_fault(unsigned long far, unsigned long esr,
>>   		/* It was write fault */
>>   		vm_flags = VM_WRITE;
>>   		mm_flags |= FAULT_FLAG_WRITE;
>> +	} else if (is_el0_atomic_instr(regs)) {
>> +		/* Force write fault */
>> +		vm_flags = VM_WRITE;
>> +		mm_flags |= FAULT_FLAG_WRITE;
>> +		force_write = true;
>>   	} else {
>>   		/* It was read fault */
>>   		vm_flags = VM_READ;
>> @@ -586,6 +621,14 @@ static int __kprobes do_page_fault(unsigned long far, unsigned long esr,
>>   	if (!vma)
>>   		goto lock_mmap;
>>   
>> +	/* vma flags don't allow write, undo force write */
>> +	if (force_write && !(vma->vm_flags & VM_WRITE)) {
>> +		vm_flags |= VM_READ;
>> +		if (!alternative_has_cap_unlikely(ARM64_HAS_EPAN))
>> +			vm_flags |= VM_EXEC;
>> +		mm_flags &= ~FAULT_FLAG_WRITE;
>> +	}
> Ah, this revert to the non-write flags doesn't look great as we
> basically duplicate the 'else' block in the original check. So, it
> probably look better as per your earlier patch to just do the
> instruction read just before the !(vma->vm_flags & flags) check,
> something like:
>
> 	if ((vma->vm_flags & VM_WRITE) && is_el0_atomic_instr(regs)) {
> 		vm_flags = VM_WRITE;
> 		mm_flags |= FAULT_FLAG_WRITE;
> 	}
>
> This way we also only read the instruction if the vma is writeable. I
> think it's fine to do this under the vma lock since we have
> pagefault_disable() for the insn read.

I think we also need to skip this for write fault and instruction fault. 
Some something like:

@@ -529,6 +557,7 @@ static int __kprobes do_page_fault(unsigned long 
far, unsigned long esr,
         unsigned int mm_flags = FAULT_FLAG_DEFAULT;
         unsigned long addr = untagged_addr(far);
         struct vm_area_struct *vma;
+       bool may_force_write = false;

         if (kprobe_page_fault(regs, esr))
                 return 0;
@@ -565,6 +594,7 @@ static int __kprobes do_page_fault(unsigned long 
far, unsigned long esr,
                 /* If EPAN is absent then exec implies read */
                 if (!alternative_has_cap_unlikely(ARM64_HAS_EPAN))
                         vm_flags |= VM_EXEC;
+               may_force_write = true;
         }

         if (is_ttbr0_addr(addr) && is_el1_permission_fault(addr, esr, 
regs)) {
@@ -586,6 +616,12 @@ static int __kprobes do_page_fault(unsigned long 
far, unsigned long esr,
         if (!vma)
                 goto lock_mmap;

+       if (may_force_write && (vma->vm_flags & VM_WRITE) &&
+           is_el0_atomic_instr(regs)) {
+               vm_flags = VM_WRITE;
+               mm_flags |= FAULT_FLAG_WRITE;
+       }
+
         if (!(vma->vm_flags & vm_flags)) {
                 vma_end_read(vma);
                 goto lock_mmap;

>