LoongArch: Fix access error when read fault on a write-only VMA

commit efb44ff upstream.

As with most architectures, allow handling of read faults in VMAs that
have VM_WRITE but without VM_READ (WRITE implies READ).

Otherwise, reading before writing a write-only memory will error while
reading after writing everything is fine.

BTW, move the VM_EXEC judgement before VM_READ/VM_WRITE to make logic a
little clearer.

Cc: stable@vger.kernel.org
Fixes: 09cfefb ("LoongArch: Add memory management")
Signed-off-by: Jiantao Shan <shanjiantao@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/loongarch/mm/fault.c

@@ -202,10 +202,10 @@ static void __kprobes __do_page_fault(struct pt_regs *regs,
 		if (!(vma->vm_flags & VM_WRITE))
 			goto bad_area;
 	} else {
-		if (!(vma->vm_flags & VM_READ) && address != exception_era(regs))
-			goto bad_area;
 		if (!(vma->vm_flags & VM_EXEC) && address == exception_era(regs))
 			goto bad_area;
+		if (!(vma->vm_flags & (VM_READ | VM_WRITE)) && address != exception_era(regs))
+			goto bad_area;
 	}
 
 	/*