riscv: Make VM_WRITE imply VM_READ

RISC-V does not presently have write-only mappings as that PTE bit pattern
is considered reserved in the privileged spec, so allow handling of read
faults in VMAs that have VM_WRITE without VM_READ in order to be consistent
with other architectures that have similar limitations.

Fixes: 2139619 ("riscv: mmap with PROT_WRITE but no PROT_READ is invalid")
Reviewed-by: Atish Patra <[email protected]>
Signed-off-by: Andrew Bresticker <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]/
Signed-off-by: Palmer Dabbelt <[email protected]>

Author: abrestic-rivos

arch/riscv/mm/fault.c

@@ -184,7 +184,8 @@ static inline bool access_error(unsigned long cause, struct vm_area_struct *vma)
 		}
 		break;
 	case EXC_LOAD_PAGE_FAULT:
-		if (!(vma->vm_flags & VM_READ)) {
+		/* Write implies read */
+		if (!(vma->vm_flags & (VM_READ | VM_WRITE))) {
 			return true;
 		}
 		break;