Re: [PATCH] mm/mprotect: reduce Committed_AS if memory protection is changed to PROT_NONE

From: liusongtang
Date: Wed Apr 27 2022 - 02:34:32 EST

On 2022/4/27 4:34, Andrew Morton wrote:

On Tue, 26 Apr 2022 19:27:05 +0800 liusongtang <liusongtang@xxxxxxxxxx> wrote:

If PROT_WRITE is set, the size of vm area will be added to Committed_AS.
However, if memory protection is changed to PROT_NONE,
the corresponding physical memory will not be used, but Committed_AS still
count the size of the PROT_NONE memory.

This patch reduce Committed_AS and free the corresponding memory if
memory protection is changed to PROT_NONE.


--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -497,6 +497,12 @@ mprotect_fixup(struct vm_area_struct *vma, struct vm_area_struct **pprev,
+ if ((newflags & (VM_READ | VM_WRITE | VM_EXEC | VM_LOCKED | VM_ACCOUNT)) == VM_ACCOUNT) {
+ zap_page_range(vma, start, end - start);
+ newflags &= ~VM_ACCOUNT;
+ vm_unacct_memory((end - start) >> PAGE_SHIFT);
+ }
* vm_flags and vm_page_prot are protected by the mmap_lock
* held in write mode.
Surprised. If userspace does mprotect(addr, len. PROT_NONE) then
mprotect(addr, len. PROT_READ), what is now at *addr? Zeroes?

1. In the case mentioned above, I think data in *addr is invalid after
mprotect(addr, len. PROT_NONE), so clear it will not cause a problem.
2. Another idea is we can check if this vm area is populated before reduce Committed_AS.