linux-kernel - Re: [PATCH 2/6] mm/mremap: For MREMAP_DONTUNMAP check security_vm_enough_memory

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Message-ID: <d25ad10c-6f67-8c11-18c3-0193b8ea14c4@arista.com>
Date:   Mon, 28 Dec 2020 19:12:21 +0000
From:   Dmitry Safonov <dima@...sta.com>
To:     Brian Geffon <bgeffon@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Hugh Dickins <hughd@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Minchan Kim <minchan@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will@...nel.org>, linux-aio@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH 2/6] mm/mremap: For MREMAP_DONTUNMAP check
 security_vm_enough_memory_mm()

On 12/28/20 6:21 PM, Brian Geffon wrote:
> This looks good to me with a small comment.
> 
>>         if (do_munmap(mm, old_addr, old_len, uf_unmap) < 0) {
>>                 /* OOM: unable to split vma, just get accounts right */
>> -               if (vm_flags & VM_ACCOUNT)
>> +               if (vm_flags & VM_ACCOUNT && !(flags & MREMAP_DONTUNMAP))
>>                         vm_acct_memory(new_len >> PAGE_SHIFT);
> 
> Checking MREMAP_DONTUNMAP in the do_munmap path is unnecessary as
> MREMAP_DONTUNMAP will have already returned by this point.

In this code it is also used as err-path. In case move_page_tables()
fails to move all page tables or .mremap() callback fails, the new VMA
is unmapped.

IOW, MREMAP_DONTUNMAP returns under:
:	if (unlikely(!err && (flags & MREMAP_DONTUNMAP))) {

-- 
          Dima