linux-kernel - Re: [PATCH 1/3] ima: Remove inode lock

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Message-ID: <CAG48ez0m4O5M8m4bLJ++gTZzsAyKgud++cBMBqAm74OLUKBFpg@mail.gmail.com>
Date: Fri, 18 Oct 2024 16:36:16 +0200
From: Jann Horn <jannh@...gle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: "Kirill A. Shutemov" <kirill@...temov.name>, Roberto Sassu <roberto.sassu@...weicloud.com>, 
	Paul Moore <paul@...l-moore.com>, ebpqwerty472123@...il.com, 
	kirill.shutemov@...ux.intel.com, zohar@...ux.ibm.com, 
	dmitry.kasatkin@...il.com, eric.snowberg@...cle.com, jmorris@...ei.org, 
	serge@...lyn.com, linux-integrity@...r.kernel.org, 
	linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org, 
	bpf@...r.kernel.org, Roberto Sassu <roberto.sassu@...wei.com>, linux-mm@...ck.org, 
	akpm@...ux-foundation.org, vbabka@...e.cz, linux-fsdevel@...r.kernel.org, 
	Liam Howlett <liam.howlett@...cle.com>
Subject: Re: [PATCH 1/3] ima: Remove inode lock

On Fri, Oct 18, 2024 at 1:00 PM Lorenzo Stoakes
<lorenzo.stoakes@...cle.com> wrote:
> On Fri, Oct 18, 2024 at 01:49:06PM +0300, Kirill A. Shutemov wrote:
> > On Fri, Oct 18, 2024 at 11:24:06AM +0200, Roberto Sassu wrote:
> > > Probably it is hard, @Kirill would there be any way to safely move
> > > security_mmap_file() out of the mmap_lock lock?
> >
> > What about something like this (untested):
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index dd4b35a25aeb..03473e77d356 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -1646,6 +1646,26 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
> >       if (pgoff + (size >> PAGE_SHIFT) < pgoff)
> >               return ret;
> >
> > +     if (mmap_read_lock_killable(mm))
> > +             return -EINTR;
> > +
> > +     vma = vma_lookup(mm, start);
> > +
> > +     if (!vma || !(vma->vm_flags & VM_SHARED)) {
> > +             mmap_read_unlock(mm);
> > +             return -EINVAL;
> > +     }
> > +
> > +     file = get_file(vma->vm_file);
> > +
> > +     mmap_read_unlock(mm);
> > +
> > +     ret = security_mmap_file(vma->vm_file, prot, flags);
>
> Accessing VMA fields without any kind of lock is... very much not advised.
>
> I'm guessing you meant to say:
>
>         ret = security_mmap_file(file, prot, flags);
>
> Here? :)
>
> I see the original code did this, but obviously was under an mmap lock.
>
> I guess given you check that the file is the same below this.... should be
> fine? Assuming nothing can come in and invalidate the security_mmap_file()
> check in the mean time somehow?
>
> Jann any thoughts?

The overall approach seems reasonable to me - it aligns this path with
the other security_mmap_file() checks, which also don't happen under
the lock.