linux-kernel - Re: [PATCH RFC 6/9] s390/pci_mmio: fully validate the VMA before calling follow

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Date:   Fri, 10 Sep 2021 14:52:12 +0000
From:   Liam Howlett <liam.howlett@...cle.com>
To:     Niklas Schnelle <schnelle@...ux.ibm.com>
CC:     David Hildenbrand <david@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH RFC 6/9] s390/pci_mmio: fully validate the VMA before
 calling follow_pte()

* Niklas Schnelle <schnelle@...ux.ibm.com> [210910 10:31]:
> On Fri, 2021-09-10 at 14:12 +0000, Liam Howlett wrote:
> > * David Hildenbrand <david@...hat.com> [210910 05:23]:
> > > On 10.09.21 10:22, Niklas Schnelle wrote:
> > > > On Thu, 2021-09-09 at 16:59 +0200, David Hildenbrand wrote:
> > > > > We should not walk/touch page tables outside of VMA boundaries when
> > > > > holding only the mmap sem in read mode. Evil user space can modify the
> > > > > VMA layout just before this function runs and e.g., trigger races with
> > > > > page table removal code since commit dd2283f2605e ("mm: mmap: zap pages
> > > > > with read mmap_sem in munmap").
> > > > > 
> > > > > find_vma() does not check if the address is >= the VMA start address;
> > > > > use vma_lookup() instead.
> > > > > 
> > > > > Fixes: dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in munmap")
> > > > > Signed-off-by: David Hildenbrand <david@...hat.com>
> > > > > ---
> > > > >   arch/s390/pci/pci_mmio.c | 4 ++--
> > > > >   1 file changed, 2 insertions(+), 2 deletions(-)
> > > > > 
> > > > > diff --git a/arch/s390/pci/pci_mmio.c b/arch/s390/pci/pci_mmio.c
> > > > > index ae683aa623ac..c5b35ea129cf 100644
> > > > > --- a/arch/s390/pci/pci_mmio.c
> > > > > +++ b/arch/s390/pci/pci_mmio.c
> > > > > @@ -159,7 +159,7 @@ SYSCALL_DEFINE3(s390_pci_mmio_write, unsigned long, mmio_addr,
> > > > >   	mmap_read_lock(current->mm);
> > > > >   	ret = -EINVAL;
> > > > > -	vma = find_vma(current->mm, mmio_addr);
> > > > > +	vma = vma_lookup(current->mm, mmio_addr);
> > > > >   	if (!vma)
> > > > >   		goto out_unlock_mmap;
> > > > >   	if (!(vma->vm_flags & (VM_IO | VM_PFNMAP)))
> > > > > @@ -298,7 +298,7 @@ SYSCALL_DEFINE3(s390_pci_mmio_read, unsigned long, mmio_addr,
> > > > >   	mmap_read_lock(current->mm);
> > > > >   	ret = -EINVAL;
> > > > > -	vma = find_vma(current->mm, mmio_addr);
> > > > > +	vma = vma_lookup(current->mm, mmio_addr);
> > > > >   	if (!vma)
> > > > >   		goto out_unlock_mmap;
> > > > >   	if (!(vma->vm_flags & (VM_IO | VM_PFNMAP)))
> > > > 
> > > > Oh wow great find thanks! If I may say so these are not great function
> > > > names. Looking at the code vma_lookup() is inded find_vma() plus the
> > > > check that the looked up address is indeed inside the vma.
> > > > 
> > > 
> > > IIRC, vma_lookup() was introduced fairly recently. Before that, this
> > > additional check was open coded (and still are in some instances). It's
> > > confusing, I agree.
> > 
> > This confusion is why I introduced vma_lookup().  My hope is to reduce
> > the users of find_vma() to only those that actually need the added
> > functionality, which are mostly in the mm code.
> 
> Ah I see, soo the confusingly similar names are in hope of one day
> making find_vma() only visible or at least used in the mm code. That
> does make more sense then. Thanks for the explanation! Maybe this would
> be a good candidate for a treewide change/coccinelle script? Then again
> I guess sometimes one really wants find_vma() and it's hard to tell
> apart.
> 

find_vma() does not describe what the code actually does, so I think it
is a good candidate for a tree wide change.  I'm not sure it would be
popular though.  I couldn't come up with a name that would be worth the
efforts.  If the name does change, then it should also change
find_vma_intersection() as well, nommu code also has a find_vma_exact().
Given the unraveling of a rename, I thought it'd be best to try and
clean up the current code and make it less error-prone with a new mm
API.