Re: [PATCH v1] x86/mm/pat: fix VM_PAT handling in COW mappings

[David Hildenbrand]

On 26.03.24 09:33, Ingo Molnar wrote:
> * David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> > On 12.03.24 20:22, Matthew Wilcox wrote:
> > > On Tue, Mar 12, 2024 at 07:11:18PM +0100, David Hildenbrand wrote:
> > > > PAT handling won't do the right thing in COW mappings: the first PTE
> > > > (or, in fact, all PTEs) can be replaced during write faults to point at
> > > > anon folios. Reliably recovering the correct PFN and cachemode using
> > > > follow_phys() from PTEs will not work in COW mappings.
> > >
> > > I guess the first question is: Why do we want to support COW mappings
> > > of VM_PAT areas?  What breaks if we just disallow it?
> >
> > Well, that was my first approach. Then I decided to be less radical (IOW
> > make my life easier by breaking less user space) and "fix it" with
> > minimal effort.
> >
> > Chances of breaking some weird user space is possible, although I believe
> > for most such mappings MAP_PRIVATE doesn't make too much sense sense.
> >
> > Nasty COW support for VM_PFNMAP mappings dates back forever. So does PAT
> > support.
> >
> > I can try finding digging through some possible user space users
> > tomorrow.
>
> I'd much prefer restricting VM_PAT areas than expanding support. Could we

Note that we're not expanding support, we're fixing what used to be
possible before but mostly broke silently.

But I agree that we should rather remove these corner cases instead of fixing
them.

> try the trivial restriction approach first, and only go with your original
> patch if that fails?

Which version would you prefer, I had two alternatives (excluding comment
changes, white-space expected to be broken).


1) Disallow when we would have set VM_PAT on is_cow_mapping()

diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
index 0d72183b5dd0..6979912b1a5d 100644
--- a/arch/x86/mm/pat/memtype.c
+++ b/arch/x86/mm/pat/memtype.c
@@ -994,6 +994,9 @@ int track_pfn_remap(struct vm_area_struct *vma, pgprot_t *prot,
                                && size == (vma->vm_end - vma->vm_start))) {
                int ret;
 
+               if (is_cow_mapping(vma->vm_flags))
+                       return -EINVAL;
+
                ret = reserve_pfn_range(paddr, size, prot, 0);
                if (ret == 0 && vma)
                        vm_flags_set(vma, VM_PAT);


2) Fallback to !VM_PAT

diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
index 0d72183b5dd0..8e97156c9be8 100644
--- a/arch/x86/mm/pat/memtype.c
+++ b/arch/x86/mm/pat/memtype.c
@@ -990,8 +990,8 @@ int track_pfn_remap(struct vm_area_struct *vma, pgprot_t *prot,
        enum page_cache_mode pcm;
 
        /* reserve the whole chunk starting from paddr */
-       if (!vma || (addr == vma->vm_start
-                               && size == (vma->vm_end - vma->vm_start))) {
+       if (!vma || (!is_cow_mapping(vma->vm_flags) && addr == vma->vm_start &&
+                    size == (vma->vm_end - vma->vm_start))) {
                int ret;
 
                ret = reserve_pfn_range(paddr, size, prot, 0);



Personally, I'd go for 2).

--
Cheers,

David / dhildenb