On 28.07.23 11:08, David Hildenbrand wrote:

On 28.07.23 04:30, John Hubbard wrote:

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On 7/27/23 14:28, David Hildenbrand wrote:

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We accidentally enforced PROT_NONE PTE/PMD permission checks for follow_page() like we do for get_user_pages() and friends. That was undesired, because follow_page() is usually only used to lookup a currently mapped page, not to actually access it. Further, follow_page() does not actually trigger fault handling, but instead simply fails.

I see that follow_page() is also completely undocumented. And that reduces us to deducing how it should be used...these things that change follow_page()'s behavior maybe should have a go at documenting it too, perhaps.

I can certainly be motivated to do that. :)

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Let's restore that behavior by conditionally setting FOLL_FORCE if FOLL_WRITE is not set. This way, for example KSM and migration code will no longer fail on PROT_NONE mapped PTEs/PMDS.

Handling this internally doesn't require us to add any new FOLL_FORCE usage outside of GUP code.

While at it, refuse to accept FOLL_FORCE: we don't even perform VMA permission checks like in check_vma_flags(), so especially FOLL_FORCE|FOLL_WRITE would be dodgy.

This issue was identified by code inspection. We'll add some documentation regarding FOLL_FORCE next.

Reported-by: Peter Xu peterx@redhat.com Fixes: 474098edac26 ("mm/gup: replace FOLL_NUMA by gup_can_follow_protnone()") Cc: stable@vger.kernel.org Signed-off-by: David Hildenbrand david@redhat.com

---

mm/gup.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/mm/gup.c b/mm/gup.c index 2493ffa10f4b..da9a5cc096ac 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -841,9 +841,17 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address, if (vma_is_secretmem(vma)) return NULL;

• if (WARN_ON_ONCE(foll_flags & FOLL_PIN))

• if (WARN_ON_ONCE(foll_flags & (FOLL_PIN | FOLL_FORCE))) return NULL;

This is not a super happy situation: follow_page() is now prohibited (see above: we should document that interface) from passing in FOLL_FORCE...

I guess you saw my patch #4.

If you take a look at the existing callers (that are fortunately very limited), you'll see that nobody cares.

Most of the FOLL flags don't make any sense for follow_page(), and limiting further (ab)use is at least to me very appealing.

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• /*

• * Traditionally, follow_page() succeeded on PROT_NONE-mapped pages
  

• * but failed follow_page(FOLL_WRITE) on R/O-mapped pages. Let's
  

• * keep these semantics by setting FOLL_FORCE if FOLL_WRITE is not set.
  

• */
  

• if (!(foll_flags & FOLL_WRITE))

• foll_flags |= FOLL_FORCE;
  

...but then we set it anyway, for special cases. It's awkward because FOLL_FORCE is not an "internal to gup" flag (yet?).

I don't yet have suggestions, other than:

1. Yes, the FOLL_NUMA made things bad.

2. And they are still very confusing, especially the new use of FOLL_FORCE.

...I'll try to let this soak in and maybe recommend something in a more productive way. :)

What I can offer that might be very appealing is the following:

Get rid of the flags parameter for follow_page() *completely*. Yes, then we can even rename FOLL_ to something reasonable in the context where it is nowadays used ;)

Internally, we'll then set

FOLL_GET | FOLL_DUMP | FOLL_FORCE

and document exactly what this functions does. Any user that needs something different should just look into using get_user_pages() instead.

I can prototype that on top of this work easily.

The end result looks something like:

/** * follow_page - look up and reference a page descriptor from a user-virtual * address * @vma: vm_area_struct mapping @address * @address: virtual address to look up * * follow_page() will look up the page mapped at the given address and * take a reference on the page. The returned page has to be released using * put_page(). * * follow_page() will not return special (like zero) pages and does not check * PTE protection: the returned page might be mapped PROT_NONE, R/O or R/W. * Consequently, follow_page() will not trigger NUMA hinting faults. * * follow_page() does not trigger page faults. If no page is mapped, or * a special (like zero) page is mapped, it returns %NULL or an error pointer. * * Note: new users with different requirements are probably better off using * one of the get_user_pages() variants or one of the walk_page_range() * variants. * * Return: the mapped (struct page *), %NULL if no mapping exists, or * an error pointer if there is a mapping to something not represented * by a page descriptor (see also vm_normal_page()) or the zero page. */ struct page *follow_page(struct vm_area_struct *vma, unsigned long address) { struct follow_page_context ctx = { NULL }; unsigned long gup_flags; struct page *page;

if (vma_is_secretmem(vma)) return NULL;

/* * FOLL_GET: We always want a reference on the returned page. * FOL_DUMP: Ignore special (like zero) pages. * FOLL_FORCE: Succeeded on PROT_NONE-mapped pages. */ gup_flags = FOLL_GET | FOLL_DUMP | FOLL_FORCE;

page = follow_page_mask(vma, address, gup_flags, &ctx); if (ctx.pgmap) put_dev_pagemap(ctx.pgmap); return page; }