On 28.07.23 11:08, David Hildenbrand wrote:
On 28.07.23 04:30, John Hubbard wrote:
On 7/27/23 14:28, David Hildenbrand wrote:
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. :)
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@xxxxxxxxxx>
Fixes: 474098edac26 ("mm/gup: replace FOLL_NUMA by gup_can_follow_protnone()")
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
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.
+ /*
+ * 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;
}
--
Cheers,
David / dhildenb
