+ mm-filemap-fix-filemap_get_folios_contig-thp-panic.patch added to mm-hotfixes-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/filemap: fix filemap_get_folios_contig THP panic
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-filemap-fix-filemap_get_folios_contig-thp-panic.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-filemap-fix-filemap_get_folios_contig-thp-panic.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Steve Sistare <steven.sistare@oracle.com>
Subject: mm/filemap: fix filemap_get_folios_contig THP panic
Date: Tue, 3 Sep 2024 07:25:17 -0700

Patch series "memfd-pin huge page fixes".

Fix multiple bugs that occur when using memfd_pin_folios with hugetlb
pages and THP.  The hugetlb bugs only bite when the page is not yet
faulted in when memfd_pin_folios is called.  The THP bug bites when the
starting offset passed to memfd_pin_folios is not huge page aligned.  See
the commit messages for details.


This patch (of 5):

memfd_pin_folios on memory backed by THP panics if the requested start
offset is not huge page aligned:

BUG: kernel NULL pointer dereference, address: 0000000000000036
RIP: 0010:filemap_get_folios_contig+0xdf/0x290
RSP: 0018:ffffc9002092fbe8 EFLAGS: 00010202
RAX: 0000000000000002 RBX: 0000000000000002 RCX: 0000000000000002

The fault occurs here, because xas_load returns a folio with value 2:

    filemap_get_folios_contig()
        for (folio = xas_load(&xas); folio && xas.xa_index <= end;
                        folio = xas_next(&xas)) {
                ...
                if (!folio_try_get(folio))   <-- BOOM

"2" is an xarray sibling entry.  We get it because memfd_pin_folios does
not round the indices passed to filemap_get_folios_contig to huge page
boundaries for THP, so we load from the middle of a huge page range see a
sibling.  (It does round for hugetlbfs, at the is_file_hugepages test).

To fix, if the folio is a sibling, then return the next index as the
starting point for the next call to filemap_get_folios_contig.

Link: https://lkml.kernel.org/r/1725373521-451395-1-git-send-email-steven.sistare@oracle.com
Link: https://lkml.kernel.org/r/1725373521-451395-2-git-send-email-steven.sistare@oracle.com
Fixes: 89c1905d9c14 ("mm/gup: introduce memfd_pin_folios() for pinning memfd folios")
Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Peter Xu <peterx@redhat.com>
Cc: Vivek Kasireddy <vivek.kasireddy@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/filemap.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/mm/filemap.c~mm-filemap-fix-filemap_get_folios_contig-thp-panic
+++ a/mm/filemap.c
@@ -2181,6 +2181,10 @@ unsigned filemap_get_folios_contig(struc
 		if (xa_is_value(folio))
 			goto update_start;
 
+		/* If we landed in the middle of a THP, continue at its end. */
+		if (xa_is_sibling(folio))
+			goto update_start;
+
 		if (!folio_try_get(folio))
 			goto retry;
 
_

Patches currently in -mm which might be from steven.sistare@oracle.com are

mm-filemap-fix-filemap_get_folios_contig-thp-panic.patch
mm-hugetlb-fix-memfd_pin_folios-free_huge_pages-leak.patch
mm-hugetlb-fix-memfd_pin_folios-resv_huge_pages-leak.patch
mm-gup-fix-memfd_pin_folios-hugetlb-page-allocation.patch
mm-gup-fix-memfd_pin_folios-alloc-race-panic.patch