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SubjectRe: [PATCH v1] mm/rmap: fix dereferencing invalid subpage pointer in try_to_migrate_one()
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I ran into a similar problem today. I can't see any issue with this patch and
it fixed the problem for me so feel free to add:

Reviewed-by: Alistair Popple <apopple@nvidia.com>
Tested-by: Alistair Popple <apopple@nvidia.com>

On Friday, 24 June 2022 6:53:32 AM AEST David Hildenbrand wrote:
> The subpage we calculate is an invalid pointer for device private pages,
> because device private pages are mapped via non-present device private
> entries, not ordinary present PTEs.
>
> Let's just not compute broken pointers and fixup later. Move the proper
> assignment of the correct subpage to the beginning of the function and
> assert that we really only have a single page in our folio.
>
> This currently results in a BUG when tying to compute anon_exclusive,
> because:
>
> [ 528.727237] BUG: unable to handle page fault for address: ffffea1fffffffc0
> [ 528.739585] #PF: supervisor read access in kernel mode
> [ 528.745324] #PF: error_code(0x0000) - not-present page
> [ 528.751062] PGD 44eaf2067 P4D 44eaf2067 PUD 0
> [ 528.756026] Oops: 0000 [#1] PREEMPT SMP NOPTI
> [ 528.760890] CPU: 120 PID: 18275 Comm: hmm-tests Not tainted 5.19.0-rc3-kfd-alex #257
> [ 528.769542] Hardware name: AMD Corporation BardPeak/BardPeak, BIOS RTY1002BDS 09/17/2021
> [ 528.778579] RIP: 0010:try_to_migrate_one+0x21a/0x1000
> [ 528.784225] Code: f6 48 89 c8 48 2b 05 45 d1 6a 01 48 c1 f8 06 48 29
> c3 48 8b 45 a8 48 c1 e3 06 48 01 cb f6 41 18 01 48 89 85 50 ff ff ff 74
> 0b <4c> 8b 33 49 c1 ee 11 41 83 e6 01 48 8b bd 48 ff ff ff e8 3f 99 02
> [ 528.805194] RSP: 0000:ffffc90003cdfaa0 EFLAGS: 00010202
> [ 528.811027] RAX: 00007ffff7ff4000 RBX: ffffea1fffffffc0 RCX: ffffeaffffffffc0
> [ 528.818995] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffc90003cdfaf8
> [ 528.826962] RBP: ffffc90003cdfb70 R08: 0000000000000000 R09: 0000000000000000
> [ 528.834930] R10: ffffc90003cdf910 R11: 0000000000000002 R12: ffff888194450540
> [ 528.842899] R13: ffff888160d057c0 R14: 0000000000000000 R15: 03ffffffffffffff
> [ 528.850865] FS: 00007ffff7fdb740(0000) GS:ffff8883b0600000(0000) knlGS:0000000000000000
> [ 528.859891] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 528.866308] CR2: ffffea1fffffffc0 CR3: 00000001562b4003 CR4: 0000000000770ee0
> [ 528.874275] PKRU: 55555554
> [ 528.877286] Call Trace:
> [ 528.880016] <TASK>
> [ 528.882356] ? lock_is_held_type+0xdf/0x130
> [ 528.887033] rmap_walk_anon+0x167/0x410
> [ 528.891316] try_to_migrate+0x90/0xd0
> [ 528.895405] ? try_to_unmap_one+0xe10/0xe10
> [ 528.900074] ? anon_vma_ctor+0x50/0x50
> [ 528.904260] ? put_anon_vma+0x10/0x10
> [ 528.908347] ? invalid_mkclean_vma+0x20/0x20
> [ 528.913114] migrate_vma_setup+0x5f4/0x750
> [ 528.917691] dmirror_devmem_fault+0x8c/0x250 [test_hmm]
> [ 528.923532] do_swap_page+0xac0/0xe50
> [ 528.927623] ? __lock_acquire+0x4b2/0x1ac0
> [ 528.932199] __handle_mm_fault+0x949/0x1440
> [ 528.936876] handle_mm_fault+0x13f/0x3e0
> [ 528.941256] do_user_addr_fault+0x215/0x740
> [ 528.945928] exc_page_fault+0x75/0x280
> [ 528.950115] asm_exc_page_fault+0x27/0x30
> [ 528.954593] RIP: 0033:0x40366b
> ...
>
> Fixes: 6c287605fd56 ("mm: remember exclusively mapped anonymous pages with PG_anon_exclusive")
> Reported-by: "Sierra Guiza, Alejandro (Alex)" <alex.sierra@amd.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
> mm/rmap.c | 27 +++++++++++++++++----------
> 1 file changed, 17 insertions(+), 10 deletions(-)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 5bcb334cd6f2..746c05acad27 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1899,8 +1899,23 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
> /* Unexpected PMD-mapped THP? */
> VM_BUG_ON_FOLIO(!pvmw.pte, folio);
>
> - subpage = folio_page(folio,
> - pte_pfn(*pvmw.pte) - folio_pfn(folio));
> + if (folio_is_zone_device(folio)) {
> + /*
> + * Our PTE is a non-present device exclusive entry and
> + * calculating the subpage as for the common case would
> + * result in an invalid pointer.
> + *
> + * Since only PAGE_SIZE pages can currently be
> + * migrated, just set it to page. This will need to be
> + * changed when hugepage migrations to device private
> + * memory are supported.
> + */
> + VM_BUG_ON_FOLIO(folio_nr_pages(folio) > 1, folio);
> + subpage = &folio->page;
> + } else {
> + subpage = folio_page(folio,
> + pte_pfn(*pvmw.pte) - folio_pfn(folio));
> + }
> address = pvmw.address;
> anon_exclusive = folio_test_anon(folio) &&
> PageAnonExclusive(subpage);
> @@ -1993,15 +2008,7 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
> /*
> * No need to invalidate here it will synchronize on
> * against the special swap migration pte.
> - *
> - * The assignment to subpage above was computed from a
> - * swap PTE which results in an invalid pointer.
> - * Since only PAGE_SIZE pages can currently be
> - * migrated, just set it to page. This will need to be
> - * changed when hugepage migrations to device private
> - * memory are supported.
> */
> - subpage = &folio->page;
> } else if (PageHWPoison(subpage)) {
> pteval = swp_entry_to_pte(make_hwpoison_entry(subpage));
> if (folio_test_hugetlb(folio)) {
>




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