Re: [PATCH] mm/hugetlb.c: fix a bug within a BUG(): inconsistent pte comparison

[Ryan Roberts]

On 30/06/2023 02:32, John Hubbard wrote:
> The following crash happens for me when running the -mm selftests
> (below). Specifically, it happens while running the uffd-stress
> subtests:
> 
> kernel BUG at mm/hugetlb.c:7249!
> invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
> CPU: 0 PID: 3238 Comm: uffd-stress Not tainted 6.4.0-hubbard-github+ #109
> Hardware name: ASUS X299-A/PRIME X299-A, BIOS 1503 08/03/2018
> RIP: 0010:huge_pte_alloc+0x12c/0x1a0
> ...
> Call Trace:
>  <TASK>
>  ? __die_body+0x63/0xb0
>  ? die+0x9f/0xc0
>  ? do_trap+0xab/0x180
>  ? huge_pte_alloc+0x12c/0x1a0
>  ? do_error_trap+0xc6/0x110
>  ? huge_pte_alloc+0x12c/0x1a0
>  ? handle_invalid_op+0x2c/0x40
>  ? huge_pte_alloc+0x12c/0x1a0
>  ? exc_invalid_op+0x33/0x50
>  ? asm_exc_invalid_op+0x16/0x20
>  ? __pfx_put_prev_task_idle+0x10/0x10
>  ? huge_pte_alloc+0x12c/0x1a0
>  hugetlb_fault+0x1a3/0x1120
>  ? finish_task_switch+0xb3/0x2a0
>  ? lock_is_held_type+0xdb/0x150
>  handle_mm_fault+0xb8a/0xd40
>  ? find_vma+0x5d/0xa0
>  do_user_addr_fault+0x257/0x5d0
>  exc_page_fault+0x7b/0x1f0
>  asm_exc_page_fault+0x22/0x30
> 
> That happens because a BUG() statement in huge_pte_alloc() attempts to
> check that a pte, if present, is a hugetlb pte, but it does so in a
> non-lockless-safe manner that leads to a false BUG() report.
> 
> We got here due to a couple of bugs, each of which by itself was not
> quite enough to cause a problem:
> 
> First of all, before commit c33c794828f2("mm: ptep_get() conversion"),
> the BUG() statement in huge_pte_alloc() was itself fragile: it relied
> upon compiler behavior to only read the pte once, despite using it twice
> in the same conditional.
> 
> Next, commit c33c794828f2 ("mm: ptep_get() conversion") broke that
> delicate situation, by causing all direct pte reads to be done via
> READ_ONCE(). And so READ_ONCE() got called twice within the same BUG()
> conditional, leading to comparing (potentially, occasionally) different
> versions of the pte, and thus to false BUG() reports.

Thanks for finding and fixing this - sorry for the issue. FWIW, I've re-reviewed
the whole ptep_get conversion patch looking for other instances of this pattern
- I didn't spot any other issues.

> 
> Fix this by taking a single snapshot of the pte before using it in the
> BUG conditional.
> 
> Now, that commit is only partially to blame here but, people doing
> bisections will invariably land there, so this will help them find a fix
> for a real crash. And also, the previous behavior was unlikely to ever
> expose this bug--it was fragile, yet not actually broken.
> 
> So that's why I chose this commit for the Fixes tag, rather than the
> commit that created the original BUG() statement.
> 
> Fixes: c33c794828f2 ("mm: ptep_get() conversion")
> Cc: Adrian Hunter <adrian.hunter@xxxxxxxxx>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Andrey Konovalov <andreyknvl@xxxxxxxxx>
> Cc: Andrey Ryabinin <ryabinin.a.a@xxxxxxxxx>
> Cc: Christian Brauner <brauner@xxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> Cc: Daniel Vetter <daniel@xxxxxxxx>
> Cc: Dave Airlie <airlied@xxxxxxxxx>
> Cc: Dimitri Sivanich <dimitri.sivanich@xxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Ian Rogers <irogers@xxxxxxxxxx>
> Cc: Jason Gunthorpe <jgg@xxxxxxxx>
> Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Cc: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>
> Cc: Muchun Song <muchun.song@xxxxxxxxx>
> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> Cc: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
> Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
> Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
> Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
> Cc: SeongJae Park <sj@xxxxxxxxxx>
> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Cc: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
> Cc: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
> ---
>  mm/hugetlb.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index bce28cca73a1..73fbeb8f979f 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -7246,7 +7246,12 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma,
>  				pte = (pte_t *)pmd_alloc(mm, pud, addr);
>  		}
>  	}
> -	BUG_ON(pte && pte_present(ptep_get(pte)) && !pte_huge(ptep_get(pte)));
> +
> +	if (pte) {
> +		pte_t pteval = ptep_get(pte);

Given the PTL is not held here, I think this should technically be
ptep_get_lockless()?

Thanks,
Ryan


> +
> +		BUG_ON(pte_present(pteval) && !pte_huge(pteval));
> +	}
>  
>  	return pte;
>  }
> 
> base-commit: bf1fa6f15553df04f2bdd06190ccd5f388ab0777