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

[James Houghton]

On Thu, Jun 29, 2023 at 9:32 PM John Hubbard <jhubbard@xxxxxxxxxx> 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.
>
> 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")

Hi John,

Good catch, and thanks for the detailed explanation. It looks like
riscv and powerpc have equivalent problems in their huge_pte_alloc
implementations, perhaps it's worth taking a look at those. (riscv
looks like it has precisely the same problem except it's a WARN, but
powerpc looks more interesting.)

Either way,

Acked-by: James Houghton <jthoughton@xxxxxxxxxx>