Re: [PATCH v3] mm/shuffle.c: Fix races in add_to_free_area_random()

[Alexander Duyck <alexander.duyck@gmail.com>]

On Wed, Mar 18, 2020 at 1:39 PM George Spelvin <lkml@sdf.org> wrote:
>
> The old code had separate "rand" and "rand_count" variables,
> which could get out of sync with bad results.
>
> In the worst case, two threads would see rand_count = 1 and
> both decrement it, resulting in rand_count = 255 and rand being
> filled with zeros for the next 255 calls.
>
> Instead, pack them both into a single, atomically updateable,
> variable.  This makes it a lot easier to reason about race
> conditions.  They are still there - the code deliberately eschews
> locking - but basically harmless on the rare occasions that
> they happen.
>
> Second, use READ_ONCE and WRITE_ONCE.  Without them, we are deep
> in the land of nasal demons.  The compiler would be free to spill
> temporaries to the static variables in arbitrarily perverse ways
> and create hard-to-find bugs.
>
> (Alternatively, we could declare the static variable "volatile",
> one of the few places in the Linux kernel that would be correct,
> but it would probably annoy Linus.)
>
> Third, use long rather than u64.  This not only keeps the state
> atomically updateable, it also speeds up the fast path on 32-bit
> machines.  Saving at least three instructions on the fast path (one
> load, one add-with-carry, and one store) is worth a second call
> to get_random_u*() per 64 bits.  The fast path of get_random_u*
> is less than the 3*64 = 192 instructions saved, and the slow path
> happens every 64 bytes so isn't affected by the change.
>
> Fourth, use left-shift rather than right.  Testing the sign bit
> produces slightly smaller/faster code than testing the lsbit.
>
> I've tried shifting both ways, and copying the desired bit to
> a boolean before shifting rather than keeping separate full-width
> r and rshift variables, but both produce larger code:
>
>                 x86_64  i386
> This code        94      95
> Explicit bool   103      99
> Lsbits           99     101
> Both             96     100
>
> In a perfect world, the x86-64 object code would be tiny,
> and dominated by the unavoidable unpredictable branch, but
> this function doesn't warrant arch-specific optimization.
>
> add_to_free_area_random:
>         shlq    rand(%rip)
>         jz      3f
> 1:
>         jnc     2f
>         jmp     add_to_free_area        # tail call
> 2:
>         jmp     add_to_free_area_tail
> 3:
>         pushq   %rdx
>         pushq   %rsi
>         pushq   %rdi
>         call    get_random_u64
>         popq    %rdi
>         popq    %rsi
>         popq    %rdx
>         stc
>         adcq    %rax,%rax       # not lea because we need carry out
>         movq    %rax, rand(%rip)
>         jmp     1b
>
> Signed-off-by: George Spelvin <lkml@sdf.org>
> Acked-by: Kees Cook <keescook@chromium.org>
> Acked-by: Dan Williams <dan.j.williams@intel.com>
> Cc: Alexander Duyck <alexander.duyck@gmail.com>
> Cc: Randy Dunlap <rdunlap@infradead.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: linux-mm@kvack.org

What kernel is this based on? You might want to rebase on the latest
linux-next as it occurs to me that this function was renamed to
shuffle_pick_tail as I had incorporated a few bits of it into the
logic for placing buddy pages and reported pages on the tail of the
list.

> ---
> v2: Rewrote commit message to explain existing races better.
>     Made local variables unsigned to avoid (technically undefined)
>     signed overflow.
> v3: Typos fixed, Acked-by, expanded commit message.
>
>  mm/shuffle.c | 26 ++++++++++++++++----------
>  1 file changed, 16 insertions(+), 10 deletions(-)
>
> diff --git a/mm/shuffle.c b/mm/shuffle.c
> index e0ed247f8d90..16c5fddc292f 100644
> --- a/mm/shuffle.c
> +++ b/mm/shuffle.c
> @@ -186,22 +186,28 @@ void __meminit __shuffle_free_memory(pg_data_t *pgdat)
>  void add_to_free_area_random(struct page *page, struct free_area *area,
>                 int migratetype)
>  {
> -       static u64 rand;
> -       static u8 rand_bits;
> +       static unsigned long rand;      /* buffered random bits */
> +       unsigned long r = READ_ONCE(rand), rshift = r << 1;
>
>         /*
> -        * The lack of locking is deliberate. If 2 threads race to
> -        * update the rand state it just adds to the entropy.
> +        * rand holds 0..BITS_PER_LONG-1 random msbits, followed by a
> +        * 1 bit, then zero-padding in the lsbits.  This allows us to
> +        * maintain the pre-generated bits and the count of bits in a
> +        * single, atomically updatable, variable.
> +        *
> +        * The lack of locking is deliberate. If two threads race to
> +        * update the rand state it just adds to the entropy.  The
> +        * worst that can happen is a random bit is used twice, or
> +        * get_random_long is called redundantly.
>          */
> -       if (rand_bits == 0) {
> -               rand_bits = 64;
> -               rand = get_random_u64();
> +       if (unlikely(rshift == 0)) {
> +               r = get_random_long();
> +               rshift = r << 1 | 1;
>         }
> +       WRITE_ONCE(rand, rshift);
>
> -       if (rand & 1)
> +       if ((long)r < 0)
>                 add_to_free_area(page, area, migratetype);
>         else
>                 add_to_free_area_tail(page, area, migratetype);
> -       rand_bits--;
> -       rand >>= 1;
>  }
> --
> 2.26.0.rc2