Re: [PATCH] hugetlb: fix subpool release race

[Joshua Hahn <joshua.hahnjy@gmail.com>]

Hello Yichong, thank you for the patch!

> unlock_or_release_subpool() drops spool->lock before checking whether the
> subpool can be freed.  However, subpool_is_free() reads fields that are
> updated under spool->lock, including count, used_hpages and rsv_hpages.
> 
> Another thread can update those fields before the first thread evaluates
> subpool_is_free(), allowing both threads to observe the final freeable
> state and release the subpool.

I don't think that the race you are mentioning can be hit in practice (i.e.
there is no user-visible change for this) since two threads cannot actually
observe the freeable state at the same time.

subpool_is_free() does an early exit on spool->count so it will never reach
the inside if block and have two threads try to decrement the
spool->hstate and free the spool twice. I'm wondering if we need the Fixes
tag in this case.

I do think that this is not obvious though, so having the subpool_is_free
check inside the locked section makes sense to me too.

Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>

Have a great day!
Joshua

> Make the free decision while still holding spool->lock.  Keep the actual
> hugetlb_acct_memory() and kfree() calls after dropping the lock.

> Fixes: 90481622d757 ("hugepages: fix use after free bug in "quota" handling")
> Signed-off-by: Yichong Chen <chenyichong@uniontech.com>
> ---
>  mm/hugetlb.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index e319c6a00555..46ab702c0fc0 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -140,12 +140,14 @@ static inline bool subpool_is_free(struct hugepage_subpool *spool)
>  static inline void unlock_or_release_subpool(struct hugepage_subpool *spool,
>  						unsigned long irq_flags)
>  {
> -	spin_unlock_irqrestore(&spool->lock, irq_flags);
> +	bool free_subpool = subpool_is_free(spool);
>  
>  	/* If no pages are used, and no other handles to the subpool
>  	 * remain, give up any reservations based on minimum size and
>  	 * free the subpool */
> -	if (subpool_is_free(spool)) {
> +	spin_unlock_irqrestore(&spool->lock, irq_flags);
> +
> +	if (free_subpool) {
>  		if (spool->min_hpages != -1)
>  			hugetlb_acct_memory(spool->hstate,
>  						-spool->min_hpages);
> -- 
> 2.51.0