Re: [PATCH] userfaultfd: release page in error path to avoid BUG_ON

[Peter Xu <peterx@redhat.com>]

On Wed, Apr 28, 2021 at 11:01:09AM -0700, Axel Rasmussen wrote:
> Consider the following sequence of events (described from the point of
> view of the commit that introduced the bug - see "Fixes:" below):
> 
> 1. Userspace issues a UFFD ioctl, which ends up calling into
>    shmem_mcopy_atomic_pte(). We successfully account the blocks, we
>    shmem_alloc_page(), but then the copy_from_user() fails. We return
>    -EFAULT. We don't release the page we allocated.
> 2. Our caller detects this error code, tries the copy_from_user() after
>    dropping the mmap_sem, and retries, calling back into
>    shmem_mcopy_atomic_pte().
> 3. Meanwhile, let's say another process filled up the tmpfs being used.
> 4. So shmem_mcopy_atomic_pte() fails to account blocks this time, and
>    immediately returns - without releasing the page. This triggers a
>    BUG_ON in our caller, which asserts that the page should always be
>    consumed, unless -EFAULT is returned.
> 
> (Later on in the commit history, -EFAULT became -ENOENT, mmap_sem became
> mmap_lock, and shmem_inode_acct_block() was added.)

I suggest you do s/EFAULT/ENOENT/ directly in above.

> 
> A malicious user (even an unprivileged one) could trigger this
> intentionally without too much trouble.
> 
> To fix this, detect if we have a "dangling" page when accounting fails,
> and if so, release it before returning.
> 
> Fixes: cb658a453b93 ("userfaultfd: shmem: avoid leaking blocks and used blocks in UFFDIO_COPY")
> Reported-by: Hugh Dickins <hughd@google.com>
> Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
> ---
>  mm/shmem.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 26c76b13ad23..46766c9d7151 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2375,8 +2375,19 @@ static int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
>  	pgoff_t offset, max_off;
>  
>  	ret = -ENOMEM;
> -	if (!shmem_inode_acct_block(inode, 1))
> +	if (!shmem_inode_acct_block(inode, 1)) {
> +		/*
> +		 * We may have got a page, returned -ENOENT triggering a retry,
> +		 * and now we find ourselves with -ENOMEM. Release the page, to
> +		 * avoid a BUG_ON in our caller.
> +		 */
> +		if (unlikely(*pagep)) {
> +			unlock_page(*pagep);

Not necessary?

> +			put_page(*pagep);
> +			*pagep = NULL;
> +		}
>  		goto out;

All "goto out" in this functions looks weird as it returns directly... so if
you're touching this after all, I suggest we do "return -ENOMEM" directly and
drop the "ret = -ENOMEM".

Thanks,

> +	}
>  
>  	if (!*pagep) {
>  		page = shmem_alloc_page(gfp, info, pgoff);
> -- 
> 2.31.1.498.g6c1eba8ee3d-goog
> 

-- 
Peter Xu