linux-kernel - Re: [PATCH] staging: android: ashmem: Remove deadlock

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Date:   Wed, 7 Mar 2018 14:48:29 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Paul Lawrence <paullawrence@...gle.com>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Ben Hutchings <ben@...adent.org.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>, devel@...verdev.osuosl.org
Subject: Re: [PATCH] staging: android: ashmem: Remove deadlock

On Wed, Mar 07, 2018 at 01:40:30PM -0800, Paul Lawrence wrote:
> Regression introduced in commit ce8a3a9e76d0193e2e8d74a06d275b3c324ca652
> ("staging: android: ashmem: Fix a race condition in pin ioctls")
> causing deadlock.
> 
> No need to hold ashmem_mutex while copying from user
> 
> Stacks are:
> 
> ashmem_mmap+0x53/0x400 drivers/staging/android/ashmem.c:379
> mmap_region+0x7dd/0xfd0 mm/mmap.c:1694
> do_mmap+0x57b/0xbe0 mm/mmap.c:1473
> 
> and
> 
> lock_acquire+0x12e/0x410 kernel/locking/lockdep.c:3756
> __might_fault+0x14a/0x1d0 mm/memory.c:4014
> copy_from_user arch/x86/include/asm/uaccess.h:705 [inline]
> ashmem_pin_unpin drivers/staging/android/ashmem.c:719 [inline]
> 
> Signed-off-by: Paul Lawrence <paullawrence@...gle.com>
> Cc: <stable@...r.kernel.org> # 4.9.x
> Cc: <stable@...r.kernel.org> # 4.4.x
> Cc: <stable@...r.kernel.org> # 3.18.x: ce8a3a9e76d01
> Cc: <stable@...r.kernel.org> # 3.18.x
> Cc: Ben Hutchings <ben@...adent.org.uk>
> ---
>  drivers/staging/android/ashmem.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c
> index 6dbba5aff191..8c55706c2cfa 100644
> --- a/drivers/staging/android/ashmem.c
> +++ b/drivers/staging/android/ashmem.c
> @@ -702,16 +702,14 @@ static int ashmem_pin_unpin(struct ashmem_area *asma, unsigned long cmd,
>  	size_t pgstart, pgend;
>  	int ret = -EINVAL;
>  
> +	if (unlikely(copy_from_user(&pin, p, sizeof(pin))))
> +		return -EFAULT;
> +
>  	mutex_lock(&ashmem_mutex);
>  
>  	if (unlikely(!asma->file))
>  		goto out_unlock;
>  
> -	if (unlikely(copy_from_user(&pin, p, sizeof(pin)))) {
> -		ret = -EFAULT;
> -		goto out_unlock;
> -	}
> -
>  	/* per custom, you can pass zero for len to mean "everything onward" */
>  	if (!pin.len)
>  		pin.len = PAGE_ALIGN(asma->size) - pin.offset;
> -- 
> 2.16.2.395.g2e18187dfd-goog
>

Hey Paul,

Looks like this same patch is already in Greg's tree:
https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging.git/commit/?id=740a5759bf222332fbb5eda42f89aa25ba38f9b2

Cheers!
Nathan Chancellor