Re: [PATCH v3] misc: fastrpc: Fix double free and races in fastrpc_req_mmap() error path

[Ekansh Gupta <ekansh.gupta@oss.qualcomm.com>]

On 30-09-2026 12:43, Yao Yiqi wrote:
> fastrpc_req_mmap() links the newly allocated buffer into fl->mmaps before
> copying the response back to userspace. If copy_to_user() fails, the
> err_assign path calls fastrpc_req_munmap_impl() to unmap the buffer from
> the DSP and, on success, to free it.
> 
> Since commit 6102ceb4eab8 ("misc: fastrpc: Remove buffer from list prior
> to unmap operation"), fastrpc_req_munmap_impl() no longer removes the
> buffer from fl->mmaps, as that is now the responsibility of the caller.
> fastrpc_req_mmap() was not updated accordingly, so the buffer is freed
> while still linked in the list. The fl->mmaps cleanup in
> fastrpc_user_free() then calls list_del() and fastrpc_buf_free() on the
> already freed buffer, resulting in a use-after-free and a double free.
> 
> Publishing the buffer before copy_to_user() returns is also racy:
> another thread of the same process can read the DSP address from the
> partially written user buffer (or via userfaultfd) and issue
> FASTRPC_IOCTL_MUNMAP, which unlinks and frees the buffer while
> fastrpc_req_mmap() is still using it.
> 
> Fix this by adding the buffer to fl->mmaps only after copy_to_user() has
> succeeded, so the buffer is not visible to a concurrent unmap while the
> response is still being copied. If unmapping the buffer fails on the
> error path, fastrpc_buf_free() is not called, so re-add the buffer to
> fl->mmaps to keep it tracked and let fastrpc_user_free() reclaim it on
> release instead of leaking the DMA allocation.
> 
> Fixes: 6102ceb4eab8 ("misc: fastrpc: Remove buffer from list prior to unmap operation")
> Cc: stable@vger.kernel.org
> Signed-off-by: Yao Yiqi <yaoyiqi3@huawei.com>

Reviewed-by: Ekansh Gupta <ekansh.gupta@oss.qualcomm.com>

> ---
> Changes in v3:
>  - Re-add the buffer to fl->mmaps if unmapping it on the error path
>    fails, since fastrpc_buf_free() is not called in that case and the
>    DMA allocation would otherwise be leaked.
> 
> Changes in v2:
>  - Link the buffer into fl->mmaps only after copy_to_user() succeeds.
>    Besides fixing the double free, this also closes the concurrent
>    FASTRPC_IOCTL_MUNMAP race, where a partially written user buffer let
>    another thread unmap and free the buffer while fastrpc_req_mmap() was
>    still using it.
> 
>  drivers/misc/fastrpc.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
> index d4fac2caca86..917a5dfe3774 100644
> --- a/drivers/misc/fastrpc.c
> +++ b/drivers/misc/fastrpc.c
> @@ -2153,22 +2153,26 @@ static int fastrpc_req_mmap(struct fastrpc_user *fl, char __user *argp)
>  		}
>  	}
>  
> -	spin_lock(&fl->lock);
> -	list_add_tail(&buf->node, &fl->mmaps);
> -	spin_unlock(&fl->lock);
> -
>  	if (copy_to_user((void __user *)argp, &req, sizeof(req))) {
>  		err = -EFAULT;
>  		goto err_assign;
>  	}
>  
> +	spin_lock(&fl->lock);
> +	list_add_tail(&buf->node, &fl->mmaps);
> +	spin_unlock(&fl->lock);
> +
>  	dev_dbg(dev, "mmap\t\tpt 0x%09lx OK [len 0x%08llx]\n",
>  		buf->raddr, buf->size);
>  
>  	return 0;
>  
>  err_assign:
> -	fastrpc_req_munmap_impl(fl, buf);
> +	if (fastrpc_req_munmap_impl(fl, buf)) {
> +		spin_lock(&fl->lock);
> +		list_add_tail(&buf->node, &fl->mmaps);
> +		spin_unlock(&fl->lock);
> +	}
>  
>  	return err;
>  }