linux-kernel - Re: [PATCH] Userfaultfd: Avoid double free of userfault_ctx and remove O

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Date:   Wed, 5 Aug 2020 13:47:58 +1000
From:   Aleksa Sarai <cyphar@...har.com>
To:     Lokesh Gidra <lokeshgidra@...gle.com>
Cc:     viro@...iv.linux.org.uk, stephen.smalley.work@...il.com,
        casey@...aufler-ca.com, jmorris@...ei.org, kaleshsingh@...gle.com,
        dancol@...col.org, surenb@...gle.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        nnk@...gle.com, jeffv@...gle.com, calin@...gle.com,
        kernel-team@...roid.com, yanfei.xu@...driver.com,
        syzbot+75867c44841cb6373570@...kaller.appspotmail.com
Subject: Re: [PATCH] Userfaultfd: Avoid double free of userfault_ctx and
 remove O_CLOEXEC

On 2020-08-04, Lokesh Gidra <lokeshgidra@...gle.com> wrote:
> when get_unused_fd_flags returns error, ctx will be freed by
> userfaultfd's release function, which is indirectly called by fput().
> Also, if anon_inode_getfile_secure() returns an error, then
> userfaultfd_ctx_put() is called, which calls mmdrop() and frees ctx.
> 
> Also, the O_CLOEXEC was inadvertently added to the call to
> get_unused_fd_flags() [1].

I disagree that it is "wrong" to do O_CLOEXEC-by-default (after all,
it's trivial to disable O_CLOEXEC, but it's non-trivial to enable it on
an existing file descriptor because it's possible for another thread to
exec() before you set the flag). Several new syscalls and fd-returning
facilities are O_CLOEXEC-by-default now (the most obvious being pidfds
and seccomp notifier fds).

At the very least there should be a new flag added that sets O_CLOEXEC.

> Adding Al Viro's suggested-by, based on [2].
> 
> [1] https://lore.kernel.org/lkml/1f69c0ab-5791-974f-8bc0-3997ab1d61ea@dancol.org/
> [2] https://lore.kernel.org/lkml/20200719165746.GJ2786714@ZenIV.linux.org.uk/
> 
> Fixes: d08ac70b1e0d (Wire UFFD up to SELinux)
> Suggested-by: Al Viro <viro@...iv.linux.org.uk>
> Reported-by: syzbot+75867c44841cb6373570@...kaller.appspotmail.com
> Signed-off-by: Lokesh Gidra <lokeshgidra@...gle.com>
> ---
>  fs/userfaultfd.c | 14 ++++----------
>  1 file changed, 4 insertions(+), 10 deletions(-)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index ae859161908f..e15eb8fdc083 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -2042,24 +2042,18 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
>  		O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS),
>  		NULL);
>  	if (IS_ERR(file)) {
> -		fd = PTR_ERR(file);
> -		goto out;
> +		userfaultfd_ctx_put(ctx);
> +		return PTR_ERR(file);
>  	}
>  
> -	fd = get_unused_fd_flags(O_RDONLY | O_CLOEXEC);
> +	fd = get_unused_fd_flags(O_RDONLY);
>  	if (fd < 0) {
>  		fput(file);
> -		goto out;
> +		return fd;
>  	}
>  
>  	ctx->owner = file_inode(file);
>  	fd_install(fd, file);
> -
> -out:
> -	if (fd < 0) {
> -		mmdrop(ctx->mm);
> -		kmem_cache_free(userfaultfd_ctx_cachep, ctx);
> -	}
>  	return fd;
>  }
>  
> -- 
> 2.28.0.163.g6104cc2f0b6-goog
> 

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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