Re: [PATCH] Userfaultfd: Avoid double free of userfault_ctx and remove O_CLOEXEC

Aleksa Sarai
2020-08-05T03:48:25Z

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On 2020-08-04, Lokesh Gidra <lokeshgidra@google.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@zeniv.linux.org.uk>
> Reported-by: syzbot+75867c44841cb6373570@syzkaller.appspotmail.com
> Signed-off-by: Lokesh Gidra <lokeshgidra@google.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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