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

Lokesh Gidra
2020-08-04T23:34:52Z

On Tue, Aug 4, 2020 at 1:58 PM Eric Biggers <ebiggers@kernel.org> wrote:
>
> On Tue, Aug 04, 2020 at 01:49:30PM -0700, Lokesh Gidra wrote:
> > On Tue, Aug 4, 2020 at 1:45 PM Eric Biggers <ebiggers@kernel.org> wrote:
> > >
> > > On Tue, Aug 04, 2020 at 01:31:55PM -0700, Lokesh Gidra 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].
> > > >
> > > > 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>
> > >
> > > What branch does this patch apply to? Neither mainline nor linux-next works.
> > >
> > On James Morris' tree (secure_uffd_v5.9 branch).
> >
>
> For those of us not "in the know", that apparently means branch secure_uffd_v5.9
> of https://git.kernel.org/pub/scm/linux/kernel/git/jmorris/linux-security.git
>
> Perhaps it would make more sense to resend your original patch series with this
> fix folded in?
>
OK. I'll resend the whole patch series with the fixes soon.

> > 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;
>
> This introduces the opposite bug: now it's hardcoded to *not* use O_CLOEXEC,
> instead of using the flag the user passed in the flags argument to the syscall.

I get your point. I agree the flags passed in to the syscall should be used.
>
> - Eric