Re: [PATCH] kdb: prevent possible null deref in kdb_msg_write

[Sumit Garg]

On Mon, 29 Jun 2020 at 21:07, Daniel Thompson
<daniel.thompson@xxxxxxxxxx> wrote:
>
> On Mon, Jun 29, 2020 at 04:50:20PM +0200, Petr Mladek wrote:
> > On Mon 2020-06-29 16:59:24, Cengiz Can wrote:
> > > `kdb_msg_write` operates on a global `struct kgdb_io *` called
> > > `dbg_io_ops`.
> > >
> > > Although it is initialized in `debug_core.c`, there's a null check in
> > > `kdb_msg_write` which implies that it can be null whenever we dereference
> > > it in this function call.
> > >
> > > Coverity scanner caught this as CID 1465042.
> > >
> > > I have modified the function to bail out if `dbg_io_ops` is not properly
> > > initialized.
> > >
> > > Signed-off-by: Cengiz Can <cengiz@xxxxxxxxxx>
> > > ---
> > >  kernel/debug/kdb/kdb_io.c | 15 ++++++++-------
> > >  1 file changed, 8 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
> > > index 683a799618ad..85e579812458 100644
> > > --- a/kernel/debug/kdb/kdb_io.c
> > > +++ b/kernel/debug/kdb/kdb_io.c
> > > @@ -549,14 +549,15 @@ static void kdb_msg_write(const char *msg, int msg_len)
> > >     if (msg_len == 0)
> > >             return;
> > >
> > > -   if (dbg_io_ops) {
> > > -           const char *cp = msg;
> > > -           int len = msg_len;
> > > +   if (!dbg_io_ops)
> > > +           return;
> >
> > This looks wrong. The message should be printed to the consoles
> > even when dbg_io_ops is NULL. I mean that the for_each_console(c)
> > cycle should always get called.
> >
> > Well, the code really looks racy. dbg_io_ops is set under
> > kgdb_registration_lock. IMHO, it should also get accessed under this lock.
> >
> > It seems that the race is possible. kdb_msg_write() is called from
> > vkdb_printf(). This function is serialized on more CPUs using
> > kdb_printf_cpu lock. But it is not serialized with
> > kgdb_register_io_module() and kgdb_unregister_io_module() calls.
>
> We can't take the lock from the trap handler itself since we cannot
> have spinlocks contended between regular threads and the debug trap
> (which could be an NMI).
>
> Instead, the call to kgdb_unregister_callbacks() at the beginning
> of kgdb_unregister_io_module() should render kdb_msg_write()
> unreachable prior to dbg_io_ops becoming NULL.
>
> As it happens I am starting to believe there is a race in this area but
> the race is between register/unregister calls rather than against the
> trap handler (if there were register/unregister races then the trap
> handler is be a potential victim of the race though).
>
>
> > But I might miss something. dbg_io_ops is dereferenced on many other
> > locations without any check.
>
> There is already a paranoid "just in case there are bugs" check in
> kgdb_io_ready() so in any case I think the check in kdb_msg_write() can
> simply be removed.
>
> As I said in my other post, if dbg_io_ops were ever NULL then the
> system is completely hosed anyway: we can never receive the keystroke
> needed to leave the debugger... and may not be able to tell anybody
> why.
>
>
> > >
> > > -           while (len--) {
> > > -                   dbg_io_ops->write_char(*cp);
> > > -                   cp++;
> > > -           }
> > > +   const char *cp = msg;
> > > +   int len = msg_len;
> > > +
> > > +   while (len--) {
> > > +           dbg_io_ops->write_char(*cp);
> > > +           cp++;
> > >     }
> > >
> > >     for_each_console(c) {
> >
> > You probably got confused by this new code:
> >
> >               if (c == dbg_io_ops->cons)
> >                       continue;
> >
> > It dereferences dbg_io_ops without NULL check. It should probably
> > get replaced by:
> >
> >               if (dbg_io_ops && c == dbg_io_ops->cons)
> >                       continue;
> >
> > Daniel, Sumit, could you please put some light on this?
>
> As above, I think the NULL check that confuses coverity can simply be
> removed.
>

+1

-Sumit

>
> Daniel.