Re: CVE-2023-52605: ACPI: extlog: fix NULL pointer dereference check

[Lee Jones]

On Mon, 11 Mar 2024, Prarit Bhargava wrote:

> On 3/10/24 04:10, Vegard Nossum wrote:
> > 
> > (Added author/maintainer to Cc)
> > 
> > On 06/03/2024 07:46, Greg Kroah-Hartman wrote:
> > > Description
> > > ===========
> > > 
> > > In the Linux kernel, the following vulnerability has been resolved:
> > > 
> > > ACPI: extlog: fix NULL pointer dereference check
> > > 
> > > The gcc plugin -fanalyzer [1] tries to detect various
> > > patterns of incorrect behaviour.  The tool reports:
> > > 
> > > drivers/acpi/acpi_extlog.c: In function ‘extlog_exit’:
> > > drivers/acpi/acpi_extlog.c:307:12: warning: check of
> > > ‘extlog_l1_addr’ for NULL after already dereferencing it
> > > [-Wanalyzer-deref-before-check]
> > >      |
> > >      |  306 |         ((struct extlog_l1_head
> > > *)extlog_l1_addr)->flags &= ~FLAG_OS_OPTIN;
> > >      |      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~
> > >      |      |                                                  |
> > >      |      |                                                  (1)
> > > pointer ‘extlog_l1_addr’ is dereferenced here
> > >      |  307 |         if (extlog_l1_addr)
> > >      |      |            ~
> > >      |      |            |
> > >      |      |            (2) pointer ‘extlog_l1_addr’ is checked for
> > > NULL here but it was already dereferenced at (1)
> > >      |
> > > 
> > > Fix the NULL pointer dereference check in extlog_exit().
> > > 
> > > The Linux kernel CVE team has assigned CVE-2023-52605 to this issue.
> > 
> > This code is in an __exit function:
> > 
> > diff --git a/drivers/acpi/acpi_extlog.c b/drivers/acpi/acpi_extlog.c
> > index e120a96e1eaee..193147769146e 100644
> > --- a/drivers/acpi/acpi_extlog.c
> > +++ b/drivers/acpi/acpi_extlog.c
> > @@ -303,9 +303,10 @@ err:
> >   static void __exit extlog_exit(void)
> >   {
> >       mce_unregister_decode_chain(&extlog_mce_dec);
> > -    ((struct extlog_l1_head *)extlog_l1_addr)->flags &= ~FLAG_OS_OPTIN;
> > -    if (extlog_l1_addr)
> > +    if (extlog_l1_addr) {
> > +        ((struct extlog_l1_head *)extlog_l1_addr)->flags &=
> > ~FLAG_OS_OPTIN;
> >           acpi_os_unmap_iomem(extlog_l1_addr, l1_size);
> > +    }
> >       if (elog_addr)
> >           acpi_os_unmap_iomem(elog_addr, elog_size);
> >       release_mem_region(elog_base, elog_size);
> > 
> > This can only run when you unload a module, which is a privileged
> > operation (restricted to CAP_SYS_MODULE).
> > 
> > Moreover, extlog_l1_addr is only ever assigned in the corresponding
> > module init function, and it looks like it will never be NULL if the
> > module was loaded successfully, at least on a recent mainline kernel.
> > 
> > Since the module exit won't be called unless module init succeeded, I
> > don't see a way to trigger this bug. Is this a vulnerability?
> > 
> 
> This is certainly not a CVE.
> 
> > It might be better to just delete the NULL check altogether.
> > 
> > As usual, I could be wrong...
> > 
> 
> When I made this code change I thought the same thing: Perhaps it's better
> to remove the NULL check given the status of the code.  I assumed that the
> check was there as a failsafe on unload.

If Rafael agrees with you both, I'd be happy to revoke its CVE status.

-- 
Lee Jones [李琼斯]