Re: [PATCH] firmware: efi: add NULL pointer checks in efivars api functions

[Arend van Spriel <arend.vanspriel@xxxxxxxxxxxx>]

On 11/13/2018 11:50 PM, Ard Biesheuvel wrote:
> Hi Arend,
>
> On Tue, 13 Nov 2018 at 12:51, Arend van Spriel
> <arend.vanspriel@xxxxxxxxxxxx> wrote:
> > Since commit ce2e6db554fa ("brcmfmac: Add support for getting nvram
> > contents from EFI variables") we have a device driver accessing the
> > efivars api (since next-20181107). Several functions in efivars api
> > assume __efivars is set, ie. will be accessed after efivars_register()
> > has been called. However, following NULL pointer access was reported
> > calling efivar_entry_size() from the brcmfmac device driver.
> >
> > [   14.177769] Unable to handle kernel NULL pointer dereference at
> >                 virtual address 00000008
> > [   14.197303] pgd = 60bfa5f1
> > [   14.211842] [00000008] *pgd=00000000
> > [   14.227373] Internal error: Oops: 5 [#1] SMP ARM
> > [   14.244244] Modules linked in: brcmfmac sha256_generic sha256_arm snd cfg80211 brcmutil soundcore snd_soc_tegra30_ahub tegra_wdt
> > [   14.269109] CPU: 1 PID: 114 Comm: kworker/1:2 Not tainted 4.20.0-rc1-next-20181107-gd881de3 #1
> > [   14.269114] Hardware name: NVIDIA Tegra SoC (Flattened Device Tree)
> > [   14.269154] Workqueue: events request_firmware_work_func
> > [   14.269177] PC is at efivar_entry_size+0x28/0x90
> > [   14.269362] LR is at brcmf_fw_complete_request+0x3f8/0x8d4 [brcmfmac]
> > [   14.269369] pc : [<c0c40718>]    lr : [<bf2a3ef4>]    psr: a00d0113
> > [   14.269374] sp : ede7fe28  ip : ee983410  fp : c1787f30
> > [   14.269378] r10: 00000000  r9 : 00000000  r8 : bf2b2258
> > [   14.269384] r7 : ee983000  r6 : c1604c48  r5 : ede7fe88  r4 : edf337c0
> > [   14.269389] r3 : 00000000  r2 : 00000000  r1 : ede7fe88  r0 : c17712c8
> > [   14.269398] Flags: NzCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment none
> > [   14.269404] Control: 10c5387d  Table: ad16804a  DAC: 00000051
> >
> > Disassembly showed that the local static variable __efivars is NULL. Likely
> > because efivars_register() is not called on this Tegra platform. So adding
> > a NULL pointer check in efivar_entry_size() and similar functions while at
> > it. In efivars_register() a couple of sanity checks have been added.
> >
> > Cc: Hans de Goede <hdegoede@xxxxxxxxxx>
> > Reported-by: Jon Hunter <jonathanh@xxxxxxxxxx>
> > Signed-off-by: Arend van Spriel <arend.vanspriel@xxxxxxxxxxxx>
> > ---
> >  drivers/firmware/efi/vars.c | 108 +++++++++++++++++++++++++++++++++++---------
> >  1 file changed, 87 insertions(+), 21 deletions(-)
> >
> > diff --git a/drivers/firmware/efi/vars.c b/drivers/firmware/efi/vars.c
> > index 9336ffd..5fbd8e7 100644
> > --- a/drivers/firmware/efi/vars.c
> > +++ b/drivers/firmware/efi/vars.c
> > @@ -318,7 +318,12 @@ struct variable_validate {
> >  static efi_status_t
> >  check_var_size(u32 attributes, unsigned long size)
> >  {
> > -       const struct efivar_operations *fops = __efivars->ops;
> > +       const struct efivar_operations *fops;
> > +
> > +       if (!__efivars)
> > +               return EFI_UNSUPPORTED;
> > +
> > +       fops = __efivars->ops;
> >
> >         if (!fops->query_variable_store)
> >                 return EFI_UNSUPPORTED;
> > @@ -329,7 +334,12 @@ struct variable_validate {
> >  static efi_status_t
> >  check_var_size_nonblocking(u32 attributes, unsigned long size)
> >  {
> > -       const struct efivar_operations *fops = __efivars->ops;
> > +       const struct efivar_operations *fops;
> > +
> > +       if (!__efivars)
> > +               return EFI_UNSUPPORTED;
> > +
> > +       fops = __efivars->ops;
> >
> >         if (!fops->query_variable_store)
> >                 return EFI_UNSUPPORTED;
> > @@ -429,13 +439,18 @@ static void dup_variable_bug(efi_char16_t *str16, efi_guid_t *vendor_guid,
> >  int efivar_init(int (*func)(efi_char16_t *, efi_guid_t, unsigned long, void *),
> >                 void *data, bool duplicates, struct list_head *head)
> >  {
> > -       const struct efivar_operations *ops = __efivars->ops;
> > +       const struct efivar_operations *ops;
> >         unsigned long variable_name_size = 1024;
> >         efi_char16_t *variable_name;
> >         efi_status_t status;
> >         efi_guid_t vendor_guid;
> >         int err = 0;
> >
> > +       if (!__efivars)
> > +               return -EFAULT;
> > +
> > +       ops = __efivars->ops;
> > +
> >         variable_name = kzalloc(variable_name_size, GFP_KERNEL);
> >         if (!variable_name) {
> >                 printk(KERN_ERR "efivars: Memory allocation failed.\n");
> > @@ -583,12 +598,14 @@ static void efivar_entry_list_del_unlock(struct efivar_entry *entry)
> >   */
> >  int __efivar_entry_delete(struct efivar_entry *entry)
> >  {
> > -       const struct efivar_operations *ops = __efivars->ops;
> >         efi_status_t status;
> >
> > -       status = ops->set_variable(entry->var.VariableName,
> > -                                  &entry->var.VendorGuid,
> > -                                  0, 0, NULL);
> > +       if (!__efivars)
> > +               return -EINVAL;
> > +
> > +       status = __efivars->ops->set_variable(entry->var.VariableName,
> > +                                             &entry->var.VendorGuid,
> > +                                             0, 0, NULL);
> >
> >         return efi_status_to_err(status);
> >  }
> > @@ -607,12 +624,17 @@ int __efivar_entry_delete(struct efivar_entry *entry)
> >   */
> >  int efivar_entry_delete(struct efivar_entry *entry)
> >  {
> > -       const struct efivar_operations *ops = __efivars->ops;
> > +       const struct efivar_operations *ops;
> >         efi_status_t status;
> >
> >         if (down_interruptible(&efivars_lock))
> >                 return -EINTR;
>
> Any reason in particular you put the check after the lock is taken? Is
> this to ensure that __efivars does not change under your feet?

That was my reasoning although I am not sure if that is likely. Is 
efivars_register() always called before any drivers are started?

Regards,
Arend