Re: [PATCH] efi: fix a race and a buffer overflow while reading efivars via sysfs

[Vladis Dronov]

Hello, Ard, all,

----- Original Message -----
> From: "Ard Biesheuvel" <ardb@xxxxxxxxxx>
> To: "Vladis Dronov" <vdronov@xxxxxxxxxx>
> Cc: "linux-efi" <linux-efi@xxxxxxxxxxxxxxx>, "Linux Kernel Mailing List" <linux-kernel@xxxxxxxxxxxxxxx>
> Sent: Tuesday, March 3, 2020 10:15:41 AM
> Subject: Re: [PATCH] efi: fix a race and a buffer overflow while reading efivars via sysfs
> 
> On Tue, 3 Mar 2020 at 09:55, Vladis Dronov <vdronov@xxxxxxxxxx> wrote:
> >
> > There is a race and a buffer overflow corrupting a kernel memory while
> > reading an efi variable with a size more than 1024 bytes via the older
> > sysfs method. This happens because accessing struct efi_variable in
> > efivar_{attr,size,data}_read() and friends is not protected from
> > a concurrent access leading to a kernel memory corruption and, at best,
> > to a crash. The race scenario is the following:
> >
> > CPU0:                                CPU1:
> > efivar_attr_read()
> >   var->DataSize = 1024;
> >   efivar_entry_get(... &var->DataSize)
> >     down_interruptible(&efivars_lock)
> >                                      efivar_attr_read() // same efi var
> >                                        var->DataSize = 1024;
> >                                        efivar_entry_get(... &var->DataSize)
> >                                          down_interruptible(&efivars_lock)
> >     virt_efi_get_variable()
> >     // returns EFI_BUFFER_TOO_SMALL but
> >     // var->DataSize is set to a real
> >     // var size more than 1024 bytes
> >     up(&efivars_lock)
> >                                          virt_efi_get_variable()
> >                                          // called with var->DataSize set
> >                                          // to a real var size, returns
> >                                          // successfully and overwrites
> >                                          // a 1024-bytes kernel buffer
> >                                          up(&efivars_lock)
> >
> > This can be reproduced by concurrent reading of an efi variable which size
> > is more than 1024 bytes:
> >
> > ts# for cpu in $(seq 0 $(nproc --ignore=1)); do ( taskset -c $cpu \
> > cat /sys/firmware/efi/vars/KEKDefault*/size & ) ; done
> >
> > Fix this by protecting struct efi_variable access by efivars_lock by using
> > efivar_entry_iter_begin()/efivar_entry_iter_end(). Brush up and unify
> > efivar_entry_[gs]et() and __efivar_entry_[gs]et() for this. This looks
> > simpler than introducing a separate lock for every struct efi_variable.
> >
> > Also fix the same race in efivar_store_raw() and efivar_show_raw(). The
> > call in efi_pstore_read_func() is protected like this already.
> >
> > Reported-by: Bob Sanders <bob.sanders@xxxxxxx> and the LTP testsuite
> > Signed-off-by: Vladis Dronov <vdronov@xxxxxxxxxx>
> 
> Wouldn't it be easier to pass a var_data_size stack variable into
> efivar_entry_get(), and only update the value in 'var' if it is <=
> 1024?
> 

I was thinking about this approach, but this way we still do not protect
var from a concurrent access. For example, efivar_data_read() can race
with itself:

// reading var size 5
efivar_data_read()
  efivar_entry_get()
                         // reading the same var
                         efivar_data_read()
                           var->DataSize = 1024;
                           efivar_entry_get()
  // var->DataSize is SUDDENLY 1024
  memcpy(buf, var->Data, var->DataSize);
  return var->DataSize;

Also efivar read functions still can race with the write function
efivar_store_raw(). Surely, the race window is much smaller but it is there.
I strongly believe we need to protect all data accesses here with a lock.

May be not in a way I suggest, may be by a per-var mutex, but I believe
this is overcomplication.

> > ---
> >  drivers/firmware/efi/efi-pstore.c |  2 +-
> >  drivers/firmware/efi/efivars.c    | 72 ++++++++++++++++++++++++-------
> >  drivers/firmware/efi/vars.c       | 47 ++++++++++++--------
> >  include/linux/efi.h               |  2 +
> >  4 files changed, 90 insertions(+), 33 deletions(-)
> >
> > diff --git a/drivers/firmware/efi/efi-pstore.c
> > b/drivers/firmware/efi/efi-pstore.c
> > index 9ea13e8d12ec..e4767a7ce973 100644
> > --- a/drivers/firmware/efi/efi-pstore.c
> > +++ b/drivers/firmware/efi/efi-pstore.c
> > @@ -161,7 +161,7 @@ static int efi_pstore_scan_sysfs_exit(struct
> > efivar_entry *pos,
> >   *
> >   * @record: pstore record to pass to callback
> >   *
> > - * You MUST call efivar_enter_iter_begin() before this function, and
> > + * You MUST call efivar_entry_iter_begin() before this function, and
> >   * efivar_entry_iter_end() afterwards.
> >   *
> >   */
> > diff --git a/drivers/firmware/efi/efivars.c
> > b/drivers/firmware/efi/efivars.c
> > index 7576450c8254..f415cf863ee0 100644
> > --- a/drivers/firmware/efi/efivars.c
> > +++ b/drivers/firmware/efi/efivars.c
> > @@ -88,9 +88,15 @@ efivar_attr_read(struct efivar_entry *entry, char *buf)
> >         if (!entry || !buf)
> >                 return -EINVAL;
> >
> > +       if (efivar_entry_iter_begin())
> > +               return -EINTR;
> > +
> >         var->DataSize = 1024;
> > -       if (efivar_entry_get(entry, &var->Attributes, &var->DataSize,
> > var->Data))
> > +       if (__efivar_entry_get(entry, &var->Attributes, &var->DataSize,
> > +                       var->Data)) {
> > +               efivar_entry_iter_end();
> >                 return -EIO;
> > +       }
> >
> >         if (var->Attributes & EFI_VARIABLE_NON_VOLATILE)
> >                 str += sprintf(str, "EFI_VARIABLE_NON_VOLATILE\n");
> > @@ -109,6 +115,8 @@ efivar_attr_read(struct efivar_entry *entry, char *buf)
> >                         "EFI_VARIABLE_TIME_BASED_AUTHENTICATED_WRITE_ACCESS\n");
> >         if (var->Attributes & EFI_VARIABLE_APPEND_WRITE)
> >                 str += sprintf(str, "EFI_VARIABLE_APPEND_WRITE\n");
> > +
> > +       efivar_entry_iter_end();
> >         return str - buf;
> >  }
> >
> > @@ -121,11 +129,19 @@ efivar_size_read(struct efivar_entry *entry, char
> > *buf)
> >         if (!entry || !buf)
> >                 return -EINVAL;
> >
> > +       if (efivar_entry_iter_begin())
> > +               return -EINTR;
> > +
> >         var->DataSize = 1024;
> > -       if (efivar_entry_get(entry, &var->Attributes, &var->DataSize,
> > var->Data))
> > +       if (__efivar_entry_get(entry, &var->Attributes, &var->DataSize,
> > +                       var->Data)) {
> > +               efivar_entry_iter_end();
> >                 return -EIO;
> > +       }
> >
> >         str += sprintf(str, "0x%lx\n", var->DataSize);
> > +
> > +       efivar_entry_iter_end();
> >         return str - buf;
> >  }
> >
> > @@ -137,11 +153,19 @@ efivar_data_read(struct efivar_entry *entry, char
> > *buf)
> >         if (!entry || !buf)
> >                 return -EINVAL;
> >
> > +       if (efivar_entry_iter_begin())
> > +               return -EINTR;
> > +
> >         var->DataSize = 1024;
> > -       if (efivar_entry_get(entry, &var->Attributes, &var->DataSize,
> > var->Data))
> > +       if (__efivar_entry_get(entry, &var->Attributes, &var->DataSize,
> > +                       var->Data)) {
> > +               efivar_entry_iter_end();
> >                 return -EIO;
> > +       }
> >
> >         memcpy(buf, var->Data, var->DataSize);
> > +
> > +       efivar_entry_iter_end();
> >         return var->DataSize;
> >  }
> >
> > @@ -197,13 +221,21 @@ efivar_store_raw(struct efivar_entry *entry, const
> > char *buf, size_t count)
> >         efi_guid_t vendor;
> >         u32 attributes;
> >         u8 *data;
> > -       int err;
> > +       int err = 0;
> > +
> > +       if (!entry || !buf)
> > +               return -EINVAL;
> > +
> > +       if (efivar_entry_iter_begin())
> > +               return -EINTR;
> >
> >         if (in_compat_syscall()) {
> >                 struct compat_efi_variable *compat;
> >
> > -               if (count != sizeof(*compat))
> > -                       return -EINVAL;
> > +               if (count != sizeof(*compat)) {
> > +                       err = -EINVAL;
> > +                       goto out;
> > +               }
> >
> >                 compat = (struct compat_efi_variable *)buf;
> >                 attributes = compat->Attributes;
> > @@ -214,12 +246,14 @@ efivar_store_raw(struct efivar_entry *entry, const
> > char *buf, size_t count)
> >
> >                 err = sanity_check(var, name, vendor, size, attributes,
> >                 data);
> >                 if (err)
> > -                       return err;
> > +                       goto out;
> >
> >                 copy_out_compat(&entry->var, compat);
> >         } else {
> > -               if (count != sizeof(struct efi_variable))
> > -                       return -EINVAL;
> > +               if (count != sizeof(struct efi_variable)) {
> > +                       err = -EINVAL;
> > +                       goto out;
> > +               }
> >
> >                 new_var = (struct efi_variable *)buf;
> >
> > @@ -231,18 +265,20 @@ efivar_store_raw(struct efivar_entry *entry, const
> > char *buf, size_t count)
> >
> >                 err = sanity_check(var, name, vendor, size, attributes,
> >                 data);
> >                 if (err)
> > -                       return err;
> > +                       goto out;
> >
> >                 memcpy(&entry->var, new_var, count);
> >         }
> >
> > -       err = efivar_entry_set(entry, attributes, size, data, NULL);
> > +       err = __efivar_entry_set(entry, attributes, size, data, NULL);
> >         if (err) {
> >                 printk(KERN_WARNING "efivars: set_variable() failed:
> >                 status=%d\n", err);
> > -               return -EIO;
> > +               err = -EIO;
> >         }
> >
> > -       return count;
> > +out:
> > +       efivar_entry_iter_end();
> > +       return err ?: count;
> >  }
> >
> >  static ssize_t
> > @@ -255,10 +291,15 @@ efivar_show_raw(struct efivar_entry *entry, char
> > *buf)
> >         if (!entry || !buf)
> >                 return 0;
> >
> > +       if (efivar_entry_iter_begin())
> > +               return -EINTR;
> > +
> >         var->DataSize = 1024;
> > -       if (efivar_entry_get(entry, &entry->var.Attributes,
> > -                            &entry->var.DataSize, entry->var.Data))
> > +       if (__efivar_entry_get(entry, &var->Attributes, &var->DataSize,
> > +                       var->Data)) {
> > +               efivar_entry_iter_end();
> >                 return -EIO;
> > +       }
> >
> >         if (in_compat_syscall()) {
> >                 compat = (struct compat_efi_variable *)buf;
> > @@ -276,6 +317,7 @@ efivar_show_raw(struct efivar_entry *entry, char *buf)
> >                 memcpy(buf, var, size);
> >         }
> >
> > +       efivar_entry_iter_end();
> >         return size;
> >  }
> >
> > diff --git a/drivers/firmware/efi/vars.c b/drivers/firmware/efi/vars.c
> > index 436d1776bc7b..4a47e20a7667 100644
> > --- a/drivers/firmware/efi/vars.c
> > +++ b/drivers/firmware/efi/vars.c
> > @@ -636,7 +636,7 @@ int efivar_entry_delete(struct efivar_entry *entry)
> >  EXPORT_SYMBOL_GPL(efivar_entry_delete);
> >
> >  /**
> > - * efivar_entry_set - call set_variable()
> > + * __efivar_entry_set - call set_variable()
> >   * @entry: entry containing the EFI variable to write
> >   * @attributes: variable attributes
> >   * @size: size of @data buffer
> > @@ -655,8 +655,12 @@ EXPORT_SYMBOL_GPL(efivar_entry_delete);
> >   * Returns 0 on success, -EINTR if we can't grab the semaphore,
> >   * -EEXIST if a lookup is performed and the entry already exists on
> >   * the list, or a converted EFI status code if set_variable() fails.
> > + *
> > + * The caller MUST call efivar_entry_iter_begin() and
> > + * efivar_entry_iter_end() before and after the invocation of this
> > + * function, respectively.
> >   */
> > -int efivar_entry_set(struct efivar_entry *entry, u32 attributes,
> > +int __efivar_entry_set(struct efivar_entry *entry, u32 attributes,
> >                      unsigned long size, void *data, struct list_head
> >                      *head)
> >  {
> >         const struct efivar_operations *ops;
> > @@ -664,9 +668,6 @@ int efivar_entry_set(struct efivar_entry *entry, u32
> > attributes,
> >         efi_char16_t *name = entry->var.VariableName;
> >         efi_guid_t vendor = entry->var.VendorGuid;
> >
> > -       if (down_interruptible(&efivars_lock))
> > -               return -EINTR;
> > -
> >         if (!__efivars) {
> >                 up(&efivars_lock);
> >                 return -EINVAL;
> > @@ -682,10 +683,28 @@ int efivar_entry_set(struct efivar_entry *entry, u32
> > attributes,
> >                 status = ops->set_variable(name, &vendor,
> >                                            attributes, size, data);
> >
> > -       up(&efivars_lock);
> > -
> >         return efi_status_to_err(status);
> > +}
> > +EXPORT_SYMBOL_GPL(__efivar_entry_set);
> >
> > +/**
> > + * efivar_entry_set - call set_variable()
> > + *
> > + * This function takes efivars_lock and calls __efivar_entry_set()
> > + */
> > +int efivar_entry_set(struct efivar_entry *entry, u32 attributes,
> > +                    unsigned long size, void *data, struct list_head
> > *head)
> > +{
> > +       int ret;
> > +
> > +       if (down_interruptible(&efivars_lock))
> > +               return -EINTR;
> > +
> > +       ret = __efivar_entry_set(entry, attributes, size, data, head);
> > +
> > +       up(&efivars_lock);
> > +
> > +       return ret;
> >  }
> >  EXPORT_SYMBOL_GPL(efivar_entry_set);
> >
> > @@ -914,22 +933,16 @@ EXPORT_SYMBOL_GPL(__efivar_entry_get);
> >  int efivar_entry_get(struct efivar_entry *entry, u32 *attributes,
> >                      unsigned long *size, void *data)
> >  {
> > -       efi_status_t status;
> > +       int ret;
> >
> >         if (down_interruptible(&efivars_lock))
> >                 return -EINTR;
> >
> > -       if (!__efivars) {
> > -               up(&efivars_lock);
> > -               return -EINVAL;
> > -       }
> > +       ret = __efivar_entry_get(entry, attributes, size, data);
> >
> > -       status = __efivars->ops->get_variable(entry->var.VariableName,
> > -                                             &entry->var.VendorGuid,
> > -                                             attributes, size, data);
> >         up(&efivars_lock);
> >
> > -       return efi_status_to_err(status);
> > +       return ret;
> >  }
> >  EXPORT_SYMBOL_GPL(efivar_entry_get);
> >
> > @@ -1071,7 +1084,7 @@ EXPORT_SYMBOL_GPL(efivar_entry_iter_end);
> >   * entry on the list. It is safe for @func to remove entries in the
> >   * list via efivar_entry_delete().
> >   *
> > - * You MUST call efivar_enter_iter_begin() before this function, and
> > + * You MUST call efivar_entry_iter_begin() before this function, and
> >   * efivar_entry_iter_end() afterwards.
> >   *
> >   * It is possible to begin iteration from an arbitrary entry within
> > diff --git a/include/linux/efi.h b/include/linux/efi.h
> > index 7efd7072cca5..5c3db088d375 100644
> > --- a/include/linux/efi.h
> > +++ b/include/linux/efi.h
> > @@ -1414,6 +1414,8 @@ int __efivar_entry_get(struct efivar_entry *entry,
> > u32 *attributes,
> >                        unsigned long *size, void *data);
> >  int efivar_entry_get(struct efivar_entry *entry, u32 *attributes,
> >                      unsigned long *size, void *data);
> > +int __efivar_entry_set(struct efivar_entry *entry, u32 attributes,
> > +                    unsigned long size, void *data, struct list_head
> > *head);
> >  int efivar_entry_set(struct efivar_entry *entry, u32 attributes,
> >                      unsigned long size, void *data, struct list_head
> >                      *head);
> >  int efivar_entry_set_get_size(struct efivar_entry *entry, u32 attributes,
> > --
> > 2.20.1