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References: <20200303085528.27658-1-vdronov@redhat.com>
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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Tue, 3 Mar 2020 10:15:41 +0100
Message-ID: <CAKv+Gu_3ZRRcoAcLTVVQe26q5x9KALmztaNQF=e=KqWaAwxtpA@mail.gmail.com>
Subject: Re: [PATCH] efi: fix a race and a buffer overflow while reading
 efivars via sysfs
To:     Vladis Dronov <vdronov@redhat.com>
Cc:     linux-efi <linux-efi@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Sender: linux-kernel-owner@vger.kernel.org
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On Tue, 3 Mar 2020 at 09:55, Vladis Dronov <vdronov@redhat.com> 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@hpe.com> and the LTP testsuite
> Signed-off-by: Vladis Dronov <vdronov@redhat.com>

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?

> ---
>  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
>