Re: [PATCH v2] efi: fix a race and a buffer overflow while reading efivars via sysfs

[joeyli]

Hi Vladis,

On Wed, Mar 04, 2020 at 04:49:36PM +0100, Vladis Dronov 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 using a local variable for a var's data buffer size so it
> does not get overwritten. Also add a sanity check to efivar_store_raw().
> 
> Reported-by: Bob Sanders <bob.sanders@xxxxxxx> and the LTP testsuite
> Signed-off-by: Vladis Dronov <vdronov@xxxxxxxxxx>
> ---
>  drivers/firmware/efi/efi-pstore.c |  2 +-
>  drivers/firmware/efi/efivars.c    | 32 ++++++++++++++++++++++---------
>  drivers/firmware/efi/vars.c       |  2 +-
>  3 files changed, 25 insertions(+), 11 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
[...snip]
> @@ -250,14 +262,16 @@ efivar_show_raw(struct efivar_entry *entry, char *buf)
>  {
>  	struct efi_variable *var = &entry->var;
>  	struct compat_efi_variable *compat;
> +	unsigned long datasize = sizeof(var->Data);
>  	size_t size;
> +	int ret;
>  
>  	if (!entry || !buf)
>  		return 0;
>  
> -	var->DataSize = 1024;
> -	if (efivar_entry_get(entry, &entry->var.Attributes,
> -			     &entry->var.DataSize, entry->var.Data))
> +	ret = efivar_entry_get(entry, &var->Attributes, &datasize, var->Data);
> +	var->DataSize = size;

The size is indeterminate here. I think that it should uses datasize?
	var->DataSize = datasize;

> +	if (ret)
>  		return -EIO;
>  
>  	if (in_compat_syscall()) {

Regards
Joey Lee