Re: [PATCH 4.19, 4.14, 4.9, 4.4 1/2] efi: Fix a race and a buffer overflow while reading efivars via sysfs

[Greg KH <greg@kroah.com>]

On Mon, Mar 16, 2020 at 02:19:37PM +0100, Vladis Dronov wrote:
> commit 286d3250c9d6437340203fb64938bea344729a0e upstream.
> 
> 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.
> 
> Fixes: e14ab23dde12b80d ("efivars: efivar_entry API")
> Reported-by: Bob Sanders <bob.sanders@hpe.com> and the LTP testsuite
> Signed-off-by: Vladis Dronov <vdronov@redhat.com>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> Cc: <stable@vger.kernel.org>
> Link: https://lore.kernel.org/r/20200305084041.24053-2-vdronov@redhat.com
> Link: https://lore.kernel.org/r/20200308080859.21568-24-ardb@kernel.org
> ---
>  drivers/firmware/efi/efivars.c | 29 ++++++++++++++++++++---------
>  1 file changed, 20 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/firmware/efi/efivars.c b/drivers/firmware/efi/efivars.c
> index 3e626fd9bd4e..c8688490f148 100644
> --- a/drivers/firmware/efi/efivars.c
> +++ b/drivers/firmware/efi/efivars.c
> @@ -139,13 +139,16 @@ static ssize_t
>  efivar_attr_read(struct efivar_entry *entry, char *buf)
>  {
>  	struct efi_variable *var = &entry->var;
> +	unsigned long size = sizeof(var->Data);
>  	char *str = buf;
> +	int ret;
>  
>  	if (!entry || !buf)
>  		return -EINVAL;
>  
> -	var->DataSize = 1024;
> -	if (efivar_entry_get(entry, &var->Attributes, &var->DataSize, var->Data))
> +	ret = efivar_entry_get(entry, &var->Attributes, &size, var->Data);
> +	var->DataSize = size;
> +	if (ret)
>  		return -EIO;
>  
>  	if (var->Attributes & EFI_VARIABLE_NON_VOLATILE)
> @@ -172,13 +175,16 @@ static ssize_t
>  efivar_size_read(struct efivar_entry *entry, char *buf)
>  {
>  	struct efi_variable *var = &entry->var;
> +	unsigned long size = sizeof(var->Data);
>  	char *str = buf;
> +	int ret;
>  
>  	if (!entry || !buf)
>  		return -EINVAL;
>  
> -	var->DataSize = 1024;
> -	if (efivar_entry_get(entry, &var->Attributes, &var->DataSize, var->Data))
> +	ret = efivar_entry_get(entry, &var->Attributes, &size, var->Data);
> +	var->DataSize = size;
> +	if (ret)
>  		return -EIO;
>  
>  	str += sprintf(str, "0x%lx\n", var->DataSize);
> @@ -189,12 +195,15 @@ static ssize_t
>  efivar_data_read(struct efivar_entry *entry, char *buf)
>  {
>  	struct efi_variable *var = &entry->var;
> +	unsigned long size = sizeof(var->Data);
> +	int ret;
>  
>  	if (!entry || !buf)
>  		return -EINVAL;
>  
> -	var->DataSize = 1024;
> -	if (efivar_entry_get(entry, &var->Attributes, &var->DataSize, var->Data))
> +	ret = efivar_entry_get(entry, &var->Attributes, &size, var->Data);
> +	var->DataSize = size;
> +	if (ret)
>  		return -EIO;
>  
>  	memcpy(buf, var->Data, var->DataSize);
> @@ -314,14 +323,16 @@ 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 = datasize;
> +	if (ret)
>  		return -EIO;
>  
>  	if (is_compat()) {
> -- 
> 2.20.1
> 

This is already in all of my stable trees, did it need to be somehow
backported differently to 4.19 and older?

thanks,

greg k-h