Re: [PATCH v20 21/25] x86/cet/shstk: Handle signals for shadow stack

["Yu, Yu-cheng" <yu-cheng.yu@intel.com>]

On 2/10/2021 11:58 AM, Kees Cook wrote:
> On Wed, Feb 10, 2021 at 09:56:59AM -0800, Yu-cheng Yu wrote:
>> To deliver a signal, create a shadow stack restore token and put the token
>> and the signal restorer address on the shadow stack.  For sigreturn, verify
>> the token and restore from it the shadow stack pointer.
>>
>> A shadow stack restore token marks a restore point of the shadow stack.
>> The token is distinctively different from any shadow stack address.
> 
> How is it different? It seems like it just has the last 2 bits
> masked/set?
> 

For example, for 64-bit apps,

A shadow stack pointer value (*ssp) has to be in some code area, but for 
a token, (*ptr_of_token) = (ptr_of_token + 8), which has to be within 
the same shadow stack area.  In cet_verify_rstor_token(), this is checked.

>> In sigreturn, restoring from a token ensures the target address is the
>> location pointed by the token.
> 
> As in, a token (real stack address with 2-bit mask) is checked against
> the real stack address? I don't see a comparison -- it only checks that
> it is < TASK_SIZE.
> 
> How does cet_restore_signal() figure into this? (As in, the MSR writes?)
> 

The kernel takes the restore address from the token.  It will not 
mistakenly take a wrong address from the shadow stack.  I will put this 
in my commit logs.

[...]

>> Introduce WRUSS, which is a kernel-mode instruction but writes directly to
>> user shadow stack.  It is used to construct the user signal stack as
>> described above.
>>
>> Currently there is no systematic facility for extending a signal context.
>> Introduce a signal context extension 'struct sc_ext', which is used to save
>> shadow stack restore token address and WAIT_ENDBR status.  WAIT_ENDBR will
>> be introduced later in the Indirect Branch Tracking (IBT) series, but add
>> that into sc_ext now to keep the struct stable in case the IBT series is
>> applied later.
>>
>> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>

[...]

>> diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
>> index d25a03215984..08e43d9b5176 100644
>> --- a/arch/x86/kernel/cet.c
>> +++ b/arch/x86/kernel/cet.c
>> @@ -19,6 +19,8 @@
>>   #include <asm/fpu/xstate.h>
>>   #include <asm/fpu/types.h>
>>   #include <asm/cet.h>
>> +#include <asm/special_insns.h>
>> +#include <uapi/asm/sigcontext.h>
>>   
>>   static void start_update_msrs(void)
>>   {
>> @@ -72,6 +74,80 @@ static unsigned long alloc_shstk(unsigned long size, int flags)
>>   	return addr;
>>   }
>>   
>> +#define TOKEN_MODE_MASK	3UL
>> +#define TOKEN_MODE_64	1UL
>> +#define IS_TOKEN_64(token) (((token) & TOKEN_MODE_MASK) == TOKEN_MODE_64)
>> +#define IS_TOKEN_32(token) (((token) & TOKEN_MODE_MASK) == 0)
>> +
>> +/*
>> + * Verify the restore token at the address of 'ssp' is
>> + * valid and then set shadow stack pointer according to the
>> + * token.
>> + */
>> +int cet_verify_rstor_token(bool ia32, unsigned long ssp,
>> +			   unsigned long *new_ssp)
>> +{
>> +	unsigned long token;
>> +
>> +	*new_ssp = 0;
>> +
>> +	if (!IS_ALIGNED(ssp, 8))
>> +		return -EINVAL;
>> +
>> +	if (get_user(token, (unsigned long __user *)ssp))
>> +		return -EFAULT;
>> +
>> +	/* Is 64-bit mode flag correct? */
>> +	if (!ia32 && !IS_TOKEN_64(token))
>> +		return -EINVAL;
>> +	else if (ia32 && !IS_TOKEN_32(token))
>> +		return -EINVAL;
>> +
>> +	token &= ~TOKEN_MODE_MASK;
>> +
>> +	/*
>> +	 * Restore address properly aligned?
>> +	 */
>> +	if ((!ia32 && !IS_ALIGNED(token, 8)) || !IS_ALIGNED(token, 4))
>> +		return -EINVAL;
>> +
>> +	/*
>> +	 * Token was placed properly?
>> +	 */
>> +	if (((ALIGN_DOWN(token, 8) - 8) != ssp) || token >= TASK_SIZE_MAX)
>> +		return -EINVAL;
>> +
>> +	*new_ssp = token;
>> +	return 0;
>> +}
>> +
>> +/*
>> + * Create a restore token on the shadow stack.
>> + * A token is always 8-byte and aligned to 8.
>> + */
>> +static int create_rstor_token(bool ia32, unsigned long ssp,
>> +			      unsigned long *new_ssp)
>> +{
>> +	unsigned long addr;
>> +
>> +	*new_ssp = 0;
>> +
>> +	if ((!ia32 && !IS_ALIGNED(ssp, 8)) || !IS_ALIGNED(ssp, 4))
>> +		return -EINVAL;
>> +
>> +	addr = ALIGN_DOWN(ssp, 8) - 8;
>> +
>> +	/* Is the token for 64-bit? */
>> +	if (!ia32)
>> +		ssp |= TOKEN_MODE_64;
>> +
>> +	if (write_user_shstk_64(addr, ssp))
>> +		return -EFAULT;
>> +
>> +	*new_ssp = addr;
>> +	return 0;
>> +}
>> +

[...]