Re: [PATCH 2/2] rseq: Kill process when unknown flags are encountered in ABI structures

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Aug 1, 2022, at 9:32 AM, Ingo Molnar mingo@kernel.org wrote:

> * Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> 
>> rseq_abi()->flags and rseq_abi()->rseq_cs->flags 29 upper bits are
>> currently unused.
>> 
>> The current behavior when those bits are set is to ignore them. This is
>> not an ideal behavior, because when future features will start using
>> those flags, if user-space fails to correctly validate that the kernel
>> indeed supports those flags (e.g. with a new sys_rseq flags bit) before
>> using them, it may incorrectly assume that the kernel will handle those
>> flags way when in fact those will be silently ignored on older kernels.
>> 
>> Validating that unused flags bits are cleared will allow a smoother
>> transition when those flags will start to be used by allowing
>> applications to fail early, and obviously, when they attempt to use the
>> new flags on an older kernel that does not support them.
>> 
>> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>> ---
>>  kernel/rseq.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/kernel/rseq.c b/kernel/rseq.c
>> index 81d7dc80787b..bda8175f8f99 100644
>> --- a/kernel/rseq.c
>> +++ b/kernel/rseq.c
>> @@ -176,7 +176,7 @@ static int rseq_need_restart(struct task_struct *t, u32
>> cs_flags)
>>  	u32 flags, event_mask;
>>  	int ret;
>>  
>> -	if (WARN_ON_ONCE(cs_flags & RSEQ_CS_NO_RESTART_FLAGS))
>> +	if (WARN_ON_ONCE(cs_flags & RSEQ_CS_NO_RESTART_FLAGS) || cs_flags)
>>  		return -EINVAL;
>>  
>>  	/* Get thread flags. */
>> @@ -184,7 +184,7 @@ static int rseq_need_restart(struct task_struct *t, u32
>> cs_flags)
>>  	if (ret)
>>  		return ret;
>>  
>> -	if (WARN_ON_ONCE(flags & RSEQ_CS_NO_RESTART_FLAGS))
>> +	if (WARN_ON_ONCE(flags & RSEQ_CS_NO_RESTART_FLAGS) || flags)
>>  		return -EINVAL;
> 
> Just to make it clear: no existing libraries/tooling out there have learned
> to rely on the old ABI that ignored unset flags, right? Only then is this
> patch ABI-safe.

The projects I know about that use rseq at the moment don't rely on the old ABI
ignoring unset flags:

- glibc initialize the rseq_abi()->flags to 0 and do not use rseq_abi()->rseq_cs->flags yet.
- tcmalloc initialize rseq_abi()->flags and rseq_abi()->rseq_cs->flags to 0.
- librseq (still only a master branch, no officially released public API yet) initialize
  rseq_abi()->flags and rseq_abi()->rseq_cs->cs_flags to 0.
- the Linux kernel selftests initialize rseq_abi()->flags and rseq_abi()->rseq_cs->cs_flags
  to 0.
- AFAIK DynamoRIO does not rely on the kernel ignoring unset flags bits.
- AFAIK CRIU does not rely on the kernel ignoring unset flags bits.

If anyone else rely on rseq ignoring those unset flags, please yell now.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com