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 10:25 AM, Florian Weimer fweimer@redhat.com wrote:

> * Ingo Molnar:
> 
>> * 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.
> 
> I believe glibc initializes the flag fields to zero before calling the
> rseq system call.  (I don't know if the rseq system call does its own
> initialization; maybe it should if it doesn't do so already.)

Initialization and following updates of rseq_abi()->flags and
rseq_abi()->rseq_cs->flags is done by user-space, so the rseq
system call does not initialize any of those fields.

Indeed glibc initialize the rseq_abi()->flags to 0, and does not
use rseq_abi()->rseq_cs->flags as of now.

Thanks,

Mathieu

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