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

Florian Weimer
2022-08-01T14:25:32Z

* 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.)

Thanks,
Florian