Re: [RFC PATCH v9 for 4.15 01/14] Restartable sequences system call

[Florian Weimer <fweimer@redhat.com>]

On 10/13/2017 07:24 PM, Andy Lutomirski wrote:
> On Fri, Oct 13, 2017 at 7:27 AM, Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
>> ----- On Oct 13, 2017, at 9:56 AM, Florian Weimer fweimer@redhat.com wrote:
>>
>>> On 10/13/2017 03:40 PM, Mathieu Desnoyers wrote:
>>>> The proposed ABI does not require to store any function pointer. For a given
>>>> rseq_finish() critical section, pointers to specific instructions (within a
>>>> function) are emitted at link-time into a struct rseq_cs:
>>>>
>>>> struct rseq_cs {
>>>>           RSEQ_FIELD_u32_u64(start_ip);
>>>>           RSEQ_FIELD_u32_u64(post_commit_ip);
>>>>           RSEQ_FIELD_u32_u64(abort_ip);
>>>>           uint32_t flags;
>>>> } __attribute__((aligned(4 * sizeof(uint64_t))));
>>>>
>>>> Then, at runtime, the fast-path stores the address of that struct rseq_cs
>>>> into the TLS struct rseq "rseq_cs" field.
>>>>
>>>> So all we store at runtime is a pointer to data, not a pointer to functions.
>>>>
>>>> But you seem to hint that having a pointer to data containing pointers to code
>>>> may still be making it easier for exploit writers. Can you elaborate on the
>>>> scenario ?
>>>
>>> I'm concerned that the exploit writer writes a totally made up struct
>>> rseq_cs object into writable memory, along with function pointers, and
>>> puts the address of that in to the rseq_cs field.
>>>
>>> This would be comparable to how C++ vtable pointers are targeted
>>> (including those in the glibc libio implementation of stdio streams).
>>>
>>> Does this answer your questions?
>>
>> Yes, it does. How about we add a "canary" field to the TLS struct rseq, e.g.:
>>
>> struct rseq {
>>          union rseq_cpu_event u;
>>          RSEQ_FIELD_u32_u64(rseq_cs);  -> pointer to struct rseq_cs
>>          uint32_t flags;
>>          uint32_t canary;   -> 32 low bits of rseq_cs ^ canary_mask
>> };
>>
>> We could then add a "uint32_t canary_mask" argument to sys_rseq, e.g.:
>>
>> SYSCALL_DEFINE3(rseq, struct rseq __user *, rseq, uint32_t, canary_mask, int, flags);
>>
>> So a thread which does not care about hardening would simply register its
>> struct rseq TLS with a canary mask of "0". Nothing changes on the fast-path.
>>
>> A thread belonging to a process that cares about hardening could use a random
>> value as canary, and pass it as canary_mask argument to the syscall. The
>> fast-path could then set the struct rseq "canary" value to
>> (32-low-bits of rseq_cs) ^ canary_mask just surrounding the critical section,
>> and set it back to 0 afterward.
>>
>> In the kernel, whenever the rseq_cs pointer would be loaded, its 32 low bits
>> would be checked to match (canary ^ canary_mask). If it differs, then the
>> kernel kills the process with SIGSEGV.
>>
>> Would that take care of your concern ?
>>
> 
> I would propose a slightly different solution: have the kernel verify
> that it jumps to a code sequence that occurs just after some
> highly-unlikely magic bytes in the text *and* that those bytes have
> some signature that matches a signature in the struct rseq that's
> passed in.

And the signature is fixed at the time of the rseq syscall?

Yes, that would be far more reliable.

Thanks,
Florian