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

[Mathieu Desnoyers]

----- On Oct 13, 2017, at 8:50 AM, Florian Weimer fweimer@xxxxxxxxxx wrote:

> On 10/13/2017 01:03 AM, Mathieu Desnoyers wrote:
>> Expose a new system call allowing each thread to register one userspace
>> memory area to be used as an ABI between kernel and user-space for two
>> purposes: user-space restartable sequences and quick access to read the
>> current CPU number value from user-space.
>> 
>> * Restartable sequences (per-cpu atomics)
>> 
>> Restartables sequences allow user-space to perform update operations on
>> per-cpu data without requiring heavy-weight atomic operations.
>> 
>> The restartable critical sections (percpu atomics) work has been started
>> by Paul Turner and Andrew Hunter. It lets the kernel handle restart of
>> critical sections. [1] [2] The re-implementation proposed here brings a
>> few simplifications to the ABI which facilitates porting to other
>> architectures and speeds up the user-space fast path.

This part:

>> A locking-based
>> fall-back, purely implemented in user-space, is proposed here to deal
>> with debugger single-stepping. This fallback interacts with rseq_start()
>> and rseq_finish(), which force retries in response to concurrent
>> lock-based activity.

should have been updated in this series to:

A second system call, cpu_opv(), is proposed as fallback to deal with debugger
single-stepping. cpu_opv() executes a sequence of operations on behalf of
user-space with preemption disabled.

> This functionality essentially relies on writable function pointers (or
> pointers to data containing function pointers), right?  Is there a way
> to make this a less attractive target for exploit writers?

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 ?

Thanks,

Mathieu


> 
> Thanks,
> Florian

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