On 10/13/2017 07:24 PM, Andy Lutomirski wrote:
On Fri, Oct 13, 2017 at 7:27 AM, Mathieu Desnoyers
<mathieu.desnoy...@efficios.com> wrote:
----- On Oct 13, 2017, at 9:56 AM, Florian Weimer fwei...@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
