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

From: Florian Weimer
Date: Fri Oct 13 2017 - 09:56:30 EST


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?

Thanks,
Florian