Messages in this thread | | | Subject | Re: [RFC PATCH v9 for 4.15 01/14] Restartable sequences system call | From | Florian Weimer <> | Date | Fri, 13 Oct 2017 19:53:50 +0200 |
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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
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