Messages in this thread | | | Date | Tue, 26 Jun 2018 16:12:12 -0400 (EDT) | From | Mathieu Desnoyers <> | Subject | Re: rseq: How to test for compat task at signal delivery |
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----- On Jun 26, 2018, at 3:55 PM, Andy Lutomirski luto@amacapital.net wrote:
> On Tue, Jun 26, 2018 at 12:50 PM Mathieu Desnoyers > <mathieu.desnoyers@efficios.com> wrote: >> >> ----- On Jun 26, 2018, at 3:32 PM, Andy Lutomirski luto@amacapital.net wrote: >> >> > On Tue, Jun 26, 2018 at 11:45 AM Mathieu Desnoyers >> > <mathieu.desnoyers@efficios.com> wrote: >> >> >> >> ----- On Jun 26, 2018, at 1:38 PM, Mathieu Desnoyers >> >> mathieu.desnoyers@efficios.com wrote: >> >> >> >> > Hi Andy, >> >> > >> >> > I would like to make the behavior rseq on compat tasks more robust >> >> > by ensuring that kernel/rseq.c:rseq_get_rseq_cs() clears the high >> >> > bits of rseq_cs->abort_ip, rseq_cs->start_ip and >> >> > rseq_cs->post_commit_offset when a 32-bit binary is run on a 64-bit >> >> > kernel. >> >> > >> >> > The intent here is that if user-space has garbage rather than zeroes >> >> > in its struct rseq_cs fields padding, the behavior will be the same >> >> > whether the binary is run on 32-bit or 64 kernels. >> >> > >> >> > I know that internally, the kernel is making a transition from >> >> > is_compat_task() to in_compat_syscall(). >> >> > >> >> > I'm fine with using in_compat_syscall() when rseq_get_rseq_cs() is >> >> > invoked from a system call, but is it OK to call it when it is >> >> > invoked from signal delivery ? AFAIU, signals can be delivered >> >> > upon return from interrupt as well. >> >> > >> >> > If not, what strategy do you recommend for arch-agnostic code ? >> >> >> >> I think what we're missing here is a new "is_compat_frame(struct ksignal *ksig)" >> >> which I could use in the rseq code. I'll prepare a patch and we can discuss >> >> from there. >> >> >> > >> > That sounds about right. >> > >> > I'm confused, though. Wouldn't it be more consistent to just segfault >> > if the high 32 bits are not clear when rseq transitions to a 32-bit >> > context? If there's garbage in 64-bit mode, the program will crash. >> > Why should 32-bit mode be any different? >> >> Currently, if a 32-bit binary puts garbage in the high bits of >> start_ip, post_commit_offset, and abort_ip in >> >> include/uapi/linux/rseq.h: >> >> struct rseq_cs { >> /* Version of this structure. */ >> __u32 version; >> /* enum rseq_cs_flags */ >> __u32 flags; >> LINUX_FIELD_u32_u64(start_ip); >> /* Offset from start_ip. */ >> LINUX_FIELD_u32_u64(post_commit_offset); >> LINUX_FIELD_u32_u64(abort_ip); >> } __attribute__((aligned(4 * sizeof(__u64)))); > > This ABI isn't real ABI until a stable kernel happens, right? So how > about just making all those fields be u64?
Good point. Unlike the rseq_cs field in the struct rseq TLS, those fields don't need to be word-sized/word-aligned, so we could simply declare them as __u64.
> >> >> A 32-bit kernel just never reads the padding, thus in reality acting >> as if those were zeroes. However, a 64-bit kernel dealing with this >> 32-bit compat task will read that padding, handling those as very >> large values. > > Sounds like a design error. Have all kernels read the fields no > matter what. A 32-bit kernel will send SIGSEGV if the high bits are > set. A 64-bit kernel running compat userspace should make sure that a > 32-bit task dies if the high bits are set.
If we end up declaring those as __u64, that approach makes sense.
> >> >> We need to improve that by introducing a consistent behavior across >> native 32-bit kernels and 32-bit compat mode on 64-bit kernels. >> >> There are two ways to achieve this: either the 32-bit kernel validates >> the padding by killing the process if padding is non-zero, or the >> 64-bit kernel treats compat mode by zeroing the high bits of padding. >> >> If we look at system call interfaces in general, I think the usual >> approach is to clear the top bits whenever a value read from a >> compat task ends up being used as a pointer. This is why I am tempted >> to go for the "clear high bits" approach rather than killing the task. > > I think the modern preference is to use fields of fixed size rather > than long when UABI is involved. > > In any event, I think the test you want is user_64bit_mode().
Currently, user_64bit_mode is only implemented on x86.
Should we introduce an architecture-agnostic user_64bit_mode(struct pt_regs *) which maps to is_compat_task() for non-x86 ? I'm just worried that ptrace code could try to use it from the context of another task and get mixed up.
> >> >> Also, validating that the top 32-bit is zeroes from a native 32-bit >> kernel requires extra loads, whereas not caring about their content >> is free, which makes me slightly prefer an approach where 32-bit >> compat mode on 64-bit kernel just clears the top bits. >> > > But performance is totally irrelvant here, right? This only affects > the abort path, unless I'm rather confused.
This load is added on return-to-userspace after a preemption, and upon signal delivery. So it's not a fast-path.
Thanks,
Mathieu
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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