Messages in this thread | | | From | Andy Lutomirski <> | Date | Tue, 26 Jun 2018 12:55:14 -0700 | Subject | Re: rseq: How to test for compat task at signal delivery |
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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?
> > 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.
> > 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().
> > 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.
--Andy
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