Messages in this thread | | | Date | Tue, 26 Jun 2018 15:50:08 -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 3:32 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))));
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.
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.
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.
Thoughts ?
Thanks,
Mathieu
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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