Messages in this thread | | | Date | Mon, 2 Jul 2018 19:00:18 -0400 (EDT) | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq_cs fields, validate user inputs |
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----- On Jul 2, 2018, at 6:45 PM, Linus Torvalds torvalds@linux-foundation.org wrote:
> On Mon, Jul 2, 2018 at 3:31 PM Mathieu Desnoyers > <mathieu.desnoyers@efficios.com> wrote: >> >> Change the rseq ABI so rseq_cs start_ip, post_commit_offset and abort_ip >> fields are seen as 64-bit fields by both 32-bit and 64-bit kernels rather >> that ignoring the 32 upper bits on 32-bit kernels. This ensures we have a >> consistent behavior for a 32-bit binary executed on 32-bit kernels and in >> compat mode on 64-bit kernels. > > Actually, now that I see this again, I react to: > > >> +static int check_rseq_cs_padding(struct task_struct *t) >> +{ >> + u32 pad; >> + int ret; >> + >> + ret = __get_user(pad, &t->rseq->rseq_cs_padding); >> + if (ret) >> + return ret; >> + if (pad) >> + return -EINVAL; >> + return 0; >> +} > > This is all wrong. > > Just make "rseq_cs" be an __u64" too. That will clean up everything, > and user space will have a much easier time filling it in too, since > it's just one field. Instead of having to remember about the "let's > fill in padding for 32-bit cases". > > Then the rseq_get_rseq_cs() will be > > __u64 rseq_cs; > > ret = get_user(rseq_cs, &t->rseq->rseq_cs); > if (ret) > return ret; > ptr = (void *)rseq_cs; > if (rseq_cs != (unsigned long)ptr) > return -EINVAL; > > and it's all good, no #ifdef's etc needed. > > Hmm?
Unfortunately, that rseq->rseq_cs field needs to be updated by user-space with single-copy atomicity. Therefore, we want 32-bit user-space to initialize the padding with 0, and only update the low bits with single-copy atomicity.
> > Sorry for the bike-shedding, but this is now the last remaining user > of that LINUX_FIELD_u32_u64, so let's just get rid of it entirely, ok? > > Then we can also get rid of that silly uapi/linux/types_32_64.h header > file entirely. > > That would be *lovely*. Simpler code, simpler and less error-prone > interfaces, and one less specialized header file.
We can easily switch from LINUX_FIELD_u32_u64 to __u64 for fields within struct rseq_cs because we have no requirement on update single-copy atomicity. However, this is not true for the rseq->rseq_cs pointer.
Thoughts ?
Thanks,
Mathieu
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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