Messages in this thread | | | Subject | Re: [PATCH v26 22/30] x86/cet/shstk: Add user-mode shadow stack support | From | "Yu, Yu-cheng" <> | Date | Thu, 29 Apr 2021 09:17:06 -0700 |
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On 4/29/2021 2:12 AM, Borislav Petkov wrote: > On Wed, Apr 28, 2021 at 11:39:00AM -0700, Yu, Yu-cheng wrote: >> Sorry about that. After that email thread, we went ahead to separate shadow >> stack and ibt into different files. I thought about the struct, the file >> names cet.h, etc. The struct still needs to include ibt status, and if it >> is shstk_desc, the name is not entirely true. One possible approach is, we >> don't make it a struct here, and put every item directly in thread_struct. >> However, the benefit of putting all in a struct is understandable (you might >> argue the opposite :-)). Please make the call, and I will do the change. > > /me looks forward into the patchset... > > So this looks like the final version of it: > > @@ -15,6 +15,7 @@ struct cet_status { > unsigned long shstk_base; > unsigned long shstk_size; > unsigned int locked:1; > + unsigned int ibt_enabled:1; > }; > > If so, that thing should be simply: > > struct cet { > unsigned long shstk_base; > unsigned long shstk_size; > unsigned int shstk_lock : 1, > ibt : 1; > } > > Is that ibt flag per thread or why is it here? I guess I'll find out. > > /me greps... > > ah yes, it is. >
The lock applies to both shadow stack and ibt. So maybe just "locked"?
>> Yes, the comments are in patch #23: Handle thread shadow stack. I wanted to >> add that in the patch that takes the path. > > That comes next, I'll look there. > >>> vm_munmap() can return other negative error values, where are you >>> handling those? >>> >> >> For other error values, the loop stops. > > And then what happens? > >>>> + cet->shstk_base = 0; >>>> + cet->shstk_size = 0; > > You clear those here without even checking whether unmap failed somehow. > And then stuff leaks but we don't care, right? > > Someone else's problem, I'm sure. >
vm_munmap() returns error as the following:
(1) -EINVAL: address/size/alignment is wrong. For shadow stack, the kernel keeps track of it, this cannot/should not happen. Should it happen, it is a bug. The kernel can probably do WARN().
(2) -ENOMEM: when doing __split_vma()/__vma_adjust(), kmem_cache_alloc() fails. Not much we can do. Perhaps WARN()?
(3) -EINTR: mmap_write_lock_killable(mm) fails. This should only happen to a pthread. When a thread is existing, its siblings are holding mm->mmap_lock. This is handled here.
Right now, in the kernel, only the munmap() syscall returns __vm_munmap() error code, otherwise the error is not checked. Within the kernel and if -EINTR is not expected, this makes sense as explained above.
Thanks for questioning. This piece needs to be correct.
Yu-cheng
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