Re: [PATCH v26 22/30] x86/cet/shstk: Add user-mode shadow stack support
From: Yu, Yu-cheng <hidden>
Date: 2021-04-29 16:17:17
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On 4/29/2021 2:12 AM, Borislav Petkov wrote:
quoted hunk ↗ jump to hunk
On Wed, Apr 28, 2021 at 11:39:00AM -0700, Yu, Yu-cheng wrote:quoted
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"?
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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.quoted
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vm_munmap() can return other negative error values, where are you handling those?For other error values, the loop stops.And then what happens?quoted
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+ 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