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
