On 1/30/22 13:18, Rick Edgecombe wrote:

Shadow stack accesses are writes from handle_mm_fault() perspective. So to
generate the correct PTE, maybe_mkwrite() will rely on the presence of
VM_SHADOW_STACK or VM_WRITE in the vma.

In future patches, when VM_SHADOW_STACK is actually creatable by
userspace, a problem could happen if a user calls
mprotect( , , PROT_WRITE) on VM_SHADOW_STACK shadow stack memory. The code
would then be confused in the event of shadow stack accesses, and create a
writable PTE for a shadow stack access. Then the process would fault in a
loop.

Prevent this from happening by blocking this kind of memory (VM_WRITE and
VM_SHADOW_STACK) from being created, instead of complicating the fault
handler logic to handle it.

Add an x86 arch_validate_flags() implementation to handle the check.
Rename the uapi/asm/mman.h header guard to be able to use it for
arch/x86/include/asm/mman.h where the arch_validate_flags() will be.

It would be great if this also said:

	There is an existing arch_validate_flags() hook for mmap() and
	mprotect() which allows architectures to reject unwanted
	->vm_flags combinations.  Add an implementation for x86.

That's somewhat implied from what is there already, but making it more
clear would be nice.  There's a much higher bar to add a new arch hook
than to just implement an existing one.

The design decision here seems to be that VM_SHADOW_STACK is itself a
pseudo-VM_WRITE flag.  Like you said: "Shadow stack accesses are writes
from handle_mm_fault()".

Very early on, this series seems to have made the decision that shadow
stacks are writable and need lots of write handling behavior, *BUT*
shouldn't have VM_WRITE set.  As a whole, that seems odd.

The alternative would be *requiring* VM_WRITE and VM_SHADOW_STACK be set
together.  I guess the downside is that pte_mkwrite() would need to be
made to work on shadow stack PTEs.

That particular design decision was never discussed.  I think it has a
really big impact on the rest of the series.  What do you think?  Was it
a good idea?  Or would the alternative be more complicated than what you
have now?