Re: [PATCH v4 16/39] x86/mm: Check Shadow Stack page fault errors

Borislav Petkov
2023-01-04T14:32:59Z

On Fri, Dec 02, 2022 at 04:35:43PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@intel.com>
>
> The CPU performs "shadow stack accesses" when it expects to encounter
> shadow stack mappings. These accesses can be implicit (via CALL/RET
> instructions) or explicit (instructions like WRSS).
>
> Shadow stacks accesses to shadow-stack mappings can see faults in normal,
> valid operation just like regular accesses to regular mappings. Shadow
> stacks need some of the same features like delayed allocation, swap and
> copy-on-write. The kernel needs to use faults to implement those features.
>
> The architecture has concepts of both shadow stack reads and shadow stack
> writes. Any shadow stack access to non-shadow stack memory will generate
> a fault with the shadow stack error code bit set.

You lost me here: by "shadow stack access to non-shadow stack memory" you mean
the explicit one using WRU*SS?

> This means that, unlike normal write protection, the fault handler needs
> to create a type of memory that can be written to (with instructions that
> generate shadow stack writes), even to fulfill a read access. So in the
> case of COW memory, the COW needs to take place even with a shadow stack
> read.

I guess I'm missing an example here: are we talking here about a user process
getting its shadow stack pages allocated and them being COW first and on the
first shstk operation, it would generate that fault?

> @@ -1331,6 +1345,30 @@ void do_user_addr_fault(struct pt_regs *regs,
>
> perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
>
> + /*
> + * When a page becomes COW it changes from a shadow stack permissioned

Unknown word [permissioned] in comment.

> + * page (Write=0,Dirty=1) to (Write=0,Dirty=0,CoW=1), which is simply
> + * read-only to the CPU. When shadow stack is enabled, a RET would
> + * normally pop the shadow stack by reading it with a "shadow stack
> + * read" access. However, in the COW case the shadow stack memory does
> + * not have shadow stack permissions, it is read-only. So it will
> + * generate a fault.
> + *
> + * For conventionally writable pages, a read can be serviced with a
> + * read only PTE, and COW would not have to happen. But for shadow
> + * stack, there isn't the concept of read-only shadow stack memory.
> + * If it is shadow stack permissioned, it can be modified via CALL and

Ditto.

> + * RET instructions. So COW needs to happen before any memory can be
> + * mapped with shadow stack permissions.
> + *
> + * Shadow stack accesses (read or write) need to be serviced with
> + * shadow stack permissioned memory, so in the case of a shadow stack

Is this some new formulation I haven't heard about yet?

"Permissioned <something>"?

> + * read access, treat it as a WRITE fault so both COW will happen and
> + * the write fault path will tickle maybe_mkwrite() and map the memory
> + * shadow stack.
> + */
> + if (error_code & X86_PF_SHSTK)
> + flags |= FAULT_FLAG_WRITE;
> if (error_code & X86_PF_WRITE)
> flags |= FAULT_FLAG_WRITE;
> if (error_code & X86_PF_INSTR)
> --
> 2.17.1
>

--
Regards/Gruss,
Boris.