Re: [PATCH v7 19/41] x86/mm: Check shadow stack page fault errors

[Borislav Petkov]

On Mon, Feb 27, 2023 at 02:29:35PM -0800, Rick Edgecombe wrote:
> @@ -1310,6 +1324,23 @@ void do_user_addr_fault(struct pt_regs *regs,
>  
>  	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
>  
> +	/*
> +	 * For conventionally writable pages, a read can be serviced with a
> +	 * read only PTE. But for shadow stack, there isn't a concept of
> +	 * read-only shadow stack memory. If it a PTE has the shadow stack

s/it //

> +	 * permission, it can be modified via CALL and RET instructions. So
> +	 * core MM needs to fault in a writable PTE and do things it already
> +	 * does for write faults.
> +	 *
> +	 * Shadow stack accesses (read or write) need to be serviced with
> +	 * shadow stack permission memory, which always include write
> +	 * permissions. So in the case of a shadow stack read access, treat it
> +	 * as a WRITE fault. This will make sure that MM will prepare
> +	 * everything (e.g., break COW) such that maybe_mkwrite() can create a
> +	 * proper shadow stack PTE.
> +	 */
> +	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.

https://people.kernel.org/tglx/notes-about-netiquette