Re: [PATCH v6 37/41] selftests/x86: Add shadow stack test

[David Hildenbrand]

On 18.02.23 22:14, Rick Edgecombe wrote:
> Add a simple selftest for exercising some shadow stack behavior:
>   - map_shadow_stack syscall and pivot
>   - Faulting in shadow stack memory
>   - Handling shadow stack violations
>   - GUP of shadow stack memory
>   - mprotect() of shadow stack memory
>   - Userfaultfd on shadow stack memory
>
> Since this test exercises a recently added syscall manually, it needs
> to find the automatically created __NR_foo defines. Per the selftest
> documentation, KHDR_INCLUDES can be used to help the selftest Makefile's
> find the headers from the kernel source. This way the new selftest can
> be built inside the kernel source tree without installing the headers
> to the system. So also add KHDR_INCLUDES as described in the selftest
> docs, to facilitate this.
>
> Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
> Tested-by: John Allen <john.allen@xxxxxxx>
> Co-developed-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
>
> ---


[...]

> +bool gup_write(void *ptr)
> +{
> +	unsigned long val;
> +
> +	lseek(fd, (unsigned long)ptr, SEEK_SET);
> +	if (write(fd, &val, sizeof(val)) < 0)
> +		return 1;

/proc/self/mem is for debug/ptrace access (FOLL_FORCE). I think you 
might also want to add tests for ordinary GUP, checking that we fail to 
obtain a write pin -- and call these tests "gup_ptrace_read" / 
"gup_ptrace_write"

An simple approach would be to trigger a read()/write() on a file opened 
via O_DIRECT, using the shadow stack as buffer. While the write() 
[reading from the page] is expected to work, a read() [writing to the 
page] has to fail.


--
Thanks,

David / dhildenb