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SubjectRe: [PATCH v6 37/41] selftests/x86: Add shadow stack test
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@intel.com>
> Tested-by: John Allen <john.allen@amd.com>
> Co-developed-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
>
> ---


[...]

> +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

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