Re: [PATCH v4 2/2] selftests/vm/pkeys: Add a regression test for setting PKRU through ptrace

Kyle Huey
2022-08-06T12:46:49Z

On Sat, Aug 6, 2022 at 1:52 AM Ingo Molnar <mingo@kernel.org> wrote:
>
>
> * Kyle Huey <me@kylehuey.com> wrote:
>
> > From: Kyle Huey <me@kylehuey.com>
> >
> > This tests PTRACE_SETREGSET with NT_X86_XSTATE modifying PKRU directly and
> > removing the PKRU bit from XSTATE_BV.
> >
> > Signed-off-by: Kyle Huey <me@kylehuey.com>
> > ---
> > tools/testing/selftests/vm/pkey-x86.h | 12 +++
> > tools/testing/selftests/vm/protection_keys.c | 88 +++++++++++++++++++-
> > 2 files changed, 98 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/testing/selftests/vm/pkey-x86.h b/tools/testing/selftests/vm/pkey-x86.h
> > index b078ce9c6d2a..72c14cd3ddc7 100644
> > --- a/tools/testing/selftests/vm/pkey-x86.h
> > +++ b/tools/testing/selftests/vm/pkey-x86.h
> > @@ -104,6 +104,18 @@ static inline int cpu_has_pkeys(void)
> > return 1;
> > }
> >
> > +static inline int cpu_max_xsave_size(void)
> > +{
> > + unsigned long XSTATE_CPUID = 0xd;
> > + unsigned int eax;
> > + unsigned int ebx;
> > + unsigned int ecx;
> > + unsigned int edx;
> > +
> > + __cpuid_count(XSTATE_CPUID, 0, eax, ebx, ecx, edx);
> > + return ecx;
> > +}
> > +
> > static inline u32 pkey_bit_position(int pkey)
> > {
> > return pkey * PKEY_BITS_PER_PKEY;
> > diff --git a/tools/testing/selftests/vm/protection_keys.c b/tools/testing/selftests/vm/protection_keys.c
> > index 291bc1e07842..27759d3ed9cd 100644
> > --- a/tools/testing/selftests/vm/protection_keys.c
> > +++ b/tools/testing/selftests/vm/protection_keys.c
> > @@ -18,12 +18,13 @@
> > * do a plain mprotect() to a mprotect_pkey() area and make sure the pkey sticks
> > *
> > * Compile like this:
> > - * gcc -o protection_keys -O2 -g -std=gnu99 -pthread -Wall protection_keys.c -lrt -ldl -lm
> > - * gcc -m32 -o protection_keys_32 -O2 -g -std=gnu99 -pthread -Wall protection_keys.c -lrt -ldl -lm
> > + * gcc -mxsave -o protection_keys -O2 -g -std=gnu99 -pthread -Wall protection_keys.c -lrt -ldl -lm
> > + * gcc -mxsave -m32 -o protection_keys_32 -O2 -g -std=gnu99 -pthread -Wall protection_keys.c -lrt -ldl -lm
> > */
> > #define _GNU_SOURCE
> > #define __SANE_USERSPACE_TYPES__
> > #include <errno.h>
> > +#include <linux/elf.h>
> > #include <linux/futex.h>
> > #include <time.h>
> > #include <sys/time.h>
> > @@ -1550,6 +1551,86 @@ void test_implicit_mprotect_exec_only_memory(int *ptr, u16 pkey)
> > do_not_expect_pkey_fault("plain read on recently PROT_EXEC area");
> > }
> >
> > +#if defined(__i386__) || defined(__x86_64__)
> > +void test_ptrace_modifies_pkru(int *ptr, u16 pkey)
> > +{
> > + pid_t child;
> > + int status, ret;
> > + int pkey_offset = pkey_reg_xstate_offset();
> > + size_t xsave_size = cpu_max_xsave_size();
> > + void *xsave;
> > + u32 *pkey_register;
> > + u64 *xstate_bv;
> > + struct iovec iov;
> > +
> > + child = fork();
> > + pkey_assert(child >= 0);
> > + dprintf3("[%d] fork() ret: %d\n", getpid(), child);
> > + if (!child) {
> > + u32 pkey_register = read_pkey_reg();
> > +
> > + ptrace(PTRACE_TRACEME, 0, 0, 0);
> > + raise(SIGSTOP);
> > +
> > + /*
> > + * need __read_pkey_reg() version so we do not do shadow_pkey_reg
> > + * checking
> > + */
> > + if (pkey_register == __read_pkey_reg())
> > + exit(1);
> > +
> > + raise(SIGSTOP);
> > +
> > + exit(__read_pkey_reg());
> > + }
> > +
> > + pkey_assert(child == waitpid(child, &status, 0));
> > + dprintf3("[%d] waitpid(%d) status: %x\n", getpid(), child, status);
> > + pkey_assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGSTOP);
> > +
> > + xsave = (void *)malloc(xsave_size);
> > + pkey_assert(xsave > 0);
> > +
> > + iov.iov_base = xsave;
> > + iov.iov_len = xsave_size;
> > + ret = ptrace(PTRACE_GETREGSET, child, (void *)NT_X86_XSTATE, &iov);
> > + pkey_assert(ret == 0);
> > +
> > + pkey_register = (u32 *)(xsave + pkey_offset);
> > + pkey_assert(*pkey_register == read_pkey_reg());
> > +
> > + *pkey_register = !read_pkey_reg();
> > +
> > + ret = ptrace(PTRACE_SETREGSET, child, (void *)NT_X86_XSTATE, &iov);
> > + pkey_assert(ret == 0);
> > +
> > + ret = ptrace(PTRACE_CONT, child, 0, 0);
> > + pkey_assert(ret == 0);
> > +
> > + pkey_assert(child == waitpid(child, &status, 0));
> > + dprintf3("[%d] waitpid(%d) status: %x\n", getpid(), child, status);
> > + pkey_assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGSTOP);
> > +
> > + ret = ptrace(PTRACE_GETREGSET, child, (void *)NT_X86_XSTATE, &iov);
> > + pkey_assert(ret == 0);
> > +
> > + xstate_bv = (u64 *)(xsave + 512);
> > + *xstate_bv &= ~(1 << 9);
> > +
> > + ret = ptrace(PTRACE_SETREGSET, child, (void *)NT_X86_XSTATE, &iov);
> > + pkey_assert(ret == 0);
> > +
> > + ret = ptrace(PTRACE_CONT, child, 0, 0);
> > + pkey_assert(ret == 0);
> > +
> > + pkey_assert(child == waitpid(child, &status, 0));
> > + dprintf3("[%d] waitpid(%d) status: %x\n", getpid(), child, status);
> > + pkey_assert(WIFEXITED(status));
> > + pkey_assert(WEXITSTATUS(status) == 0);
> > + free(xsave);
>
> LGTM.
>
> May I ask for a bit more in terms of testing the ABI: writing some
> non-trivial (not all-zero and not all-ones) value into the PKRU register,
> forcing the child task to go through a FPU save/restore context switch
> and then reading it back and verifying the value, or something like that?

Can you elaborate a bit on what you mean here? I'm not sure what "a
FPU save/restore context switch" is. The XSTATE (and everything else)
will be saved/restored at the ptrace stops (for the raise(SIGSTOP)s)
already.

- Kyle

> Thanks,
>
> Ingo