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

Ingo Molnar
2022-08-06T18:55:26Z

* Kyle Huey <me@kylehuey.com> wrote:

> 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.

Yeah, here I meant that the ptraced child actually has to execute to carry
the new values - and AFAICS that already happens in your testcase, as
there's a PTRACE_CONT+waitpid() between the PTRACE_SETREGSET and the second
PTRACE_GETREGSET call, right?

If so, then the testcase should be mostly fine, except would it make sense
to use something less trivial than clearing the permission bitmask:

> > > + xstate_bv = (u64 *)(xsave + 512);
> > > + *xstate_bv &= ~(1 << 9);

if I'm reading the code right? A 01010101 bitmask perhaps?

Thanks,

Ingo