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    Subject[PATCH 5.13 777/800] selftests/vm/pkeys: refill shadow register after implicit kernel write
    Date
    From: Dave Hansen <dave.hansen@linux.intel.com>

    [ Upstream commit 6039ca254979694c5362dfebadd105e286c397bb ]

    The pkey test code keeps a "shadow" of the pkey register around. This
    ensures that any bugs which might write to the register can be caught more
    quickly.

    Generally, userspace has a good idea when the kernel is going to write to
    the register. For instance, alloc_pkey() is passed a permission mask.
    The caller of alloc_pkey() can update the shadow based on the return value
    and the mask.

    But, the kernel can also modify the pkey register in a more sneaky way.
    For mprotect(PROT_EXEC) mappings, the kernel will allocate a pkey and
    write the pkey register to create an execute-only mapping. The kernel
    never tells userspace what key it uses for this.

    This can cause the test to fail with messages like:

    protection_keys_64.2: pkey-helpers.h:132: _read_pkey_reg: Assertion `pkey_reg == shadow_pkey_reg' failed.

    because the shadow was not updated with the new kernel-set value.

    Forcibly update the shadow value immediately after an mprotect().

    Link: https://lkml.kernel.org/r/20210611164200.EF76AB73@viggo.jf.intel.com
    Fixes: 6af17cf89e99 ("x86/pkeys/selftests: Add PROT_EXEC test")
    Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Tested-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
    Cc: Ram Pai <linuxram@us.ibm.com>
    Cc: Sandipan Das <sandipan@linux.ibm.com>
    Cc: Florian Weimer <fweimer@redhat.com>
    Cc: "Desnes A. Nunes do Rosario" <desnesn@linux.vnet.ibm.com>
    Cc: Ingo Molnar <mingo@kernel.org>
    Cc: Thiago Jung Bauermann <bauerman@linux.ibm.com>
    Cc: Michael Ellerman <mpe@ellerman.id.au>
    Cc: Michal Hocko <mhocko@kernel.org>
    Cc: Michal Suchanek <msuchanek@suse.de>
    Cc: Shuah Khan <shuah@kernel.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    tools/testing/selftests/vm/protection_keys.c | 7 +++++++
    1 file changed, 7 insertions(+)

    diff --git a/tools/testing/selftests/vm/protection_keys.c b/tools/testing/selftests/vm/protection_keys.c
    index 356d62fca27f..87eecd5ba577 100644
    --- a/tools/testing/selftests/vm/protection_keys.c
    +++ b/tools/testing/selftests/vm/protection_keys.c
    @@ -1448,6 +1448,13 @@ void test_implicit_mprotect_exec_only_memory(int *ptr, u16 pkey)
    ret = mprotect(p1, PAGE_SIZE, PROT_EXEC);
    pkey_assert(!ret);

    + /*
    + * Reset the shadow, assuming that the above mprotect()
    + * correctly changed PKRU, but to an unknown value since
    + * the actual alllocated pkey is unknown.
    + */
    + shadow_pkey_reg = __read_pkey_reg();
    +
    dprintf2("pkey_reg: %016llx\n", read_pkey_reg());

    /* Make sure this is an *instruction* fault */
    --
    2.30.2


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