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    Subject[PATCH AUTOSEL 4.9 41/90] x86/pkeys: Add check for pkey "overflow"
    Date
    From: Dave Hansen <dave.hansen@linux.intel.com>

    [ Upstream commit 16171bffc829272d5e6014bad48f680cb50943d9 ]

    Alex Shi reported the pkey macros above arch_set_user_pkey_access()
    to be unused. They are unused, and even refer to a nonexistent
    CONFIG option.

    But, they might have served a good use, which was to ensure that
    the code does not try to set values that would not fit in the
    PKRU register. As it stands, a too-large 'pkey' value would
    be likely to silently overflow the u32 new_pkru_bits.

    Add a check to look for overflows. Also add a comment to remind
    any future developer to closely examine the types used to store
    pkey values if arch_max_pkey() ever changes.

    This boots and passes the x86 pkey selftests.

    Reported-by: Alex Shi <alex.shi@linux.alibaba.com>
    Signed-off-by: Dave Hansen <dave.hansen@intel.com>
    Signed-off-by: Borislav Petkov <bp@suse.de>
    Link: https://lkml.kernel.org/r/20200122165346.AD4DA150@viggo.jf.intel.com
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    arch/x86/include/asm/pkeys.h | 5 +++++
    arch/x86/kernel/fpu/xstate.c | 9 +++++++--
    2 files changed, 12 insertions(+), 2 deletions(-)

    diff --git a/arch/x86/include/asm/pkeys.h b/arch/x86/include/asm/pkeys.h
    index c50d6dcf4a227..4e7273e176cb7 100644
    --- a/arch/x86/include/asm/pkeys.h
    +++ b/arch/x86/include/asm/pkeys.h
    @@ -3,6 +3,11 @@

    #define ARCH_DEFAULT_PKEY 0

    +/*
    + * If more than 16 keys are ever supported, a thorough audit
    + * will be necessary to ensure that the types that store key
    + * numbers and masks have sufficient capacity.
    + */
    #define arch_max_pkey() (boot_cpu_has(X86_FEATURE_OSPKE) ? 16 : 1)

    extern int arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
    diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
    index e9d7f461b7fa5..dbd396c913488 100644
    --- a/arch/x86/kernel/fpu/xstate.c
    +++ b/arch/x86/kernel/fpu/xstate.c
    @@ -871,8 +871,6 @@ const void *get_xsave_field_ptr(int xsave_state)

    #ifdef CONFIG_ARCH_HAS_PKEYS

    -#define NR_VALID_PKRU_BITS (CONFIG_NR_PROTECTION_KEYS * 2)
    -#define PKRU_VALID_MASK (NR_VALID_PKRU_BITS - 1)
    /*
    * This will go out and modify PKRU register to set the access
    * rights for @pkey to @init_val.
    @@ -891,6 +889,13 @@ int arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
    if (!boot_cpu_has(X86_FEATURE_OSPKE))
    return -EINVAL;

    + /*
    + * This code should only be called with valid 'pkey'
    + * values originating from in-kernel users. Complain
    + * if a bad value is observed.
    + */
    + WARN_ON_ONCE(pkey >= arch_max_pkey());
    +
    /* Set the bits we need in PKRU: */
    if (init_val & PKEY_DISABLE_ACCESS)
    new_pkru_bits |= PKRU_AD_BIT;
    --
    2.25.1
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