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    Subject[PATCH v4 38/39] x86/shstk: Add ARCH_SHSTK_UNLOCK
    Date
    From: Mike Rapoport <rppt@linux.ibm.com>

    Userspace loaders may lock features before a CRIU restore operation has
    the chance to set them to whatever state is required by the process
    being restored. Allow a way for CRIU to unlock features. Add it as an
    arch_prctl() like the other shadow stack operations, but restrict it being
    called by the ptrace arch_pctl() interface.

    Tested-by: Pengfei Xu <pengfei.xu@intel.com>
    Tested-by: John Allen <john.allen@amd.com>
    Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
    [Merged into recent API changes, added commit log and docs]
    Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
    ---

    v4:
    - Add to docs that it is ptrace only.
    - Remove "CET" references

    v3:
    - Depend on CONFIG_CHECKPOINT_RESTORE (Kees)

    Documentation/x86/shstk.rst | 4 ++++
    arch/x86/include/uapi/asm/prctl.h | 1 +
    arch/x86/kernel/process_64.c | 1 +
    arch/x86/kernel/shstk.c | 9 +++++++--
    4 files changed, 13 insertions(+), 2 deletions(-)

    diff --git a/Documentation/x86/shstk.rst b/Documentation/x86/shstk.rst
    index 8e0b2fe83ef8..0d7d1ccfff06 100644
    --- a/Documentation/x86/shstk.rst
    +++ b/Documentation/x86/shstk.rst
    @@ -73,6 +73,10 @@ arch_prctl(ARCH_SHSTK_LOCK, unsigned long features)
    are ignored. The mask is ORed with the existing value. So any feature bits
    set here cannot be enabled or disabled afterwards.

    +arch_prctl(ARCH_SHSTK_UNLOCK, unsigned long features)
    + Unlock features. 'features' is a mask of all features to unlock. All
    + bits set are processed, unset bits are ignored. Only works via ptrace.
    +
    The return values are as following:
    On success, return 0. On error, errno can be::

    diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
    index f13751c6bae4..0c95688cf58e 100644
    --- a/arch/x86/include/uapi/asm/prctl.h
    +++ b/arch/x86/include/uapi/asm/prctl.h
    @@ -30,6 +30,7 @@
    #define ARCH_SHSTK_ENABLE 0x5001
    #define ARCH_SHSTK_DISABLE 0x5002
    #define ARCH_SHSTK_LOCK 0x5003
    +#define ARCH_SHSTK_UNLOCK 0x5004

    /* ARCH_SHSTK_ features bits */
    #define ARCH_SHSTK_SHSTK (1ULL << 0)
    diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
    index 4ddd7d9209e1..2be6e01fb144 100644
    --- a/arch/x86/kernel/process_64.c
    +++ b/arch/x86/kernel/process_64.c
    @@ -921,6 +921,7 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
    case ARCH_SHSTK_ENABLE:
    case ARCH_SHSTK_DISABLE:
    case ARCH_SHSTK_LOCK:
    + case ARCH_SHSTK_UNLOCK:
    return shstk_prctl(task, option, arg2);
    default:
    ret = -EINVAL;
    diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
    index 5d91e653f77a..95579f7bace3 100644
    --- a/arch/x86/kernel/shstk.c
    +++ b/arch/x86/kernel/shstk.c
    @@ -455,9 +455,14 @@ long shstk_prctl(struct task_struct *task, int option, unsigned long features)
    return 0;
    }

    - /* Don't allow via ptrace */
    - if (task != current)
    + /* Only allow via ptrace */
    + if (task != current) {
    + if (option == ARCH_SHSTK_UNLOCK && IS_ENABLED(CONFIG_CHECKPOINT_RESTORE)) {
    + task->thread.features_locked &= ~features;
    + return 0;
    + }
    return -EINVAL;
    + }

    /* Do not allow to change locked features */
    if (features & task->thread.features_locked)
    --
    2.17.1
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