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    SubjectRe: [PATCH v4 39/39] x86/shstk: Add ARCH_SHSTK_STATUS
    On Fri, Dec 02, 2022 at 04:36:06PM -0800, Rick Edgecombe wrote:
    > CRIU and GDB need to get the current shadow stack and WRSS enablement
    > status. This information is already available via /proc/pid/status, but
    > this is incovienent for CRIU because it involves parsing the text output
    > in an area of the code where this is difficult. Provide a status
    > arch_prctl(), ARCH_SHSTK_STATUS for retrieving the status. Have arg2 be a
    > userspace address, and make the new arch_prctl simply copy the features
    > out to userspace.
    >
    > Tested-by: Pengfei Xu <pengfei.xu@intel.com>
    > Requested-by: Mike Rapoport <rppt@kernel.org>
    > Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
    > ---
    >
    > v4:
    > - New patch
    >
    > Documentation/x86/shstk.rst | 6 ++++++
    > arch/x86/include/asm/shstk.h | 4 ++--
    > arch/x86/include/uapi/asm/prctl.h | 1 +
    > arch/x86/kernel/process_64.c | 1 +
    > arch/x86/kernel/shstk.c | 8 +++++++-
    > 5 files changed, 17 insertions(+), 3 deletions(-)
    >
    > diff --git a/Documentation/x86/shstk.rst b/Documentation/x86/shstk.rst
    > index 0d7d1ccfff06..b3eb87046c27 100644
    > --- a/Documentation/x86/shstk.rst
    > +++ b/Documentation/x86/shstk.rst
    > @@ -77,6 +77,11 @@ 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.
    >
    > +arch_prctl(ARCH_SHSTK_STATUS, unsigned long addr)
    > + Copy the currently enabled features to the address passed in addr. The
    > + features are described using the bits passed into the others in
    > + 'features'.
    > +
    > The return values are as following:
    > On success, return 0. On error, errno can be::
    >
    > @@ -84,6 +89,7 @@ The return values are as following:
    > -EOPNOTSUPP if the feature is not supported by the hardware or
    > disabled by kernel parameter.
    > -EINVAL arguments (non existing feature, etc)
    > + -EFAULT if could not copy information back to userspace
    >
    > The feature's bits supported are::
    >
    > diff --git a/arch/x86/include/asm/shstk.h b/arch/x86/include/asm/shstk.h
    > index c82f22fd5e6d..23bfb63c597d 100644
    > --- a/arch/x86/include/asm/shstk.h
    > +++ b/arch/x86/include/asm/shstk.h
    > @@ -15,7 +15,7 @@ struct thread_shstk {
    > u64 size;
    > };
    >
    > -long shstk_prctl(struct task_struct *task, int option, unsigned long features);
    > +long shstk_prctl(struct task_struct *task, int option, unsigned long arg2);
    > void reset_thread_features(void);
    > int shstk_alloc_thread_stack(struct task_struct *p, unsigned long clone_flags,
    > unsigned long stack_size,
    > @@ -31,7 +31,7 @@ static inline bool shstk_enabled(void)
    > }
    > #else
    > static inline long shstk_prctl(struct task_struct *task, int option,
    > - unsigned long features) { return -EINVAL; }
    > + unsigned long arg2) { return -EINVAL; }
    > static inline void reset_thread_features(void) {}
    > static inline int shstk_alloc_thread_stack(struct task_struct *p,
    > unsigned long clone_flags,
    > diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
    > index 0c95688cf58e..abe3fe6db6d2 100644
    > --- a/arch/x86/include/uapi/asm/prctl.h
    > +++ b/arch/x86/include/uapi/asm/prctl.h
    > @@ -31,6 +31,7 @@
    > #define ARCH_SHSTK_DISABLE 0x5002
    > #define ARCH_SHSTK_LOCK 0x5003
    > #define ARCH_SHSTK_UNLOCK 0x5004
    > +#define ARCH_SHSTK_STATUS 0x5005
    >
    > /* 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 2be6e01fb144..5dcf5426241b 100644
    > --- a/arch/x86/kernel/process_64.c
    > +++ b/arch/x86/kernel/process_64.c
    > @@ -922,6 +922,7 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
    > case ARCH_SHSTK_DISABLE:
    > case ARCH_SHSTK_LOCK:
    > case ARCH_SHSTK_UNLOCK:
    > + case ARCH_SHSTK_STATUS:
    > return shstk_prctl(task, option, arg2);
    > default:
    > ret = -EINVAL;
    > diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
    > index 95579f7bace3..05f8dcc19dbc 100644
    > --- a/arch/x86/kernel/shstk.c
    > +++ b/arch/x86/kernel/shstk.c
    > @@ -448,8 +448,14 @@ SYSCALL_DEFINE3(map_shadow_stack, unsigned long, addr, unsigned long, size, unsi
    > return alloc_shstk(addr, aligned_size, size, set_tok);
    > }
    >
    > -long shstk_prctl(struct task_struct *task, int option, unsigned long features)
    > +long shstk_prctl(struct task_struct *task, int option, unsigned long arg2)
    > {
    > + unsigned long features = arg2;
    > +
    > + if (option == ARCH_SHSTK_STATUS) {
    > + return put_user(task->thread.features, (unsigned long __user *)arg2);
    > + }

    Doesn't need the braces, but that's fine by me.

    Reviewed-by: Kees Cook <keescook@chromium.org>

    > +
    > if (option == ARCH_SHSTK_LOCK) {
    > task->thread.features_locked |= features;
    > return 0;
    > --
    > 2.17.1
    >

    --
    Kees Cook

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