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    SubjectRe: [RFC PATCH v4 26/27] x86/cet/shstk: Add arch_prctl functions for Shadow Stack
    On Fri, Sep 21, 2018 at 08:03:50AM -0700, Yu-cheng Yu wrote:
    > arch_prctl(ARCH_CET_STATUS, unsigned long *addr)
    > Return CET feature status.
    >
    > The parameter 'addr' is a pointer to a user buffer.
    > On returning to the caller, the kernel fills the following
    > information:
    >
    > *addr = SHSTK/IBT status
    > *(addr + 1) = SHSTK base address
    > *(addr + 2) = SHSTK size

    The subtle detail here is that x32 binaries will get 64-bit value, which
    is not entirely obvious. I think, it might be better to define
    a structure type for it as a part of UAPI, for example:

    struct user_cet_status {
    __u32 struct_size;
    __u32 features;
    __kernel_ulong_t shstk_base;
    __kernel_ulong_t shstk_size;
    };

    Adding "struct_size" field along with appropriate checks will also
    allow for possible extensions, if they ever appear.

    > arch_prctl(ARCH_CET_DISABLE, unsigned long features)
    > Disable CET features specified in 'features'. Return
    > -EPERM if CET is locked.

    While x86_64 and x32 will have 64-bit space for feature bits, IA-32 will
    have only 32 bits.

    > arch_prctl(ARCH_CET_LOCK)
    > Lock in CET feature.
    >
    > arch_prctl(ARCH_CET_ALLOC_SHSTK, unsigned long *addr)
    > Allocate a new SHSTK.
    >
    > The parameter 'addr' is a pointer to a user buffer and indicates
    > the desired SHSTK size to allocate. On returning to the caller
    > the buffer contains the address of the new SHSTK.

    Again, on x32 that will be a pointer to a 64-bit value, which is not
    entirely obvious from this description.

    It's not clear whether inability to enable some CET feature in runtime
    is unavailable by design or by omission; same for setting (an allocated)
    shadow stack as task's shadow stack.

    >
    > Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
    > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
    > ---
    > arch/x86/include/asm/cet.h | 5 ++
    > arch/x86/include/uapi/asm/prctl.h | 5 ++
    > arch/x86/kernel/Makefile | 2 +-
    > arch/x86/kernel/cet.c | 27 +++++++++++
    > arch/x86/kernel/cet_prctl.c | 79 +++++++++++++++++++++++++++++++
    > arch/x86/kernel/process.c | 5 ++
    > 6 files changed, 122 insertions(+), 1 deletion(-)
    > create mode 100644 arch/x86/kernel/cet_prctl.c
    >
    > diff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h
    > index b7b33e1026bb..212bd68e31d3 100644
    > --- a/arch/x86/include/asm/cet.h
    > +++ b/arch/x86/include/asm/cet.h
    > @@ -12,19 +12,24 @@ struct task_struct;
    > struct cet_status {
    > unsigned long shstk_base;
    > unsigned long shstk_size;
    > + unsigned int locked:1;
    > unsigned int shstk_enabled:1;
    > };
    >
    > #ifdef CONFIG_X86_INTEL_CET
    > +int prctl_cet(int option, unsigned long arg2);
    > int cet_setup_shstk(void);
    > int cet_setup_thread_shstk(struct task_struct *p);
    > +int cet_alloc_shstk(unsigned long *arg);
    > void cet_disable_shstk(void);
    > void cet_disable_free_shstk(struct task_struct *p);
    > int cet_restore_signal(unsigned long ssp);
    > int cet_setup_signal(bool ia32, unsigned long rstor, unsigned long *new_ssp);
    > #else
    > +static inline int prctl_cet(int option, unsigned long arg2) { return 0; }

    Why 0 and not -EINVAL?

    > static inline int cet_setup_shstk(void) { return 0; }

    0 here also looks strange.

    > static inline int cet_setup_thread_shstk(struct task_struct *p) { return 0; }

    And here.

    > +static inline int cet_alloc_shstk(unsigned long *arg) { return -EINVAL; }
    > static inline void cet_disable_shstk(void) {}
    > static inline void cet_disable_free_shstk(struct task_struct *p) {}
    > static inline int cet_restore_signal(unsigned long ssp) { return 0; }
    > diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
    > index 5a6aac9fa41f..3aec1088e01d 100644
    > --- a/arch/x86/include/uapi/asm/prctl.h
    > +++ b/arch/x86/include/uapi/asm/prctl.h
    > @@ -14,4 +14,9 @@
    > #define ARCH_MAP_VDSO_32 0x2002
    > #define ARCH_MAP_VDSO_64 0x2003
    >
    > +#define ARCH_CET_STATUS 0x3001
    > +#define ARCH_CET_DISABLE 0x3002
    > +#define ARCH_CET_LOCK 0x3003
    > +#define ARCH_CET_ALLOC_SHSTK 0x3004
    > +
    > #endif /* _ASM_X86_PRCTL_H */
    > diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
    > index 36b14ef410c8..b9e6cdc6b4f7 100644
    > --- a/arch/x86/kernel/Makefile
    > +++ b/arch/x86/kernel/Makefile
    > @@ -139,7 +139,7 @@ obj-$(CONFIG_UNWINDER_ORC) += unwind_orc.o
    > obj-$(CONFIG_UNWINDER_FRAME_POINTER) += unwind_frame.o
    > obj-$(CONFIG_UNWINDER_GUESS) += unwind_guess.o
    >
    > -obj-$(CONFIG_X86_INTEL_CET) += cet.o
    > +obj-$(CONFIG_X86_INTEL_CET) += cet.o cet_prctl.o
    >
    > obj-$(CONFIG_ARCH_HAS_PROGRAM_PROPERTIES) += elf.o
    >
    > diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
    > index ce0b3b7b1160..1c2689738604 100644
    > --- a/arch/x86/kernel/cet.c
    > +++ b/arch/x86/kernel/cet.c
    > @@ -110,6 +110,33 @@ static int create_rstor_token(bool ia32, unsigned long ssp,
    > return 0;
    > }
    >
    > +int cet_alloc_shstk(unsigned long *arg)
    > +{
    > + unsigned long len = *arg;
    > + unsigned long addr;
    > + unsigned long token;
    > + unsigned long ssp;
    > +
    > + addr = do_mmap_locked(0, len, PROT_READ,
    > + MAP_ANONYMOUS | MAP_PRIVATE, VM_SHSTK);
    > + if (addr >= TASK_SIZE_MAX)
    > + return -ENOMEM;
    > +
    > + /* Restore token is 8 bytes and aligned to 8 bytes */
    > + ssp = addr + len;
    > + token = ssp;
    > +
    > + if (!in_ia32_syscall())
    > + token |= 1;

    This pair of check and bit or'ing definitely asks for a macro or a
    wrapper function.

    > + ssp -= 8;
    > +
    > + if (write_user_shstk_64(ssp, token))
    > + return -EINVAL;

    Shouldn't addr be unmapped on error?

    > + *arg = addr;
    > + return 0;
    > +}
    > +
    > int cet_setup_shstk(void)
    > {
    > unsigned long addr, size;
    > diff --git a/arch/x86/kernel/cet_prctl.c b/arch/x86/kernel/cet_prctl.c
    > new file mode 100644
    > index 000000000000..c4b7c19f5040
    > --- /dev/null
    > +++ b/arch/x86/kernel/cet_prctl.c
    > @@ -0,0 +1,79 @@
    > +/* SPDX-License-Identifier: GPL-2.0 */
    > +
    > +#include <linux/errno.h>
    > +#include <linux/uaccess.h>
    > +#include <linux/prctl.h>
    > +#include <linux/compat.h>
    > +#include <asm/processor.h>
    > +#include <asm/prctl.h>
    > +#include <asm/elf.h>
    > +#include <asm/elf_property.h>
    > +#include <asm/cet.h>
    > +
    > +/* See Documentation/x86/intel_cet.txt. */
    > +
    > +static int handle_get_status(unsigned long arg2)
    > +{
    > + unsigned int features = 0;
    > + unsigned long shstk_base, shstk_size;
    > + unsigned long buf[3];
    > +
    > + if (current->thread.cet.shstk_enabled)
    > + features |= GNU_PROPERTY_X86_FEATURE_1_SHSTK;
    > +
    > + shstk_base = current->thread.cet.shstk_base;
    > + shstk_size = current->thread.cet.shstk_size;
    > +
    > + buf[0] = (unsigned long)features;
    > + buf[1] = shstk_base;
    > + buf[2] = shstk_size;
    > + return copy_to_user((unsigned long __user *)arg2, buf,
    > + sizeof(buf));
    > +}
    > +
    > +static int handle_alloc_shstk(unsigned long arg2)
    > +{
    > + int err = 0;
    > + unsigned long shstk_size = 0;
    > +
    > + if (get_user(shstk_size, (unsigned long __user *)arg2))
    > + return -EFAULT;
    > +
    > + err = cet_alloc_shstk(&shstk_size);
    > + if (err)
    > + return err;
    > +
    > + if (put_user(shstk_size, (unsigned long __user *)arg2))
    Again, leaking allocated stack.

    > + return -EFAULT;
    > +
    > + return 0;
    > +}
    > +
    > +int prctl_cet(int option, unsigned long arg2)
    > +{
    > + if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
    > + return -EINVAL;
    > +
    > + switch (option) {
    > + case ARCH_CET_STATUS:
    > + return handle_get_status(arg2);
    > +
    > + case ARCH_CET_DISABLE:
    > + if (current->thread.cet.locked)
    > + return -EPERM;
    > + if (arg2 & GNU_PROPERTY_X86_FEATURE_1_SHSTK)
    > + cet_disable_free_shstk(current);

    The rest of bits in arg2 should be 0, otherwise this interface won't be
    possible to extend.

    > + return 0;
    > +
    > + case ARCH_CET_LOCK:
    > + current->thread.cet.locked = 1;
    > + return 0;
    > +
    > + case ARCH_CET_ALLOC_SHSTK:
    > + return handle_alloc_shstk(arg2);
    > +
    > + default:
    > + return -EINVAL;
    > + }
    > +}
    > diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
    > index 440f012ef925..251b8714f9a3 100644
    > --- a/arch/x86/kernel/process.c
    > +++ b/arch/x86/kernel/process.c
    > @@ -792,6 +792,11 @@ long do_arch_prctl_common(struct task_struct *task, int option,
    > return get_cpuid_mode();
    > case ARCH_SET_CPUID:
    > return set_cpuid_mode(task, cpuid_enabled);
    > + case ARCH_CET_STATUS:
    > + case ARCH_CET_DISABLE:
    > + case ARCH_CET_LOCK:
    > + case ARCH_CET_ALLOC_SHSTK:
    > + return prctl_cet(option, cpuid_enabled);

    It's probably a good opportunity to change the strange name for an argument
    of a dispatch call.

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