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SubjectRe: [PATCHv13 06/16] x86/mm: Provide arch_prctl() interface for LAM
Date
On Tue, 2022-12-27 at 06:08 +0300, Kirill A. Shutemov wrote:
> Add a couple of arch_prctl() handles:
>
> - ARCH_ENABLE_TAGGED_ADDR enabled LAM. The argument is required
> number
> of tag bits. It is rounded up to the nearest LAM mode that can
> provide it. For now only LAM_U57 is supported, with 6 tag bits.
>
> - ARCH_GET_UNTAG_MASK returns untag mask. It can indicates where tag
> bits located in the address.
>
> - ARCH_GET_MAX_TAG_BITS returns the maximum tag bits user can
> request.
> Zero if LAM is not supported.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
> arch/x86/include/asm/mmu.h | 2 ++
> arch/x86/include/uapi/asm/prctl.h | 4 +++
> arch/x86/kernel/process.c | 3 +++
> arch/x86/kernel/process_64.c | 44
> ++++++++++++++++++++++++++++++-
> 4 files changed, 52 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/mmu.h b/arch/x86/include/asm/mmu.h
> index ed72fcd2292d..54e4a3e9b5c5 100644
> --- a/arch/x86/include/asm/mmu.h
> +++ b/arch/x86/include/asm/mmu.h
> @@ -12,6 +12,8 @@
> #define MM_CONTEXT_UPROBE_IA32 0
> /* vsyscall page is accessible on this MM */
> #define MM_CONTEXT_HAS_VSYSCALL 1
> +/* Do not allow changing LAM mode */
> +#define MM_CONTEXT_LOCK_LAM 2
>
> /*
> * x86 has arch-specific MMU state beyond what lives in mm_struct.
> diff --git a/arch/x86/include/uapi/asm/prctl.h
> b/arch/x86/include/uapi/asm/prctl.h
> index 500b96e71f18..a31e27b95b19 100644
> --- a/arch/x86/include/uapi/asm/prctl.h
> +++ b/arch/x86/include/uapi/asm/prctl.h
> @@ -20,4 +20,8 @@
> #define ARCH_MAP_VDSO_32 0x2002
> #define ARCH_MAP_VDSO_64 0x2003
>
> +#define ARCH_GET_UNTAG_MASK 0x4001
> +#define ARCH_ENABLE_TAGGED_ADDR 0x4002
> +#define ARCH_GET_MAX_TAG_BITS 0x4003
> +
> #endif /* _ASM_X86_PRCTL_H */
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index ef6bde1d40d8..cc0677f58f42 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -162,6 +162,9 @@ int copy_thread(struct task_struct *p, const
> struct kernel_clone_args *args)
>
> savesegment(es, p->thread.es);
> savesegment(ds, p->thread.ds);
> +
> + if (p->mm && (clone_flags & (CLONE_VM | CLONE_VFORK)) ==
> CLONE_VM)
> + set_bit(MM_CONTEXT_LOCK_LAM, &p->mm->context.flags);
> #else
> p->thread.sp0 = (unsigned long) (childregs + 1);
> savesegment(gs, p->thread.gs);
> diff --git a/arch/x86/kernel/process_64.c
> b/arch/x86/kernel/process_64.c
> index 8b06034e8c70..fef127ed79b6 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -743,6 +743,39 @@ static long prctl_map_vdso(const struct
> vdso_image *image, unsigned long addr)
> }
> #endif
>
> +#define LAM_U57_BITS 6
> +
> +static int prctl_enable_tagged_addr(struct mm_struct *mm, unsigned
> long nr_bits)
> +{
> + if (!cpu_feature_enabled(X86_FEATURE_LAM))
> + return -ENODEV;
> +
> + if (test_bit(MM_CONTEXT_LOCK_LAM, &mm->context.flags))
> + return -EBUSY;

Since this bit doesn't get set on vfork, you might be able to work
around this by enabling LAM in a vforked task, then enabling it again
after returning to the parent. But why do we need this check anyway?

> +
> + if (mmap_write_lock_killable(mm))
> + return -EINTR;
> +
> + if (!nr_bits) {
> + mmap_write_unlock(mm);
> + return -EINVAL;
> + } else if (nr_bits <= LAM_U57_BITS) {
> + mm->context.lam_cr3_mask = X86_CR3_LAM_U57;
> + mm->context.untag_mask = ~GENMASK(62, 57);
> + } else {
> + mmap_write_unlock(mm);
> + return -EINVAL;
> + }
> +
> + write_cr3(__read_cr3() | mm->context.lam_cr3_mask);

mm might not be from the current task if it came from
PTRACE_ARCH_PRCTL, so then this would write to the wrong CR3. Maybe for
simplicity just return an error if task != current.

> + set_tlbstate_cr3_lam_mask(mm->context.lam_cr3_mask);
> + set_bit(MM_CONTEXT_LOCK_LAM, &mm->context.flags);
> +
> + mmap_write_unlock(mm);
> +
> + return 0;
> +}
> +
> long do_arch_prctl_64(struct task_struct *task, int option, unsigned
> long arg2)
> {
> int ret = 0;
> @@ -830,7 +863,16 @@ long do_arch_prctl_64(struct task_struct *task,
> int option, unsigned long arg2)
> case ARCH_MAP_VDSO_64:
> return prctl_map_vdso(&vdso_image_64, arg2);
> #endif
> -
> + case ARCH_GET_UNTAG_MASK:
> + return put_user(task->mm->context.untag_mask,
> + (unsigned long __user *)arg2);
> + case ARCH_ENABLE_TAGGED_ADDR:
> + return prctl_enable_tagged_addr(task->mm, arg2);
> + case ARCH_GET_MAX_TAG_BITS:
> + if (!cpu_feature_enabled(X86_FEATURE_LAM))
> + return put_user(0, (unsigned long __user
> *)arg2);
> + else
> + return put_user(LAM_U57_BITS, (unsigned long
> __user *)arg2);
> default:
> ret = -EINVAL;
> break;
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