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    Subject[PATCH 03/19] x86, mpx: use new tsk_get_xsave_addr()
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    Date

    From: Dave Hansen <dave.hansen@linux.intel.com>

    The MPX registers (bndcsr/bndcfgu/bndstatus) are not directly
    accessible via normal instructions. They essentially act as
    if they were floating point registers and are saved/restored
    along with those registers.

    There are two main paths in the MPX code where we care about
    the contents of these registers:
    1. #BR (bounds) faults
    2. the prctl() code where we are setting MPX up

    Both of those paths _might_ be called without the FPU having
    been used. That means that 'tsk->thread.fpu.state' might
    never be allocated.

    Also, fpu_save_init() is not preempt-safe. It was a bug to
    call it without disabling preemption. The new
    tsk_get_xsave_addr() calls unlazy_fpu() instead and properly
    disables preemption.

    Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
    Cc: Oleg Nesterov <oleg@redhat.com>
    Cc: bp@alien8.de
    Cc: Rik van Riel <riel@redhat.com>
    Cc: Suresh Siddha <sbsiddha@gmail.com>
    Cc: Andy Lutomirski <luto@amacapital.net>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Ingo Molnar <mingo@redhat.com>
    Cc: "H. Peter Anvin" <hpa@zytor.com>
    Cc: Fenghua Yu <fenghua.yu@intel.com>
    Cc: the arch/x86 maintainers <x86@kernel.org>
    Cc: linux-kernel <linux-kernel@vger.kernel.org>
    ---

    b/arch/x86/include/asm/mpx.h | 8 ++++----
    b/arch/x86/kernel/traps.c | 15 +++++++--------
    b/arch/x86/mm/mpx.c | 23 +++++++++++------------
    3 files changed, 22 insertions(+), 24 deletions(-)

    diff -puN arch/x86/include/asm/mpx.h~use-new-tsk_get_xsave_addr arch/x86/include/asm/mpx.h
    --- a/arch/x86/include/asm/mpx.h~use-new-tsk_get_xsave_addr 2015-05-08 11:46:11.385599446 -0700
    +++ b/arch/x86/include/asm/mpx.h 2015-05-08 11:46:11.392599762 -0700
    @@ -60,8 +60,8 @@

    #ifdef CONFIG_X86_INTEL_MPX
    siginfo_t *mpx_generate_siginfo(struct pt_regs *regs,
    - struct xsave_struct *xsave_buf);
    -int mpx_handle_bd_fault(struct xsave_struct *xsave_buf);
    + struct task_struct *tsk);
    +int mpx_handle_bd_fault(struct task_struct *tsk);
    static inline int kernel_managing_mpx_tables(struct mm_struct *mm)
    {
    return (mm->bd_addr != MPX_INVALID_BOUNDS_DIR);
    @@ -78,11 +78,11 @@ void mpx_notify_unmap(struct mm_struct *
    unsigned long start, unsigned long end);
    #else
    static inline siginfo_t *mpx_generate_siginfo(struct pt_regs *regs,
    - struct xsave_struct *xsave_buf)
    + struct task_struct *tsk)
    {
    return NULL;
    }
    -static inline int mpx_handle_bd_fault(struct xsave_struct *xsave_buf)
    +static inline int mpx_handle_bd_fault(struct task_struct *tsk)
    {
    return -EINVAL;
    }
    diff -puN arch/x86/kernel/traps.c~use-new-tsk_get_xsave_addr arch/x86/kernel/traps.c
    --- a/arch/x86/kernel/traps.c~use-new-tsk_get_xsave_addr 2015-05-08 11:46:11.387599536 -0700
    +++ b/arch/x86/kernel/traps.c 2015-05-08 11:46:11.392599762 -0700
    @@ -61,6 +61,7 @@
    #include <asm/mach_traps.h>
    #include <asm/alternative.h>
    #include <asm/mpx.h>
    +#include <asm/xsave.h>

    #ifdef CONFIG_X86_64
    #include <asm/x86_init.h>
    @@ -372,7 +373,6 @@ dotraplinkage void do_double_fault(struc
    dotraplinkage void do_bounds(struct pt_regs *regs, long error_code)
    {
    struct task_struct *tsk = current;
    - struct xsave_struct *xsave_buf;
    enum ctx_state prev_state;
    struct bndcsr *bndcsr;
    siginfo_t *info;
    @@ -393,12 +393,11 @@ dotraplinkage void do_bounds(struct pt_r

    /*
    * We need to look at BNDSTATUS to resolve this exception.
    - * It is not directly accessible, though, so we need to
    - * do an xsave and then pull it out of the xsave buffer.
    + * A NULL here might mean that it is in its 'init state',
    + * which is all zeros which indicates MPX was not
    + * responsible for the exception.
    */
    - fpu_save_init(&tsk->thread.fpu);
    - xsave_buf = &(tsk->thread.fpu.state->xsave);
    - bndcsr = get_xsave_addr(xsave_buf, XSTATE_BNDCSR);
    + bndcsr = get_xsave_field(XSTATE_BNDCSR);
    if (!bndcsr)
    goto exit_trap;

    @@ -409,11 +408,11 @@ dotraplinkage void do_bounds(struct pt_r
    */
    switch (bndcsr->bndstatus & MPX_BNDSTA_ERROR_CODE) {
    case 2: /* Bound directory has invalid entry. */
    - if (mpx_handle_bd_fault(xsave_buf))
    + if (mpx_handle_bd_fault(tsk))
    goto exit_trap;
    break; /* Success, it was handled */
    case 1: /* Bound violation. */
    - info = mpx_generate_siginfo(regs, xsave_buf);
    + info = mpx_generate_siginfo(regs, tsk);
    if (IS_ERR(info)) {
    /*
    * We failed to decode the MPX instruction. Act as if
    diff -puN arch/x86/mm/mpx.c~use-new-tsk_get_xsave_addr arch/x86/mm/mpx.c
    --- a/arch/x86/mm/mpx.c~use-new-tsk_get_xsave_addr 2015-05-08 11:46:11.388599581 -0700
    +++ b/arch/x86/mm/mpx.c 2015-05-08 11:46:11.393599807 -0700
    @@ -273,7 +273,7 @@ bad_opcode:
    * The caller is expected to kfree() the returned siginfo_t.
    */
    siginfo_t *mpx_generate_siginfo(struct pt_regs *regs,
    - struct xsave_struct *xsave_buf)
    + struct task_struct *tsk)
    {
    struct bndreg *bndregs, *bndreg;
    siginfo_t *info = NULL;
    @@ -295,8 +295,8 @@ siginfo_t *mpx_generate_siginfo(struct p
    err = -EINVAL;
    goto err_out;
    }
    - /* get the bndregs _area_ of the xsave structure */
    - bndregs = get_xsave_addr(xsave_buf, XSTATE_BNDREGS);
    + /* get bndregs field from urrent task's xsave area */
    + bndregs = get_xsave_field(XSTATE_BNDREGS);
    if (!bndregs) {
    err = -EINVAL;
    goto err_out;
    @@ -358,8 +358,7 @@ static __user void *task_get_bounds_dir(
    * The bounds directory pointer is stored in a register
    * only accessible if we first do an xsave.
    */
    - fpu_save_init(&tsk->thread.fpu);
    - bndcsr = get_xsave_addr(&tsk->thread.fpu.state->xsave, XSTATE_BNDCSR);
    + bndcsr = get_xsave_field(XSTATE_BNDCSR);
    if (!bndcsr)
    return MPX_INVALID_BOUNDS_DIR;

    @@ -390,9 +389,9 @@ int mpx_enable_management(struct task_st
    * directory into XSAVE/XRSTOR Save Area and enable MPX through
    * XRSTOR instruction.
    *
    - * fpu_xsave() is expected to be very expensive. Storing the bounds
    - * directory here means that we do not have to do xsave in the unmap
    - * path; we can just use mm->bd_addr instead.
    + * xsaves are expected to be very expensive. Storing the bounds
    + * directory here means that we do not have to do xsave in the
    + * unmap path; we can just use mm->bd_addr instead.
    */
    bd_base = task_get_bounds_dir(tsk);
    down_write(&mm->mmap_sem);
    @@ -498,12 +497,12 @@ out_unmap:
    * bound table is 16KB. With 64-bit mode, the size of BD is 2GB,
    * and the size of each bound table is 4MB.
    */
    -static int do_mpx_bt_fault(struct xsave_struct *xsave_buf)
    +static int do_mpx_bt_fault(struct task_struct *tsk)
    {
    unsigned long bd_entry, bd_base;
    struct bndcsr *bndcsr;

    - bndcsr = get_xsave_addr(xsave_buf, XSTATE_BNDCSR);
    + bndcsr = get_xsave_field(XSTATE_BNDCSR);
    if (!bndcsr)
    return -EINVAL;
    /*
    @@ -526,7 +525,7 @@ static int do_mpx_bt_fault(struct xsave_
    return allocate_bt((long __user *)bd_entry);
    }

    -int mpx_handle_bd_fault(struct xsave_struct *xsave_buf)
    +int mpx_handle_bd_fault(struct task_struct *tsk)
    {
    /*
    * Userspace never asked us to manage the bounds tables,
    @@ -535,7 +534,7 @@ int mpx_handle_bd_fault(struct xsave_str
    if (!kernel_managing_mpx_tables(current->mm))
    return -EINVAL;

    - if (do_mpx_bt_fault(xsave_buf)) {
    + if (do_mpx_bt_fault(tsk)) {
    force_sig(SIGSEGV, current);
    /*
    * The force_sig() is essentially "handling" this
    _

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