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    Subject[tip:x86/vdso] x86-64: Add user_64bit_mode paravirt op
    Commit-ID:  318f5a2a672152328c9fb4dead504b89ec738a43
    Gitweb: http://git.kernel.org/tip/318f5a2a672152328c9fb4dead504b89ec738a43
    Author: Andy Lutomirski <luto@MIT.EDU>
    AuthorDate: Wed, 3 Aug 2011 09:31:53 -0400
    Committer: H. Peter Anvin <hpa@linux.intel.com>
    CommitDate: Thu, 4 Aug 2011 16:13:49 -0700

    x86-64: Add user_64bit_mode paravirt op

    Three places in the kernel assume that the only long mode CPL 3
    selector is __USER_CS. This is not true on Xen -- Xen's sysretq
    changes cs to the magic value 0xe033.

    Two of the places are corner cases, but as of "x86-64: Improve
    vsyscall emulation CS and RIP handling"
    (c9712944b2a12373cb6ff8059afcfb7e826a6c54), vsyscalls will segfault
    if called with Xen's extra CS selector. This causes a panic when
    older init builds die.

    It seems impossible to make Xen use __USER_CS reliably without
    taking a performance hit on every system call, so this fixes the
    tests instead with a new paravirt op. It's a little ugly because
    ptrace.h can't include paravirt.h.

    Signed-off-by: Andy Lutomirski <luto@mit.edu>
    Link: http://lkml.kernel.org/r/f4fcb3947340d9e96ce1054a432f183f9da9db83.1312378163.git.luto@mit.edu
    Reported-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
    Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
    ---
    arch/x86/include/asm/desc.h | 4 ++--
    arch/x86/include/asm/paravirt_types.h | 6 ++++++
    arch/x86/include/asm/ptrace.h | 19 +++++++++++++++++++
    arch/x86/kernel/paravirt.c | 4 ++++
    arch/x86/kernel/step.c | 2 +-
    arch/x86/kernel/vsyscall_64.c | 6 +-----
    arch/x86/mm/fault.c | 2 +-
    arch/x86/xen/enlighten.c | 4 ++++
    8 files changed, 38 insertions(+), 9 deletions(-)

    diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
    index 7b439d9..41935fa 100644
    --- a/arch/x86/include/asm/desc.h
    +++ b/arch/x86/include/asm/desc.h
    @@ -27,8 +27,8 @@ static inline void fill_ldt(struct desc_struct *desc, const struct user_desc *in

    desc->base2 = (info->base_addr & 0xff000000) >> 24;
    /*
    - * Don't allow setting of the lm bit. It is useless anyway
    - * because 64bit system calls require __USER_CS:
    + * Don't allow setting of the lm bit. It would confuse
    + * user_64bit_mode and would get overridden by sysret anyway.
    */
    desc->l = 0;
    }
    diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h
    index 8288509..96a0f80 100644
    --- a/arch/x86/include/asm/paravirt_types.h
    +++ b/arch/x86/include/asm/paravirt_types.h
    @@ -41,6 +41,7 @@

    #include <asm/desc_defs.h>
    #include <asm/kmap_types.h>
    +#include <asm/pgtable_types.h>

    struct page;
    struct thread_struct;
    @@ -63,6 +64,11 @@ struct paravirt_callee_save {
    struct pv_info {
    unsigned int kernel_rpl;
    int shared_kernel_pmd;
    +
    +#ifdef CONFIG_X86_64
    + u16 extra_user_64bit_cs; /* __USER_CS if none */
    +#endif
    +
    int paravirt_enabled;
    const char *name;
    };
    diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
    index 94e7618..3566454 100644
    --- a/arch/x86/include/asm/ptrace.h
    +++ b/arch/x86/include/asm/ptrace.h
    @@ -131,6 +131,9 @@ struct pt_regs {
    #ifdef __KERNEL__

    #include <linux/init.h>
    +#ifdef CONFIG_PARAVIRT
    +#include <asm/paravirt_types.h>
    +#endif

    struct cpuinfo_x86;
    struct task_struct;
    @@ -187,6 +190,22 @@ static inline int v8086_mode(struct pt_regs *regs)
    #endif
    }

    +#ifdef CONFIG_X86_64
    +static inline bool user_64bit_mode(struct pt_regs *regs)
    +{
    +#ifndef CONFIG_PARAVIRT
    + /*
    + * On non-paravirt systems, this is the only long mode CPL 3
    + * selector. We do not allow long mode selectors in the LDT.
    + */
    + return regs->cs == __USER_CS;
    +#else
    + /* Headers are too twisted for this to go in paravirt.h. */
    + return regs->cs == __USER_CS || regs->cs == pv_info.extra_user_64bit_cs;
    +#endif
    +}
    +#endif
    +
    /*
    * X86_32 CPUs don't save ss and esp if the CPU is already in kernel mode
    * when it traps. The previous stack will be directly underneath the saved
    diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
    index 869e1ae..681f159 100644
    --- a/arch/x86/kernel/paravirt.c
    +++ b/arch/x86/kernel/paravirt.c
    @@ -299,6 +299,10 @@ struct pv_info pv_info = {
    .paravirt_enabled = 0,
    .kernel_rpl = 0,
    .shared_kernel_pmd = 1, /* Only used when CONFIG_X86_PAE is set */
    +
    +#ifdef CONFIG_X86_64
    + .extra_user_64bit_cs = __USER_CS,
    +#endif
    };

    struct pv_init_ops pv_init_ops = {
    diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
    index 7977f0c..c346d11 100644
    --- a/arch/x86/kernel/step.c
    +++ b/arch/x86/kernel/step.c
    @@ -74,7 +74,7 @@ static int is_setting_trap_flag(struct task_struct *child, struct pt_regs *regs)

    #ifdef CONFIG_X86_64
    case 0x40 ... 0x4f:
    - if (regs->cs != __USER_CS)
    + if (!user_64bit_mode(regs))
    /* 32-bit mode: register increment */
    return 0;
    /* 64-bit mode: REX prefix */
    diff --git a/arch/x86/kernel/vsyscall_64.c b/arch/x86/kernel/vsyscall_64.c
    index dda7dff..1725930 100644
    --- a/arch/x86/kernel/vsyscall_64.c
    +++ b/arch/x86/kernel/vsyscall_64.c
    @@ -127,11 +127,7 @@ void dotraplinkage do_emulate_vsyscall(struct pt_regs *regs, long error_code)

    local_irq_enable();

    - /*
    - * Real 64-bit user mode code has cs == __USER_CS. Anything else
    - * is bogus.
    - */
    - if (regs->cs != __USER_CS) {
    + if (!user_64bit_mode(regs)) {
    /*
    * If we trapped from kernel mode, we might as well OOPS now
    * instead of returning to some random address and OOPSing
    diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
    index 2dbf6bf..c1d0182 100644
    --- a/arch/x86/mm/fault.c
    +++ b/arch/x86/mm/fault.c
    @@ -105,7 +105,7 @@ check_prefetch_opcode(struct pt_regs *regs, unsigned char *instr,
    * but for now it's good enough to assume that long
    * mode only uses well known segments or kernel.
    */
    - return (!user_mode(regs)) || (regs->cs == __USER_CS);
    + return (!user_mode(regs) || user_64bit_mode(regs));
    #endif
    case 0x60:
    /* 0x64 thru 0x67 are valid prefixes in all modes. */
    diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
    index 5525163..78fe33d 100644
    --- a/arch/x86/xen/enlighten.c
    +++ b/arch/x86/xen/enlighten.c
    @@ -937,6 +937,10 @@ static const struct pv_info xen_info __initconst = {
    .paravirt_enabled = 1,
    .shared_kernel_pmd = 0,

    +#ifdef CONFIG_X86_64
    + .extra_user_64bit_cs = FLAT_USER_CS64,
    +#endif
    +
    .name = "Xen",
    };


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