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    Subject[PATCH V3 04/49] x86/xen: Add xenpv_restore_regs_and_return_to_usermode()
    Date
    From: Lai Jiangshan <laijs@linux.alibaba.com>

    While in the native case, PER_CPU_VAR(cpu_tss_rw + TSS_sp0) is the
    trampoline stack. But XEN pv doesn't use trampoline stack, so
    PER_CPU_VAR(cpu_tss_rw + TSS_sp0) is also the kernel stack. Hence source
    and destination stacks are identical in that case, which means reusing
    swapgs_restore_regs_and_return_to_usermode() in XEN pv would cause %rsp
    to move up to the top of the kernel stack and leave the IRET frame below
    %rsp, which is dangerous to be corrupted if #NMI / #MC hit as either of
    these events occurring in the middle of the stack pushing would clobber
    data on the (original) stack.

    And swapgs_restore_regs_and_return_to_usermode() pushing the IRET frame
    on to the original address is useless and error-prone when there is any
    future attempt to modify the code.

    Fixes: 7f2590a110b8 ("x86/entry/64: Use a per-CPU trampoline stack for IDT entries")
    Cc: Jan Beulich <jbeulich@suse.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Juergen Gross <jgross@suse.com>
    Cc: Peter Anvin <hpa@zytor.com>
    Cc: xen-devel@lists.xenproject.org>
    Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
    Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
    ---
    arch/x86/entry/entry_64.S | 9 ++++++---
    arch/x86/entry/entry_64_compat.S | 7 ++++---
    arch/x86/xen/xen-asm.S | 27 +++++++++++++++++++++++++++
    3 files changed, 37 insertions(+), 6 deletions(-)

    diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
    index 9d468c8877e2..0dde5a253dda 100644
    --- a/arch/x86/entry/entry_64.S
    +++ b/arch/x86/entry/entry_64.S
    @@ -119,7 +119,7 @@ SYM_INNER_LABEL(entry_SYSCALL_64_after_hwframe, SYM_L_GLOBAL)
    * In the Xen PV case we must use iret anyway.
    */

    - ALTERNATIVE "", "jmp swapgs_restore_regs_and_return_to_usermode", \
    + ALTERNATIVE "", "jmp xenpv_restore_regs_and_return_to_usermode", \
    X86_FEATURE_XENPV

    movq RCX(%rsp), %rcx
    @@ -286,7 +286,8 @@ SYM_CODE_START(ret_from_fork)
    UNWIND_HINT_REGS
    movq %rsp, %rdi
    call syscall_exit_to_user_mode /* returns with IRQs disabled */
    - jmp swapgs_restore_regs_and_return_to_usermode
    + ALTERNATIVE "jmp swapgs_restore_regs_and_return_to_usermode", \
    + "jmp xenpv_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV

    1:
    /* kernel thread */
    @@ -566,6 +567,7 @@ __irqentry_text_start:
    __irqentry_text_end:

    SYM_CODE_START_LOCAL(common_interrupt_return)
    +SYM_INNER_LABEL(xenpv_restore_regs_and_return_to_usermode, SYM_L_WEAK) /* placeholder */
    SYM_INNER_LABEL(swapgs_restore_regs_and_return_to_usermode, SYM_L_GLOBAL)
    #ifdef CONFIG_DEBUG_ENTRY
    /* Assert that pt_regs indicates user mode. */
    @@ -1055,7 +1057,8 @@ SYM_CODE_START_LOCAL(error_return)
    DEBUG_ENTRY_ASSERT_IRQS_OFF
    testb $3, CS(%rsp)
    jz restore_regs_and_return_to_kernel
    - jmp swapgs_restore_regs_and_return_to_usermode
    + ALTERNATIVE "jmp swapgs_restore_regs_and_return_to_usermode", \
    + "jmp xenpv_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
    SYM_CODE_END(error_return)

    /*
    diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
    index 0051cf5c792d..2a4d9532dfd5 100644
    --- a/arch/x86/entry/entry_64_compat.S
    +++ b/arch/x86/entry/entry_64_compat.S
    @@ -139,7 +139,7 @@ SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
    call do_SYSENTER_32
    /* XEN PV guests always use IRET path */
    ALTERNATIVE "testl %eax, %eax; jz swapgs_restore_regs_and_return_to_usermode", \
    - "jmp swapgs_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
    + "jmp xenpv_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
    jmp sysret32_from_system_call

    .Lsysenter_fix_flags:
    @@ -256,7 +256,7 @@ SYM_INNER_LABEL(entry_SYSCALL_compat_after_hwframe, SYM_L_GLOBAL)
    call do_fast_syscall_32
    /* XEN PV guests always use IRET path */
    ALTERNATIVE "testl %eax, %eax; jz swapgs_restore_regs_and_return_to_usermode", \
    - "jmp swapgs_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
    + "jmp xenpv_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV

    /* Opportunistic SYSRET */
    sysret32_from_system_call:
    @@ -411,5 +411,6 @@ SYM_CODE_START(entry_INT80_compat)

    movq %rsp, %rdi
    call do_int80_syscall_32
    - jmp swapgs_restore_regs_and_return_to_usermode
    + ALTERNATIVE "jmp swapgs_restore_regs_and_return_to_usermode", \
    + "jmp xenpv_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
    SYM_CODE_END(entry_INT80_compat)
    diff --git a/arch/x86/xen/xen-asm.S b/arch/x86/xen/xen-asm.S
    index 220dd9678494..032be1bab113 100644
    --- a/arch/x86/xen/xen-asm.S
    +++ b/arch/x86/xen/xen-asm.S
    @@ -20,6 +20,7 @@

    #include <linux/init.h>
    #include <linux/linkage.h>
    +#include <../entry/calling.h>

    .pushsection .noinstr.text, "ax"
    /*
    @@ -192,6 +193,32 @@ SYM_CODE_START(xen_iret)
    jmp hypercall_iret
    SYM_CODE_END(xen_iret)

    +/*
    + * XEN pv doesn't use trampoline stack, PER_CPU_VAR(cpu_tss_rw + TSS_sp0) is
    + * also the kernel stack. Reusing swapgs_restore_regs_and_return_to_usermode()
    + * in XEN pv would cause %rsp to move up to the top of the kernel stack and
    + * leave the IRET frame below %rsp, which is dangerous to be corrupted if #NMI
    + * interrupts. And swapgs_restore_regs_and_return_to_usermode() pushing the IRET
    + * frame at the same address is useless.
    + */
    +SYM_CODE_START(xenpv_restore_regs_and_return_to_usermode)
    + UNWIND_HINT_REGS
    +#ifdef CONFIG_DEBUG_ENTRY
    + /* Assert that pt_regs indicates user mode. */
    + testb $3, CS(%rsp)
    + jnz 1f
    + ud2
    +1:
    +#endif
    + POP_REGS
    +
    + /* stackleak_erase() can work safely on the kernel stack. */
    + STACKLEAK_ERASE_NOCLOBBER
    +
    + addq $8, %rsp /* skip regs->orig_ax */
    + jmp xen_iret
    +SYM_CODE_END(xenpv_restore_regs_and_return_to_usermode)
    +
    /*
    * Xen handles syscall callbacks much like ordinary exceptions, which
    * means we have:
    --
    2.19.1.6.gb485710b
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