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    Subject[tip: x86/urgent] x86/xen: Add xenpv_restore_regs_and_return_to_usermode()
    The following commit has been merged into the x86/urgent branch of tip:

    Commit-ID: 5c8f6a2e316efebb3ba93d8c1af258155dcf5632
    Gitweb: https://git.kernel.org/tip/5c8f6a2e316efebb3ba93d8c1af258155dcf5632
    Author: Lai Jiangshan <laijs@linux.alibaba.com>
    AuthorDate: Fri, 26 Nov 2021 18:11:23 +08:00
    Committer: Borislav Petkov <bp@suse.de>
    CommitterDate: Fri, 03 Dec 2021 19:21:15 +01:00

    x86/xen: Add xenpv_restore_regs_and_return_to_usermode()

    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.

    In that case, source and destination stacks are identical, which means
    that 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.

    This is dangerous as it can 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, with XEN pv, 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.

    [ bp: Massage commit message. ]

    Fixes: 7f2590a110b8 ("x86/entry/64: Use a per-CPU trampoline stack for IDT entries")
    Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
    Signed-off-by: Borislav Petkov <bp@suse.de>
    Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
    Link: https://lkml.kernel.org/r/20211126101209.8613-4-jiangshanlai@gmail.com
    ---
    arch/x86/entry/entry_64.S | 4 ++++
    arch/x86/xen/xen-asm.S | 20 ++++++++++++++++++++
    2 files changed, 24 insertions(+)

    diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
    index f9e1c06..97b1f84 100644
    --- a/arch/x86/entry/entry_64.S
    +++ b/arch/x86/entry/entry_64.S
    @@ -574,6 +574,10 @@ SYM_INNER_LABEL(swapgs_restore_regs_and_return_to_usermode, SYM_L_GLOBAL)
    ud2
    1:
    #endif
    +#ifdef CONFIG_XEN_PV
    + ALTERNATIVE "", "jmp xenpv_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
    +#endif
    +
    POP_REGS pop_rdi=0

    /*
    diff --git a/arch/x86/xen/xen-asm.S b/arch/x86/xen/xen-asm.S
    index 220dd96..444d824 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"
    /*
    @@ -193,6 +194,25 @@ SYM_CODE_START(xen_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
    + 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:
    * - kernel gs
    \
     
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