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    Subject[PATCH 4.14 029/114] x86/entry/64: Fix unwind hints in kernel exit path
    Date
    From: Josh Poimboeuf <jpoimboe@redhat.com>

    commit 1fb143634a38095b641a3a21220774799772dc4c upstream.

    In swapgs_restore_regs_and_return_to_usermode, after the stack is
    switched to the trampoline stack, the existing UNWIND_HINT_REGS hint is
    no longer valid, which can result in the following ORC unwinder warning:

    WARNING: can't dereference registers at 000000003aeb0cdd for ip swapgs_restore_regs_and_return_to_usermode+0x93/0xa0

    For full correctness, we could try to add complicated unwind hints so
    the unwinder could continue to find the registers, but when when it's
    this close to kernel exit, unwind hints aren't really needed anymore and
    it's fine to just use an empty hint which tells the unwinder to stop.

    For consistency, also move the UNWIND_HINT_EMPTY in
    entry_SYSCALL_64_after_hwframe to a similar location.

    Fixes: 3e3b9293d392 ("x86/entry/64: Return to userspace from the trampoline stack")
    Reported-by: Vince Weaver <vincent.weaver@maine.edu>
    Reported-by: Dave Jones <dsj@fb.com>
    Reported-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
    Reported-by: Joe Mario <jmario@redhat.com>
    Reported-by: Jann Horn <jannh@google.com>
    Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
    Reviewed-by: Miroslav Benes <mbenes@suse.cz>
    Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Link: https://lore.kernel.org/r/60ea8f562987ed2d9ace2977502fe481c0d7c9a0.1587808742.git.jpoimboe@redhat.com
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/x86/entry/entry_64.S | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    --- a/arch/x86/entry/entry_64.S
    +++ b/arch/x86/entry/entry_64.S
    @@ -302,7 +302,6 @@ GLOBAL(entry_SYSCALL_64_after_hwframe)
    */
    syscall_return_via_sysret:
    /* rcx and r11 are already restored (see code above) */
    - UNWIND_HINT_EMPTY
    POP_REGS pop_rdi=0 skip_r11rcx=1

    /*
    @@ -311,6 +310,7 @@ syscall_return_via_sysret:
    */
    movq %rsp, %rdi
    movq PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %rsp
    + UNWIND_HINT_EMPTY

    pushq RSP-RDI(%rdi) /* RSP */
    pushq (%rdi) /* RDI */
    @@ -606,6 +606,7 @@ GLOBAL(swapgs_restore_regs_and_return_to
    */
    movq %rsp, %rdi
    movq PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %rsp
    + UNWIND_HINT_EMPTY

    /* Copy the IRET frame to the trampoline stack. */
    pushq 6*8(%rdi) /* SS */

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