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    Subject[PATCH 3.2 051/104] x86/alternatives: Make optimize_nops() interrupt safe and synced
    3.2.101-rc1 review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Thomas Gleixner <tglx@linutronix.de>

    commit 66c117d7fa2ae429911e60d84bf31a90b2b96189 upstream.

    Richard reported the following crash:

    [ 0.036000] BUG: unable to handle kernel paging request at 55501e06
    [ 0.036000] IP: [<c0aae48b>] common_interrupt+0xb/0x38
    [ 0.036000] Call Trace:
    [ 0.036000] [<c0409c80>] ? add_nops+0x90/0xa0
    [ 0.036000] [<c040a054>] apply_alternatives+0x274/0x630

    Chuck decoded:

    " 0: 8d 90 90 83 04 24 lea 0x24048390(%eax),%edx
    6: 80 fc 0f cmp $0xf,%ah
    9: a8 0f test $0xf,%al
    >> b: a0 06 1e 50 55 mov 0x55501e06,%al
    10: 57 push %edi
    11: 56 push %esi

    Interrupt 0x30 occurred while the alternatives code was replacing the
    initial 0x90,0x90,0x90 NOPs (from the ASM_CLAC macro) with the
    optimized version, 0x8d,0x76,0x00. Only the first byte has been
    replaced so far, and it makes a mess out of the insn decoding."

    optimize_nops() is buggy in two aspects:

    - It's not disabling interrupts across the modification
    - It's lacking a sync_core() call

    Add both.

    Fixes: 4fd4b6e5537c 'x86/alternatives: Use optimized NOPs for padding'
    Reported-and-tested-by: "Richard W.M. Jones" <rjones@redhat.com>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Cc: Richard W.M. Jones <rjones@redhat.com>
    Cc: Chuck Ebbert <cebbert.lkml@gmail.com>
    Cc: Borislav Petkov <bp@alien8.de>
    Link: http://lkml.kernel.org/r/alpine.DEB.2.11.1509031232340.15006@nanos
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    arch/x86/kernel/alternative.c | 5 +++++
    1 file changed, 5 insertions(+)

    --- a/arch/x86/kernel/alternative.c
    +++ b/arch/x86/kernel/alternative.c
    @@ -323,10 +323,15 @@ done:

    static void __init_or_module optimize_nops(struct alt_instr *a, u8 *instr)
    {
    + unsigned long flags;
    +
    if (instr[0] != 0x90)
    return;

    + local_irq_save(flags);
    add_nops(instr + (a->instrlen - a->padlen), a->padlen);
    + sync_core();
    + local_irq_restore(flags);

    DUMP_BYTES(instr, a->instrlen, "%p: [%d:%d) optimized NOPs: ",
    instr, a->instrlen - a->padlen, a->padlen);
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