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    Subject[PATCH v5 01/23] Fix "x86/alternatives: Lockdep-enforce text_mutex in text_poke*()"
    Date
    text_mutex is currently expected to be held before text_poke() is
    called, but kgdb does not take the mutex, and instead *supposedly*
    ensures the lock is not taken and will not be acquired by any other core
    while text_poke() is running.

    The reason for the "supposedly" comment is that it is not entirely clear
    that this would be the case if gdb_do_roundup is zero.

    Create two wrapper functions, text_poke() and text_poke_kgdb(), which do
    or do not run the lockdep assertion respectively.

    While we are at it, change the return code of text_poke() to something
    meaningful. One day, callers might actually respect it and the existing
    BUG_ON() when patching fails could be removed. For kgdb, the return
    value can actually be used.

    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: Kees Cook <keescook@chromium.org>
    Cc: Dave Hansen <dave.hansen@intel.com>
    Cc: Masami Hiramatsu <mhiramat@kernel.org>
    Fixes: 9222f606506c ("x86/alternatives: Lockdep-enforce text_mutex in text_poke*()")
    Suggested-by: Peter Zijlstra <peterz@infradead.org>
    Acked-by: Jiri Kosina <jkosina@suse.cz>
    Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
    Signed-off-by: Nadav Amit <namit@vmware.com>
    Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
    ---
    arch/x86/include/asm/text-patching.h | 1 +
    arch/x86/kernel/alternative.c | 52 ++++++++++++++++++++--------
    arch/x86/kernel/kgdb.c | 11 +++---
    3 files changed, 45 insertions(+), 19 deletions(-)

    diff --git a/arch/x86/include/asm/text-patching.h b/arch/x86/include/asm/text-patching.h
    index e85ff65c43c3..f8fc8e86cf01 100644
    --- a/arch/x86/include/asm/text-patching.h
    +++ b/arch/x86/include/asm/text-patching.h
    @@ -35,6 +35,7 @@ extern void *text_poke_early(void *addr, const void *opcode, size_t len);
    * inconsistent instruction while you patch.
    */
    extern void *text_poke(void *addr, const void *opcode, size_t len);
    +extern void *text_poke_kgdb(void *addr, const void *opcode, size_t len);
    extern int poke_int3_handler(struct pt_regs *regs);
    extern void *text_poke_bp(void *addr, const void *opcode, size_t len, void *handler);
    extern int after_bootmem;
    diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
    index 9a79c7808f9c..0a814d73547a 100644
    --- a/arch/x86/kernel/alternative.c
    +++ b/arch/x86/kernel/alternative.c
    @@ -679,18 +679,7 @@ void *__init_or_module text_poke_early(void *addr, const void *opcode,
    return addr;
    }

    -/**
    - * text_poke - Update instructions on a live kernel
    - * @addr: address to modify
    - * @opcode: source of the copy
    - * @len: length to copy
    - *
    - * Only atomic text poke/set should be allowed when not doing early patching.
    - * It means the size must be writable atomically and the address must be aligned
    - * in a way that permits an atomic write. It also makes sure we fit on a single
    - * page.
    - */
    -void *text_poke(void *addr, const void *opcode, size_t len)
    +static void *__text_poke(void *addr, const void *opcode, size_t len)
    {
    unsigned long flags;
    char *vaddr;
    @@ -703,8 +692,6 @@ void *text_poke(void *addr, const void *opcode, size_t len)
    */
    BUG_ON(!after_bootmem);

    - lockdep_assert_held(&text_mutex);
    -
    if (!core_kernel_text((unsigned long)addr)) {
    pages[0] = vmalloc_to_page(addr);
    pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
    @@ -733,6 +720,43 @@ void *text_poke(void *addr, const void *opcode, size_t len)
    return addr;
    }

    +/**
    + * text_poke - Update instructions on a live kernel
    + * @addr: address to modify
    + * @opcode: source of the copy
    + * @len: length to copy
    + *
    + * Only atomic text poke/set should be allowed when not doing early patching.
    + * It means the size must be writable atomically and the address must be aligned
    + * in a way that permits an atomic write. It also makes sure we fit on a single
    + * page.
    + */
    +void *text_poke(void *addr, const void *opcode, size_t len)
    +{
    + lockdep_assert_held(&text_mutex);
    +
    + return __text_poke(addr, opcode, len);
    +}
    +
    +/**
    + * text_poke_kgdb - Update instructions on a live kernel by kgdb
    + * @addr: address to modify
    + * @opcode: source of the copy
    + * @len: length to copy
    + *
    + * Only atomic text poke/set should be allowed when not doing early patching.
    + * It means the size must be writable atomically and the address must be aligned
    + * in a way that permits an atomic write. It also makes sure we fit on a single
    + * page.
    + *
    + * Context: should only be used by kgdb, which ensures no other core is running,
    + * despite the fact it does not hold the text_mutex.
    + */
    +void *text_poke_kgdb(void *addr, const void *opcode, size_t len)
    +{
    + return __text_poke(addr, opcode, len);
    +}
    +
    static void do_sync_core(void *info)
    {
    sync_core();
    diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c
    index 4ff6b4cdb941..2b203ee5b879 100644
    --- a/arch/x86/kernel/kgdb.c
    +++ b/arch/x86/kernel/kgdb.c
    @@ -759,13 +759,13 @@ int kgdb_arch_set_breakpoint(struct kgdb_bkpt *bpt)
    if (!err)
    return err;
    /*
    - * It is safe to call text_poke() because normal kernel execution
    + * It is safe to call text_poke_kgdb() because normal kernel execution
    * is stopped on all cores, so long as the text_mutex is not locked.
    */
    if (mutex_is_locked(&text_mutex))
    return -EBUSY;
    - text_poke((void *)bpt->bpt_addr, arch_kgdb_ops.gdb_bpt_instr,
    - BREAK_INSTR_SIZE);
    + text_poke_kgdb((void *)bpt->bpt_addr, arch_kgdb_ops.gdb_bpt_instr,
    + BREAK_INSTR_SIZE);
    err = probe_kernel_read(opc, (char *)bpt->bpt_addr, BREAK_INSTR_SIZE);
    if (err)
    return err;
    @@ -784,12 +784,13 @@ int kgdb_arch_remove_breakpoint(struct kgdb_bkpt *bpt)
    if (bpt->type != BP_POKE_BREAKPOINT)
    goto knl_write;
    /*
    - * It is safe to call text_poke() because normal kernel execution
    + * It is safe to call text_poke_kgdb() because normal kernel execution
    * is stopped on all cores, so long as the text_mutex is not locked.
    */
    if (mutex_is_locked(&text_mutex))
    goto knl_write;
    - text_poke((void *)bpt->bpt_addr, bpt->saved_instr, BREAK_INSTR_SIZE);
    + text_poke_kgdb((void *)bpt->bpt_addr, bpt->saved_instr,
    + BREAK_INSTR_SIZE);
    err = probe_kernel_read(opc, (char *)bpt->bpt_addr, BREAK_INSTR_SIZE);
    if (err || memcmp(opc, bpt->saved_instr, BREAK_INSTR_SIZE))
    goto knl_write;
    --
    2.17.1
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