Re: [RFC PATCH 1/6] x86/alternative: assert text_mutex is taken
From: Masami Hiramatsu <mhiramat@kernel.org>
Date: 2018-08-29 08:59:43
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On Wed, 29 Aug 2018 01:11:42 -0700 Nadav Amit [off-list ref] wrote:
Use lockdep to ensure that text_mutex is taken when text_poke() is called. Actually it is not always taken, specifically when it is called by kgdb, so take the lock in these cases.
Can we really take a mutex in kgdb context?
kgdb_arch_remove_breakpoint
<- dbg_deactivate_sw_breakpoints
<- kgdb_reenter_check
<- kgdb_handle_exception
<- __kgdb_notify
<- kgdb_ll_trap
<- do_int3
<- kgdb_notify
<- die notifier
kgdb_arch_set_breakpoint
<- dbg_activate_sw_breakpoints
<- kgdb_reenter_check
<- kgdb_handle_exception
...
Both seems called in exception context, so we can not take a mutex lock.
I think kgdb needs a special path.
Thanks,
quoted hunk ↗ jump to hunk
Cc: Andy Lutomirski <luto@kernel.org> Cc: Masami Hiramatsu <mhiramat@kernel.org> Cc: Kees Cook <redacted> Suggested-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Nadav Amit <redacted> --- arch/x86/kernel/alternative.c | 1 + arch/x86/kernel/kgdb.c | 9 +++++++++ 2 files changed, 10 insertions(+)diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c index 014f214da581..916c11b410c4 100644 --- a/arch/x86/kernel/alternative.c +++ b/arch/x86/kernel/alternative.c@@ -699,6 +699,7 @@ void *text_poke(void *addr, const void *opcode, size_t len) * pages as they are not yet initialized. */ BUG_ON(!after_bootmem); + lockdep_assert_held(&text_mutex); if (!core_kernel_text((unsigned long)addr)) { pages[0] = vmalloc_to_page(addr);diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c index 8e36f249646e..60b99c76086c 100644 --- a/arch/x86/kernel/kgdb.c +++ b/arch/x86/kernel/kgdb.c@@ -768,8 +768,12 @@ int kgdb_arch_set_breakpoint(struct kgdb_bkpt *bpt) */ if (mutex_is_locked(&text_mutex)) return -EBUSY; + + /* Take the mutex to avoid lockdep assertion failures. */ + mutex_lock(&text_mutex); text_poke((void *)bpt->bpt_addr, arch_kgdb_ops.gdb_bpt_instr, BREAK_INSTR_SIZE); + mutex_unlock(&text_mutex); err = probe_kernel_read(opc, (char *)bpt->bpt_addr, BREAK_INSTR_SIZE); if (err) return err;@@ -793,7 +797,12 @@ int kgdb_arch_remove_breakpoint(struct kgdb_bkpt *bpt) */ if (mutex_is_locked(&text_mutex)) goto knl_write; + + /* Take the mutex to avoid lockdep assertion failures. */ + mutex_lock(&text_mutex); text_poke((void *)bpt->bpt_addr, bpt->saved_instr, BREAK_INSTR_SIZE); + mutex_unlock(&text_mutex); + 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
-- Masami Hiramatsu [off-list ref]