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Date: Mon, 12 Nov 2018 11:54:19 +0900
From: Masami Hiramatsu <mhiramat@...nel.org>
To: Nadav Amit <namit@...are.com>
Cc: Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
<x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
Thomas Gleixner <tglx@...utronix.de>,
Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Jiri Kosina <jkosina@...e.cz>,
Andy Lutomirski <luto@...nel.org>,
Kees Cook <keescook@...omium.org>,
Dave Hansen <dave.hansen@...el.com>,
Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH v4 01/10] Fix
"x86/alternatives: Lockdep-enforce text_mutex in text_poke*()"
On Sat, 10 Nov 2018 15:17:23 -0800
Nadav Amit <namit@...are.com> wrote:
> text_mutex is currently expected to be held before text_poke() is
> called, but we 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.
>
> This patch creates 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.
Hm, this looks reasonable and good to me.
Reviewed-by: Masami Hiramatsu <mhiramat@...nel.org>
Thank you!
>
> Cc: Jiri Kosina <jkosina@...e.cz>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Dave Hansen <dave.hansen@...el.com>
> Cc: Masami Hiramatsu <mhiramat@...nel.org>
> Fixes: 9222f606506c ("x86/alternatives: Lockdep-enforce text_mutex in text_poke*()")
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Nadav Amit <namit@...are.com>
> ---
> arch/x86/include/asm/text-patching.h | 3 +-
> arch/x86/kernel/alternative.c | 72 +++++++++++++++++++++-------
> arch/x86/kernel/kgdb.c | 15 ++++--
> 3 files changed, 66 insertions(+), 24 deletions(-)
>
> diff --git a/arch/x86/include/asm/text-patching.h b/arch/x86/include/asm/text-patching.h
> index e85ff65c43c3..5a2600370763 100644
> --- a/arch/x86/include/asm/text-patching.h
> +++ b/arch/x86/include/asm/text-patching.h
> @@ -34,7 +34,8 @@ extern void *text_poke_early(void *addr, const void *opcode, size_t len);
> * On the local CPU you need to be protected again NMI or MCE handlers seeing an
> * inconsistent instruction while you patch.
> */
> -extern void *text_poke(void *addr, const void *opcode, size_t len);
> +extern int text_poke(void *addr, const void *opcode, size_t len);
> +extern int 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 ebeac487a20c..ebe9210dc92e 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -678,23 +678,12 @@ 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 int __text_poke(void *addr, const void *opcode, size_t len)
> {
> unsigned long flags;
> char *vaddr;
> struct page *pages[2];
> - int i;
> + int i, r = 0;
>
> /*
> * While boot memory allocator is runnig we cannot use struct
> @@ -702,8 +691,6 @@ 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);
> @@ -712,7 +699,8 @@ void *text_poke(void *addr, const void *opcode, size_t len)
> WARN_ON(!PageReserved(pages[0]));
> pages[1] = virt_to_page(addr + PAGE_SIZE);
> }
> - BUG_ON(!pages[0]);
> + if (!pages[0])
> + return -EFAULT;
> local_irq_save(flags);
> set_fixmap(FIX_TEXT_POKE0, page_to_phys(pages[0]));
> if (pages[1])
> @@ -727,9 +715,57 @@ void *text_poke(void *addr, const void *opcode, size_t len)
> /* Could also do a CLFLUSH here to speed up CPU recovery; but
> that causes hangs on some VIA CPUs. */
> for (i = 0; i < len; i++)
> - BUG_ON(((char *)addr)[i] != ((char *)opcode)[i]);
> + if (((char *)addr)[i] != ((char *)opcode)[i])
> + r = -EFAULT;
> local_irq_restore(flags);
> - return addr;
> + return r;
> +}
> +
> +/**
> + * 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.
> + */
> +int text_poke(void *addr, const void *opcode, size_t len)
> +{
> + int r;
> +
> + lockdep_assert_held(&text_mutex);
> +
> + r = __text_poke(addr, opcode, len);
> +
> + /*
> + * TODO: change the callers to consider the return value and remove this
> + * historical assertion.
> + */
> + BUG_ON(r);
> +
> + return r;
> +}
> +
> +/**
> + * 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.
> + */
> +int text_poke_kgdb(void *addr, const void *opcode, size_t len)
> +{
> + return __text_poke(addr, opcode, len);
> }
>
> static void do_sync_core(void *info)
> diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c
> index 8e36f249646e..8091b2e381d4 100644
> --- a/arch/x86/kernel/kgdb.c
> +++ b/arch/x86/kernel/kgdb.c
> @@ -763,13 +763,15 @@ 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);
> + err = text_poke_kgdb((void *)bpt->bpt_addr, arch_kgdb_ops.gdb_bpt_instr,
> + BREAK_INSTR_SIZE);
> + if (err)
> + return err;
> err = probe_kernel_read(opc, (char *)bpt->bpt_addr, BREAK_INSTR_SIZE);
> if (err)
> return err;
> @@ -788,12 +790,15 @@ 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);
> + err = text_poke_kgdb((void *)bpt->bpt_addr, bpt->saved_instr,
> + BREAK_INSTR_SIZE);
> + if (err)
> + return err;
> 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 <mhiramat@...nel.org>
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