linux-kernel - Re: [uaccess] 780464aed0: WARNING:at_arch/x86/include/asm/uaccess.h:#strnlen

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Date:   Tue, 5 Mar 2019 18:01:26 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        kernel test robot <lkp@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Shuah Khan <shuah@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Changbin Du <changbin.du@...il.com>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...nel.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Nadav Amit <namit@...are.com>,
        Joel Fernandes <joel@...lfernandes.org>, yhs@...com, lkp@...org
Subject: Re: [uaccess] 780464aed0:
 WARNING:at_arch/x86/include/asm/uaccess.h:#strnlen_user/0x

On Tue, 5 Mar 2019 17:22:41 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:

> On Tue, 5 Mar 2019 11:36:35 +0900
> Masami Hiramatsu <mhiramat@...nel.org> wrote:
> 
> > On Mon, 4 Mar 2019 10:59:22 -0800
> > Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> > 
> > I think the better way to do this is allowing strncpy_from_user()
> > if some conditions are match, like
> > 
> >  - strncpy_from_user() will be able to copy user memory with set_fs(USER_DS)
> >  - strncpy_from_user() can copy kernel memory with set_fs(KERNEL_DS)
> >  - strncpy_from_user() can access unsafe memory in IRQ context if
> >    pagefault is disabled.
> > 
> > This is almost done, except for CONFIG_DEBUG_ATOMIC_SLEEP=y on x86.
> > 
> > So, what about adding a condition to WARN_ON_IN_IRQ() like below
> > instead of introducing user_access_ok() ?

OK, here is the patch. Does it work for us?

====
x86/uaccess: Verify access_ok() context strictly

From: Masami Hiramatsu <mhiramat@...nel.org>

WARN_ON_IN_IRQ() assumes that the access_ok() and following
user memory access can sleep. But this assumption is not
always correct; when the pagefault is disabled, following
memory access will just returns -EFAULT and never sleep.

Add pagefault_disabled() check in WARN_ON_ONCE() so that
it can ignore the case we call it with disabling pagefault.
For this purpose, this modified pagefault_disabled() as
an inline function.

Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
---
 arch/x86/include/asm/uaccess.h |    4 +++-
 include/linux/uaccess.h        |    5 ++++-
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 780f2b4..b98a552 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -70,7 +70,9 @@ static inline bool __chk_range_not_ok(unsigned long addr, unsigned long size, un
 })
 
 #ifdef CONFIG_DEBUG_ATOMIC_SLEEP
-# define WARN_ON_IN_IRQ()	WARN_ON_ONCE(!in_task())
+static inline bool pagefault_disabled(void);
+# define WARN_ON_IN_IRQ()	\
+	WARN_ON_ONCE(!in_task() && !pagefault_disabled())
 #else
 # define WARN_ON_IN_IRQ()
 #endif
diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 37b226e..ef3032d 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -203,7 +203,10 @@ static inline void pagefault_enable(void)
 /*
  * Is the pagefault handler disabled? If so, user access methods will not sleep.
  */
-#define pagefault_disabled() (current->pagefault_disabled != 0)
+static inline bool pagefault_disabled(void)
+{
+	return current->pagefault_disabled != 0;
+}
 
 /*
  * The pagefault handler is in general disabled by pagefault_disable() or