[PATCH] exit: Allow oops_limit to be disabled

[PATCH] exit: Allow oops_limit to be disabled

[Kees Cook]

In preparation for keeping oops_limit logic in sync with warn_limit,
have oops_limit == 0 disable checking the Oops counter.

Cc: Jann Horn <jannh@google.com>
Cc: Seth Jenkins <sethjenkins@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: Eric Biggers <ebiggers@google.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
I've changed the warn_limit logic to match...
---
 Documentation/admin-guide/sysctl/kernel.rst | 5 +++--
 kernel/exit.c                               | 2 +-
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index 09f3fb2f8585..a31d8d81ea07 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -671,8 +671,9 @@ oops_limit
 ==========
 
 Number of kernel oopses after which the kernel should panic when
-``panic_on_oops`` is not set. Setting this to 0 or 1 has the same effect
-as setting ``panic_on_oops=1``.
+``panic_on_oops`` is not set. Setting this to 0 disables checking
+the count. Setting this to  1 has the same effect as setting
+``panic_on_oops=1``. The default value is 10000.
 
 
 osrelease, ostype & version
diff --git a/kernel/exit.c b/kernel/exit.c
index dc1a32149f94..deffb8e4b1b2 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -954,7 +954,7 @@ void __noreturn make_task_dead(int signr)
 	 * To make sure this can't happen, place an upper bound on how often the
 	 * kernel may oops without panic().
 	 */
-	if (atomic_inc_return(&oops_count) >= READ_ONCE(oops_limit))
+	if (atomic_inc_return(&oops_count) >= READ_ONCE(oops_limit) && oops_limit)
 		panic("Oopsed too often (kernel.oops_limit is %d)", oops_limit);
 
 	/*
-- 
2.34.1

Re: [PATCH] exit: Allow oops_limit to be disabled

[Peter Zijlstra]

On Fri, Dec 02, 2022 at 01:06:21PM -0800, Kees Cook wrote:

> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -954,7 +954,7 @@ void __noreturn make_task_dead(int signr)
>  	 * To make sure this can't happen, place an upper bound on how often the
>  	 * kernel may oops without panic().
>  	 */
> -	if (atomic_inc_return(&oops_count) >= READ_ONCE(oops_limit))
> +	if (atomic_inc_return(&oops_count) >= READ_ONCE(oops_limit) && oops_limit)
>  		panic("Oopsed too often (kernel.oops_limit is %d)", oops_limit);
>  

That's dodgy, please write as:

	limit = READ_ONCE(oops_limit);
	if (atomic_inc_return(&oops_count) >= limit && limit)

So we don't explicitly add a reload that negates the whole READ_ONCE().

Re: [PATCH] exit: Allow oops_limit to be disabled

[Kees Cook]

On Fri, Dec 16, 2022 at 03:05:13PM +0100, Peter Zijlstra wrote:
> On Fri, Dec 02, 2022 at 01:06:21PM -0800, Kees Cook wrote:
> 
> > --- a/kernel/exit.c
> > +++ b/kernel/exit.c
> > @@ -954,7 +954,7 @@ void __noreturn make_task_dead(int signr)
> >  	 * To make sure this can't happen, place an upper bound on how often the
> >  	 * kernel may oops without panic().
> >  	 */
> > -	if (atomic_inc_return(&oops_count) >= READ_ONCE(oops_limit))
> > +	if (atomic_inc_return(&oops_count) >= READ_ONCE(oops_limit) && oops_limit)
> >  		panic("Oopsed too often (kernel.oops_limit is %d)", oops_limit);
> >  
> 
> That's dodgy, please write as:
> 
> 	limit = READ_ONCE(oops_limit);
> 	if (atomic_inc_return(&oops_count) >= limit && limit)
> 
> So we don't explicitly add a reload that negates the whole READ_ONCE().

Yup, that's more correct. Sent:
https://lore.kernel.org/lkml/20221216203024.never.640-kees@kernel.org

-- 
Kees Cook