Re: [PATCH v2 1/2] auditsc: audit_krule mask accesses need bounds checking

Eric Paris
2014-05-29T02:44:08Z

On Wed, 2014-05-28 at 19:27 -0700, Andy Lutomirski wrote:
> On Wed, May 28, 2014 at 7:23 PM, Eric Paris <eparis@redhat.com> wrote:
> > On Wed, 2014-05-28 at 18:44 -0700, Andy Lutomirski wrote:
> >> Fixes an easy DoS and possible information disclosure.
> >>
> >> This does nothing about the broken state of x32 auditing.
> >>
> >> Cc: stable@vger.kernel.org
> >> Signed-off-by: Andy Lutomirski <luto@amacapital.net>
> >> ---
> >> kernel/auditsc.c | 27 ++++++++++++++++++---------
> >> 1 file changed, 18 insertions(+), 9 deletions(-)
> >>
> >> diff --git a/kernel/auditsc.c b/kernel/auditsc.c
> >> index f251a5e..7ccd9db 100644
> >> --- a/kernel/auditsc.c
> >> +++ b/kernel/auditsc.c
> >> @@ -728,6 +728,22 @@ static enum audit_state audit_filter_task(struct task_struct *tsk, char **key)
> >> return AUDIT_BUILD_CONTEXT;
> >> }
> >>
> >> +static bool audit_in_mask(const struct audit_krule *rule, unsigned long val)
> >> +{
> >> + int word, bit;
> >> +
> >> + if (val > 0xffffffff)
> >> + return false;
> >
> > Why is this necessary?
>
> To avoid an integer overflow. Admittedly, this particular overflow
> won't cause a crash, but it will cause incorrect results.

You know this code pre-dates git? I admit, I'm shocked no one ever
noticed it before! This is ANCIENT. And clearly broken.

I'll likely ask Richard to add a WARN_ONCE() in both this place, and
below in word > AUDIT_BITMASK_SIZE so we might know if we ever need a
larger bitmask to store syscall numbers....

It'd be nice if lib had a efficient bitmask implementation...

> >
> >> +
> >> + word = AUDIT_WORD(val);
> >> + if (word >= AUDIT_BITMASK_SIZE)
> >> + return false;
> >
> > Since this covers it and it extensible...
> >
> >> +
> >> + bit = AUDIT_BIT(val);
> >> +
> >> + return rule->mask[word] & bit;
> >
> > Returning an int as a bool creates worse code than just returning the
> > int. (although in this case if the compiler chooses to inline it might
> > be smart enough not to actually convert this int to a bool and make
> > worse assembly...) I'd suggest the function here return an int. bools
> > usually make the assembly worse...
>
> I'm ambivalent. The right assembly would use flags on x86, not an int
> or a bool, and I don't know what the compiler will do.

Also, clearly X86_X32 was implemented without audit support, so we
shouldn't config it in. What do you think of this?

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 25d2c6f..fa852e2 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -129,7 +129,7 @@ config X86
select HAVE_IRQ_EXIT_ON_IRQ_STACK if X86_64
select HAVE_CC_STACKPROTECTOR
select GENERIC_CPU_AUTOPROBE
- select HAVE_ARCH_AUDITSYSCALL
+ select HAVE_ARCH_AUDITSYSCALL if !X86_X32

config INSTRUCTION_DECODER
def_bool y