On Thu, 2015-12-03 at 13:52 -0500, Aaron Conole wrote:
> Dmitry Vyukov <dvyukov@xxxxxxxxxx> writes:
> > On Thu, Dec 3, 2015 at 6:02 PM, Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> > > On Thu, Dec 3, 2015 at 7:55 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> > > > On Thu, Dec 3, 2015 at 3:48 PM, Eric Dumazet wrote:
> > > > > >
> > > > > > No, I don't. But pr_debug always computes its arguments. See no_printk
> > > > > > in printk.h. So this use-after-free happens for all users.
> > > > >
> > > > > Hmm.
> > > > >
> > > > > pr_debug() should be a nop unless either DEBUG or
> > > > > CONFIG_DYNAMIC_DEBUG are set
> > > > >
> > > > > On our production kernels, pr_debug() is a nop.
> > > > >
> > > > > Can you double check ? Thanks !
> > > >
> > > >
> > > > Why should it be nop? no_printk thing in printk.h pretty much
> > > > explicitly makes it not a nop...
>
> Because it was until commit 5264f2f75d8. It also violates my reading of
> the following from printk.h:
>
> * All of these will print unconditionally, although note that pr_debug()
> * and other debug macros are compiled out unless either DEBUG is defined
> * or CONFIG_DYNAMIC_DEBUG is set.
>
> > > >
> > > > Double-checked: debug_post_sfx leads to some generated code:
> > > >
> > > > debug_post_sfx();
> > > > ffffffff8229f256: 48 8b 85 58 fe ff ff mov -0x1a8(%rbp),%rax
> > > > ffffffff8229f25d: 48 85 c0 test %rax,%rax
> > > > ffffffff8229f260: 74 24 je
> > > > ffffffff8229f286
> > > > ffffffff8229f262: 8b b0 a8 00 00 00 mov 0xa8(%rax),%esi
> > > > ffffffff8229f268: 48 8b 85 60 fe ff ff mov -0x1a0(%rbp),%rax
> > > > ffffffff8229f26f: 44 89 85 74 fe ff ff mov %r8d,-0x18c(%rbp)
> > > > ffffffff8229f276: 48 8b 78 20 mov 0x20(%rax),%rdi
> > > > ffffffff8229f27a: e8 71 28 01 00 callq
> > > > ffffffff822b1af0
> > > > ffffffff8229f27f: 44 8b 85 74 fe ff ff mov -0x18c(%rbp),%r8d
> > > >
> > > > return error;
> > > > }
> > > > ffffffff8229f286: 48 81 c4 a0 01 00 00 add $0x1a0,%rsp
> > > > ffffffff8229f28d: 44 89 c0 mov %r8d,%eax
> > > > ffffffff8229f290: 5b pop %rbx
> > > > ffffffff8229f291: 41 5c pop %r12
> > > > ffffffff8229f293: 41 5d pop %r13
> > > > ffffffff8229f295: 41 5e pop %r14
> > > > ffffffff8229f297: 41 5f pop %r15
> > > > ffffffff8229f299: 5d pop %rbp
> > > > ffffffff8229f29a: c3 retq
> > >
> > > This is a serious concern, because we let in the past lot of patches
> > > converting traditional
>
> +1
>
> > > #ifdef DEBUG
> > > # define some_hand_coded_ugly_debug() printk( ...._
> > > #else
> > > # define some_hand_coded_ugly_debug()
> > > #endif
> > >
> > > On the premise pr_debug() would be a nop.
> > >
> > > It seems it is not always the case. This is a very serious problem.
>
> +1
>
> > > We probably have hundred of potential bugs, because few people
> > > actually make sure all debugging stuff is correct,
> > > like comments can be wrong because they are not updated properly as
> > > time flies.
> > >
> > > It is definitely a nop for many cases.
> > >
> > > +void eric_test_pr_debug(struct sock *sk)
> > > +{
> > > + if (atomic_read(&sk->sk_omem_alloc))
> > > + pr_debug("%s: optmem leakage for sock %p\n",
> > > + __func__, sk);
> > > +}
> > >
> > > ->
> > >
> > > 0000000000004740 :
> > > 4740: e8 00 00 00 00 callq 4745
> > > 4741: R_X86_64_PC32 __fentry__-0x4
> > > 4745: 55 push %rbp
> > > 4746: 8b 87 24 01 00 00 mov 0x124(%rdi),%eax //
> > > atomic_read() but nothing follows
> > > 474c: 48 89 e5 mov %rsp,%rbp
> > > 474f: 5d pop %rbp
> > > 4750: c3 retq
> >
> >
> >
> > I would expect that it is nop when argument evaluation does not have
> > side-effects. For example, for a load of a variable compiler will most
> > likely elide it (though, it does not have to elide it, because the
> > load is spelled in the code, so it can also legally emit the load and
> > doesn't use the result).
> > But if argument computation has side-effect (or compiler can't prove
> > otherwise), it must emit code. It must emit code for function calls
> > when the function is defined in a different translation unit, and for
> > volatile accesses (most likely including atomic accesses), etc
>
> This isn't 100% true. As you state, in order to reach the return 0, all
> side effects must be evaluated. Load generally does not have side
> effects, so it can be safely elided, but function() must be emitted.
>
> However, that is _not_ required to get the desired warning emission on a
> printf argument function, see http://pastebin.com/UHuaydkj for an
> example.
>
> I think that as a minimum, the following patch should be evaluted, but am
> unsure to whom I should submit it (after I test):
Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> (cc'd)
> diff --git a/include/linux/printk.h b/include/linux/printk.h
> index 9729565..cd24d2d 100644
> --- a/include/linux/printk.h
> +++ b/include/linux/printk.h
> @@ -286,7 +286,7 @@ extern asmlinkage void dump_stack(void) __cold;
> printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
> #else
> #define pr_debug(fmt, ...) \
> - no_printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
> + ({ if(0) printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__); 0;})
More common is to use do {} while (0) instead of a
statement expression.
I think it'd be good to change pr_debug and variants to
do { if (0) no_printk(...) } while (0)
or some other form that completely eliminates all the
side-effects/function evaluations.
I think the same should be true when CONFIG_PRINTK is
not enabled.
https://lkml.org/lkml/2014/12/3/696
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