Messages in this thread | | | From | Dmitry Vyukov <> | Date | Mon, 21 Jan 2019 13:42:43 +0100 | Subject | Re: [PATCH] kcov: convert kcov.refcount to refcount_t |
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On Mon, Jan 21, 2019 at 1:38 PM Mark Rutland <mark.rutland@arm.com> wrote: > > On Wed, Jan 16, 2019 at 12:27:09PM +0200, Elena Reshetova wrote: > > atomic_t variables are currently used to implement reference > > counters with the following properties: > > - counter is initialized to 1 using atomic_set() > > - a resource is freed upon counter reaching zero > > - once counter reaches zero, its further > > increments aren't allowed > > - counter schema uses basic atomic operations > > (set, inc, inc_not_zero, dec_and_test, etc.) > > > > Such atomic variables should be converted to a newly provided > > refcount_t type and API that prevents accidental counter overflows > > and underflows. This is important since overflows and underflows > > can lead to use-after-free situation and be exploitable. > > > > The variable kcov.refcount is used as pure reference counter. > > Convert it to refcount_t and fix up the operations. > > > > **Important note for maintainers: > > > > Some functions from refcount_t API defined in lib/refcount.c > > have different memory ordering guarantees than their atomic > > counterparts. > > The full comparison can be seen in > > https://lkml.org/lkml/2017/11/15/57 and it is hopefully soon > > in state to be merged to the documentation tree. > > Normally the differences should not matter since refcount_t provides > > enough guarantees to satisfy the refcounting use cases, but in > > some rare cases it might matter. > > Please double check that you don't have some undocumented > > memory guarantees for this variable usage. > > > > For the kcov.refcount it might make a difference > > in following places: > > - kcov_put(): decrement in refcount_dec_and_test() only > > provides RELEASE ordering and control dependency on success > > vs. fully ordered atomic counterpart > > > > Suggested-by: Kees Cook <keescook@chromium.org> > > Reviewed-by: David Windsor <dwindsor@gmail.com> > > Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com> > > Signed-off-by: Elena Reshetova <elena.reshetova@intel.com> > > Just to check, has this been tested with CONFIG_REFCOUNT_FULL and > something poking kcov? > > Given lib/refcount.c is instrumented, the refcount_*() calls will > recurse back into the kcov code. It looks like that's fine, given these > are only manipulated in setup/teardown paths, but it would be nice to be > sure.
A simple program using KCOV is available here: https://elixir.bootlin.com/linux/v5.0-rc3/source/Documentation/dev-tools/kcov.rst#L42 or here (it's like strace but collects and prints KCOV coverage): https://github.com/google/syzkaller/blob/master/tools/kcovtrace/kcovtrace.c
> Thanks, > Mark. > > > --- > > kernel/kcov.c | 9 +++++---- > > 1 file changed, 5 insertions(+), 4 deletions(-) > > > > diff --git a/kernel/kcov.c b/kernel/kcov.c > > index c2277db..051e86e 100644 > > --- a/kernel/kcov.c > > +++ b/kernel/kcov.c > > @@ -20,6 +20,7 @@ > > #include <linux/debugfs.h> > > #include <linux/uaccess.h> > > #include <linux/kcov.h> > > +#include <linux/refcount.h> > > #include <asm/setup.h> > > > > /* Number of 64-bit words written per one comparison: */ > > @@ -44,7 +45,7 @@ struct kcov { > > * - opened file descriptor > > * - task with enabled coverage (we can't unwire it from another task) > > */ > > - atomic_t refcount; > > + refcount_t refcount; > > /* The lock protects mode, size, area and t. */ > > spinlock_t lock; > > enum kcov_mode mode; > > @@ -228,12 +229,12 @@ EXPORT_SYMBOL(__sanitizer_cov_trace_switch); > > > > static void kcov_get(struct kcov *kcov) > > { > > - atomic_inc(&kcov->refcount); > > + refcount_inc(&kcov->refcount); > > } > > > > static void kcov_put(struct kcov *kcov) > > { > > - if (atomic_dec_and_test(&kcov->refcount)) { > > + if (refcount_dec_and_test(&kcov->refcount)) { > > vfree(kcov->area); > > kfree(kcov); > > } > > @@ -312,7 +313,7 @@ static int kcov_open(struct inode *inode, struct file *filep) > > if (!kcov) > > return -ENOMEM; > > kcov->mode = KCOV_MODE_DISABLED; > > - atomic_set(&kcov->refcount, 1); > > + refcount_set(&kcov->refcount, 1); > > spin_lock_init(&kcov->lock); > > filep->private_data = kcov; > > return nonseekable_open(inode, filep); > > -- > > 2.7.4 > >
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