6.6-stable review patch. If anyone has any objections, please let me know. ------------------ From: Aleksandr Nogikh <nogikh@xxxxxxxxxx> commit 01c8f9806bde438ca1c8cbbc439f0a14a6694f6c upstream. In kcov_remote_start()/kcov_remote_stop(), we swap the previous KCOV metadata of the current task into a per-CPU variable. However, the kcov_mode_enabled(mode) check is not sufficient in the case of remote KCOV coverage: current->kcov_mode always remains KCOV_MODE_DISABLED for remote KCOV objects. If the original task that has invoked the KCOV_REMOTE_ENABLE ioctl happens to get interrupted and kcov_remote_start() is called, it ultimately leads to kcov_remote_stop() NOT restoring the original KCOV reference. So when the task exits, all registered remote KCOV handles remain active forever. The most uncomfortable effect (at least for syzkaller) is that the bug prevents the reuse of the same /sys/kernel/debug/kcov descriptor. If we obtain it in the parent process and then e.g. drop some capabilities and continuously fork to execute individual programs, at some point current->kcov of the forked process is lost, kcov_task_exit() takes no action, and all KCOV_REMOTE_ENABLE ioctls calls from subsequent forks fail. And, yes, the efficiency is also affected if we keep on losing remote kcov objects. a) kcov_remote_map keeps on growing forever. b) (If I'm not mistaken), we're also not freeing the memory referenced by kcov->area. Fix it by introducing a special kcov_mode that is assigned to the task that owns a KCOV remote object. It makes kcov_mode_enabled() return true and yet does not trigger coverage collection in __sanitizer_cov_trace_pc() and write_comp_data(). [nogikh@xxxxxxxxxx: replace WRITE_ONCE() with an ordinary assignment] Link: https://lkml.kernel.org/r/20240614171221.2837584-1-nogikh@xxxxxxxxxx Link: https://lkml.kernel.org/r/20240611133229.527822-1-nogikh@xxxxxxxxxx Fixes: 5ff3b30ab57d ("kcov: collect coverage from interrupts") Signed-off-by: Aleksandr Nogikh <nogikh@xxxxxxxxxx> Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx> Reviewed-by: Andrey Konovalov <andreyknvl@xxxxxxxxx> Tested-by: Andrey Konovalov <andreyknvl@xxxxxxxxx> Cc: Alexander Potapenko <glider@xxxxxxxxxx> Cc: Arnd Bergmann <arnd@xxxxxxxx> Cc: Marco Elver <elver@xxxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- include/linux/kcov.h | 2 ++ kernel/kcov.c | 1 + 2 files changed, 3 insertions(+) --- a/include/linux/kcov.h +++ b/include/linux/kcov.h @@ -21,6 +21,8 @@ enum kcov_mode { KCOV_MODE_TRACE_PC = 2, /* Collecting comparison operands mode. */ KCOV_MODE_TRACE_CMP = 3, + /* The process owns a KCOV remote reference. */ + KCOV_MODE_REMOTE = 4, }; #define KCOV_IN_CTXSW (1 << 30) --- a/kernel/kcov.c +++ b/kernel/kcov.c @@ -631,6 +631,7 @@ static int kcov_ioctl_locked(struct kcov return -EINVAL; kcov->mode = mode; t->kcov = kcov; + t->kcov_mode = KCOV_MODE_REMOTE; kcov->t = t; kcov->remote = true; kcov->remote_size = remote_arg->area_size;
