On Wed, Sep 09, 2020 at 10:31:03PM +1000, Alexey Kardashevskiy wrote: > > > On 09/09/2020 21:50, Paul E. McKenney wrote: > > On Wed, Sep 09, 2020 at 07:24:11PM +1000, Alexey Kardashevskiy wrote: > >> > >> > >> On 09/09/2020 00:43, Alexey Kardashevskiy wrote: > >>> init_srcu_struct_nodes() is called with is_static==true only internally > >>> and when this happens, the srcu->sda is not initialized in > >>> init_srcu_struct_fields() and we crash on dereferencing @sdp. > >>> > >>> This fixes the crash by moving "if (is_static)" out of the loop which > >>> only does useful work for is_static=false case anyway. > >>> > >>> Found by syzkaller. > >>> > >>> Signed-off-by: Alexey Kardashevskiy <aik@xxxxxxxxx> > >>> --- > >>> kernel/rcu/srcutree.c | 5 +++-- > >>> 1 file changed, 3 insertions(+), 2 deletions(-) > >>> > >>> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c > >>> index c100acf332ed..49b54a50bde8 100644 > >>> --- a/kernel/rcu/srcutree.c > >>> +++ b/kernel/rcu/srcutree.c > >>> @@ -135,6 +135,9 @@ static void init_srcu_struct_nodes(struct srcu_struct *ssp, bool is_static) > >>> levelspread[level - 1]; > >>> } > >>> > >>> + if (is_static) > >>> + return; > >> > >> Actually, this is needed here too: > >> > >> if (!ssp->sda) > >> return; > >> > >> as > >> ssp->sda = alloc_percpu(struct srcu_data) > >> > >> can fail if the process is killed too soon - it is quite easy to get > >> this situation with syzkaller (syscalls fuzzer) > >> > >> Makes sense? > > > > Just to make sure that I understand, these failures occur when the task > > running init_srcu_struct_nodes() is killed, correct? > > There are multiple user tasks (8) which open /dev/kvm, do 1 ioctl - > KVM_CREATE_VM - and terminate, running on 8 vcpus, 8 VMs, crashes every > 20min or so, less tasks or vcpus - and the problem does not appear. > > > > > > Or has someone managed to invoke (say) synchronize_srcu() on a > > dynamically allocated srcu_struct before invoking init_srcu_struct() on > > that srcu_struct? > > Nah, none of that :) > > init_srcu_struct_nodes() assumes ssp->sda!=NULL but alloc_percpu() fails > here: > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/percpu.c#n1734 > === > } else if (mutex_lock_killable(&pcpu_alloc_mutex)) { > pcpu_memcg_post_alloc_hook(objcg, NULL, 0, size); > return NULL; > === > > I am still up to reading that osr-rcuusage.pdf to provide better > analysis :) Thanks, Ah, got it! Does the following patch help? There will likely also need to be changes to cleanup_srcu_struct(), but first let's see if I understand the problem. ;-) Thanx, Paul ------------------------------------------------------------------------ diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index c13348e..6f7880a 100644 --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -177,11 +177,13 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static) INIT_DELAYED_WORK(&ssp->work, process_srcu); if (!is_static) ssp->sda = alloc_percpu(struct srcu_data); + if (!ssp->sda) + return -ENOMEM; init_srcu_struct_nodes(ssp, is_static); ssp->srcu_gp_seq_needed_exp = 0; ssp->srcu_last_gp_end = ktime_get_mono_fast_ns(); smp_store_release(&ssp->srcu_gp_seq_needed, 0); /* Init done. */ - return ssp->sda ? 0 : -ENOMEM; + return 0; } #ifdef CONFIG_DEBUG_LOCK_ALLOC
