On Thu, Jun 18, 2026 at 06:27:43PM -0400, Michael Bommarito wrote: > user_event_enabler_destroy() removes an enabler from the per-mm > mm->enablers list with list_del_rcu() and then frees it immediately with > kfree(). That list is walked locklessly by user_event_mm_dup() during > fork(), under rcu_read_lock() only: > > rcu_read_lock(); > list_for_each_entry_rcu(enabler, &old_mm->enablers, mm_enablers_link) > ... > > user_event_mm_dup() does not take event_mutex. The per-enabler destroy > path user_events_ioctl_unreg() (DIAG_IOCSUNREG) takes event_mutex but > nothing that excludes the dup walk. Threads that share an mm share one > user_event_mm and one enabler list, so an unregister on one thread can > free an enabler while another thread is forking and user_event_mm_dup() > is mid-walk. The walk then dereferences the freed enabler (for example > enabler->event in user_event_enabler_dup()). > > This is reachable by an unprivileged task that can open user_events_data: > a single multithreaded process that registers an enabler and then > concurrently unregisters it and calls fork() triggers the race. KASAN > reports a slab-use-after-free read in user_event_enabler_dup() called > from user_event_mm_dup() and copy_process() during clone(); with > kasan.fault=panic the kernel panics. > > Free the enabler after a grace period with kfree_rcu(), matching the > list_del_rcu() removal and the rcu_read_lock() readers in > user_event_mm_dup(). Add an rcu_head to struct user_event_enabler for > this. The error path in user_event_enabler_create() keeps using kfree() > because that enabler is freed before it is published to the RCU list. > > Cc: [email protected] > Fixes: 7235759084a4 ("tracing/user_events: Use remote writes for event > enablement") > Assisted-by: Claude:claude-opus-4-8 > Signed-off-by: Michael Bommarito <[email protected]> > --- > > Notes: > KASAN on the unpatched tree (v7.1, x86-64, CONFIG_KASAN=y, SMP): > > BUG: KASAN: slab-use-after-free in user_event_enabler_dup+0x50a/0x540 > Read of size 8 (enabler->event, 16 bytes into a freed kmalloc-cg-64): > user_event_enabler_dup > user_event_mm_dup > copy_process > __do_sys_clone > Allocated by the registering task; freed on another CPU via the > DIAG_IOCSUNREG path. With kasan.fault=panic the access panics. > > After the patch the same reproducer runs cleanly (no splat, no panic) > across the full window, and a serialized control (same paths, no > concurrency) is clean on both stock and patched. > > Re-ran tools/testing/selftests/user_events on stock and patched, both > clean: abi_test pass:6/6, dyn_test pass:4/4, ftrace_test pass:6/6. > > kernel/trace/trace_events_user.c | 10 +++++++++- > 1 file changed, 9 insertions(+), 1 deletion(-) > > diff --git a/kernel/trace/trace_events_user.c > b/kernel/trace/trace_events_user.c > index c4ba484f7b38b..412ca1e3a40cf 100644 > --- a/kernel/trace/trace_events_user.c > +++ b/kernel/trace/trace_events_user.c > @@ -109,6 +109,9 @@ struct user_event_enabler { > > /* Track enable bit, flags, etc. Aligned for bitops. */ > unsigned long values; > + > + /* Defer free so RCU list readers (user_event_mm_dup) are safe. */ > + struct rcu_head rcu; > }; > > /* Bits 0-5 are for the bit to update upon enable/disable (0-63 allowed) */ > @@ -404,7 +407,12 @@ static void user_event_enabler_destroy(struct > user_event_enabler *enabler, > /* No longer tracking the event via the enabler */ > user_event_put(enabler->event, locked); > > - kfree(enabler); > + /* > + * The enabler is removed from an RCU-traversed list > + * (user_event_mm_dup walks mm->enablers under rcu_read_lock only), > + * so the backing memory must outlive a grace period. > + */ > + kfree_rcu(enabler, rcu); > } > > static int user_event_mm_fault_in(struct user_event_mm *mm, unsigned long > uaddr, > -- > 2.53.0
Thanks for fixing this! Acked-by: Beau Belgrave <[email protected]> Thanks, -Beau
