From: Steven Rostedt (VMware) rostedt@goodmis.org
[ Upstream commit befe6d946551d65cddbd32b9cb0170b0249fd5ed ]
The list of tracepoint callbacks is managed by an array that is protected by RCU. To update this array, a new array is allocated, the updates are copied over to the new array, and then the list of functions for the tracepoint is switched over to the new array. After a completion of an RCU grace period, the old array is freed.
This process happens for both adding a callback as well as removing one. But on removing a callback, if the new array fails to be allocated, the callback is not removed, and may be used after it is freed by the clients of the tracepoint.
There's really no reason to fail if the allocation for a new array fails when removing a function. Instead, the function can simply be replaced by a stub function that could be cleaned up on the next modification of the array. That is, instead of calling the function registered to the tracepoint, it would call a stub function in its place.
Link: https://lore.kernel.org/r/20201115055256.65625-1-mmullins@mmlx.us Link: https://lore.kernel.org/r/20201116175107.02db396d@gandalf.local.home Link: https://lore.kernel.org/r/20201117211836.54acaef2@oasis.local.home Link: https://lkml.kernel.org/r/20201118093405.7a6d2290@gandalf.local.home
[ Note, this version does use undefined compiler behavior (assuming that a stub function with no parameters or return, can be called by a location that thinks it has parameters but still no return value. Static calls do the same thing, so this trick is not without precedent.
There's another solution that uses RCU tricks and is more complex, but can be an alternative if this solution becomes an issue.
Link: https://lore.kernel.org/lkml/20210127170721.58bce7cc@gandalf.local.home/ ]
Cc: Peter Zijlstra peterz@infradead.org Cc: Josh Poimboeuf jpoimboe@redhat.com Cc: Mathieu Desnoyers mathieu.desnoyers@efficios.com Cc: Ingo Molnar mingo@redhat.com Cc: Alexei Starovoitov ast@kernel.org Cc: Daniel Borkmann daniel@iogearbox.net Cc: Dmitry Vyukov dvyukov@google.com Cc: Martin KaFai Lau kafai@fb.com Cc: Song Liu songliubraving@fb.com Cc: Yonghong Song yhs@fb.com Cc: Andrii Nakryiko andriin@fb.com Cc: John Fastabend john.fastabend@gmail.com Cc: KP Singh kpsingh@chromium.org Cc: netdev netdev@vger.kernel.org Cc: bpf bpf@vger.kernel.org Cc: Kees Cook keescook@chromium.org Cc: Florian Weimer fw@deneb.enyo.de Fixes: 97e1c18e8d17b ("tracing: Kernel Tracepoints") Reported-by: syzbot+83aa762ef23b6f0d1991@syzkaller.appspotmail.com Reported-by: syzbot+d29e58bb557324e55e5e@syzkaller.appspotmail.com Reported-by: Matt Mullins mmullins@mmlx.us Signed-off-by: Steven Rostedt (VMware) rostedt@goodmis.org Tested-by: Matt Mullins mmullins@mmlx.us Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/tracepoint.c | 80 ++++++++++++++++++++++++++++++++++++--------- 1 file changed, 64 insertions(+), 16 deletions(-)
diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c index 73956eaff8a9c..be51df4508cbe 100644 --- a/kernel/tracepoint.c +++ b/kernel/tracepoint.c @@ -53,6 +53,12 @@ struct tp_probes { struct tracepoint_func probes[0]; };
+/* Called in removal of a func but failed to allocate a new tp_funcs */ +static void tp_stub_func(void) +{ + return; +} + static inline void *allocate_probes(int count) { struct tp_probes *p = kmalloc(struct_size(p, probes, count), @@ -131,6 +137,7 @@ func_add(struct tracepoint_func **funcs, struct tracepoint_func *tp_func, { struct tracepoint_func *old, *new; int nr_probes = 0; + int stub_funcs = 0; int pos = -1;
if (WARN_ON(!tp_func->func)) @@ -147,14 +154,34 @@ func_add(struct tracepoint_func **funcs, struct tracepoint_func *tp_func, if (old[nr_probes].func == tp_func->func && old[nr_probes].data == tp_func->data) return ERR_PTR(-EEXIST); + if (old[nr_probes].func == tp_stub_func) + stub_funcs++; } } - /* + 2 : one for new probe, one for NULL func */ - new = allocate_probes(nr_probes + 2); + /* + 2 : one for new probe, one for NULL func - stub functions */ + new = allocate_probes(nr_probes + 2 - stub_funcs); if (new == NULL) return ERR_PTR(-ENOMEM); if (old) { - if (pos < 0) { + if (stub_funcs) { + /* Need to copy one at a time to remove stubs */ + int probes = 0; + + pos = -1; + for (nr_probes = 0; old[nr_probes].func; nr_probes++) { + if (old[nr_probes].func == tp_stub_func) + continue; + if (pos < 0 && old[nr_probes].prio < prio) + pos = probes++; + new[probes++] = old[nr_probes]; + } + nr_probes = probes; + if (pos < 0) + pos = probes; + else + nr_probes--; /* Account for insertion */ + + } else if (pos < 0) { pos = nr_probes; memcpy(new, old, nr_probes * sizeof(struct tracepoint_func)); } else { @@ -188,8 +215,9 @@ static void *func_remove(struct tracepoint_func **funcs, /* (N -> M), (N > 1, M >= 0) probes */ if (tp_func->func) { for (nr_probes = 0; old[nr_probes].func; nr_probes++) { - if (old[nr_probes].func == tp_func->func && - old[nr_probes].data == tp_func->data) + if ((old[nr_probes].func == tp_func->func && + old[nr_probes].data == tp_func->data) || + old[nr_probes].func == tp_stub_func) nr_del++; } } @@ -208,14 +236,32 @@ static void *func_remove(struct tracepoint_func **funcs, /* N -> M, (N > 1, M > 0) */ /* + 1 for NULL */ new = allocate_probes(nr_probes - nr_del + 1); - if (new == NULL) - return ERR_PTR(-ENOMEM); - for (i = 0; old[i].func; i++) - if (old[i].func != tp_func->func - || old[i].data != tp_func->data) - new[j++] = old[i]; - new[nr_probes - nr_del].func = NULL; - *funcs = new; + if (new) { + for (i = 0; old[i].func; i++) + if ((old[i].func != tp_func->func + || old[i].data != tp_func->data) + && old[i].func != tp_stub_func) + new[j++] = old[i]; + new[nr_probes - nr_del].func = NULL; + *funcs = new; + } else { + /* + * Failed to allocate, replace the old function + * with calls to tp_stub_func. + */ + for (i = 0; old[i].func; i++) + if (old[i].func == tp_func->func && + old[i].data == tp_func->data) { + old[i].func = tp_stub_func; + /* Set the prio to the next event. */ + if (old[i + 1].func) + old[i].prio = + old[i + 1].prio; + else + old[i].prio = -1; + } + *funcs = old; + } } debug_print_probes(*funcs); return old; @@ -271,10 +317,12 @@ static int tracepoint_remove_func(struct tracepoint *tp, tp_funcs = rcu_dereference_protected(tp->funcs, lockdep_is_held(&tracepoints_mutex)); old = func_remove(&tp_funcs, func); - if (IS_ERR(old)) { - WARN_ON_ONCE(PTR_ERR(old) != -ENOMEM); + if (WARN_ON_ONCE(IS_ERR(old))) return PTR_ERR(old); - } + + if (tp_funcs == old) + /* Failed allocating new tp_funcs, replaced func with stub */ + return 0;
if (!tp_funcs) { /* Removed last function */