commit a1140cb215fa13dcec06d12ba0c3ee105633b7c4 upstream.

Our syzbot instance reported memory leaks in do_seccomp() [0], similar to the report [1]. It shows that we miss freeing struct seccomp_filter and some objects included in it.

We can reproduce the issue with the program below [2] which calls one seccomp() and two clone() syscalls.

The first clone()d child exits earlier than its parent and sends a signal to kill it during the second clone(), more precisely before the fatal_signal_pending() test in copy_process(). When the parent receives the signal, it has to destroy the embryonic process and return -EINTR to user space. In the failure path, we have to call seccomp_filter_release() to decrement the filter's refcount.

Initially, we called it in free_task() called from the failure path, but the commit 3a15fb6ed92c ("seccomp: release filter after task is fully dead") moved it to release_task() to notify user space as early as possible that the filter is no longer used.

To keep the change and current seccomp refcount semantics, let's move copy_seccomp() just after the signal check and add a WARN_ON_ONCE() in free_task() for future debugging.

[0]: unreferenced object 0xffff8880063add00 (size 256): comm "repro_seccomp", pid 230, jiffies 4294687090 (age 9.914s) hex dump (first 32 bytes): 01 00 00 00 01 00 00 00 00 00 00 00 00 00 00 00 ................ ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ................ backtrace: do_seccomp (./include/linux/slab.h:600 ./include/linux/slab.h:733 kernel/seccomp.c:666 kernel/seccomp.c:708 kernel/seccomp.c:1871 kernel/seccomp.c:1991) do_syscall_64 (arch/x86/entry/common.c:50 arch/x86/entry/common.c:80) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:120) unreferenced object 0xffffc90000035000 (size 4096): comm "repro_seccomp", pid 230, jiffies 4294687090 (age 9.915s) hex dump (first 32 bytes): 01 00 00 00 00 00 00 00 00 00 00 00 05 00 00 00 ................ 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ backtrace: __vmalloc_node_range (mm/vmalloc.c:3226) __vmalloc_node (mm/vmalloc.c:3261 (discriminator 4)) bpf_prog_alloc_no_stats (kernel/bpf/core.c:91) bpf_prog_alloc (kernel/bpf/core.c:129) bpf_prog_create_from_user (net/core/filter.c:1414) do_seccomp (kernel/seccomp.c:671 kernel/seccomp.c:708 kernel/seccomp.c:1871 kernel/seccomp.c:1991) do_syscall_64 (arch/x86/entry/common.c:50 arch/x86/entry/common.c:80) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:120) unreferenced object 0xffff888003fa1000 (size 1024): comm "repro_seccomp", pid 230, jiffies 4294687090 (age 9.915s) hex dump (first 32 bytes): 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ backtrace: bpf_prog_alloc_no_stats (./include/linux/slab.h:600 ./include/linux/slab.h:733 kernel/bpf/core.c:95) bpf_prog_alloc (kernel/bpf/core.c:129) bpf_prog_create_from_user (net/core/filter.c:1414) do_seccomp (kernel/seccomp.c:671 kernel/seccomp.c:708 kernel/seccomp.c:1871 kernel/seccomp.c:1991) do_syscall_64 (arch/x86/entry/common.c:50 arch/x86/entry/common.c:80) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:120) unreferenced object 0xffff888006360240 (size 16): comm "repro_seccomp", pid 230, jiffies 4294687090 (age 9.915s) hex dump (first 16 bytes): 01 00 37 00 76 65 72 6c e0 83 01 06 80 88 ff ff ..7.verl........ backtrace: bpf_prog_store_orig_filter (net/core/filter.c:1137) bpf_prog_create_from_user (net/core/filter.c:1428) do_seccomp (kernel/seccomp.c:671 kernel/seccomp.c:708 kernel/seccomp.c:1871 kernel/seccomp.c:1991) do_syscall_64 (arch/x86/entry/common.c:50 arch/x86/entry/common.c:80) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:120) unreferenced object 0xffff8880060183e0 (size 8): comm "repro_seccomp", pid 230, jiffies 4294687090 (age 9.915s) hex dump (first 8 bytes): 06 00 00 00 00 00 ff 7f ........ backtrace: kmemdup (mm/util.c:129) bpf_prog_store_orig_filter (net/core/filter.c:1144) bpf_prog_create_from_user (net/core/filter.c:1428) do_seccomp (kernel/seccomp.c:671 kernel/seccomp.c:708 kernel/seccomp.c:1871 kernel/seccomp.c:1991) do_syscall_64 (arch/x86/entry/common.c:50 arch/x86/entry/common.c:80) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:120)

[1]: https://syzkaller.appspot.com/bug?id=2809bb0ac77ad9aa3f4afe42d6a610aba594a98...

[2]:

void main(void) { struct sock_filter filter[] = { BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW), }; struct sock_fprog fprog = { .len = sizeof(filter) / sizeof(filter[0]), .filter = filter, }; long i, pid;

syscall(__NR_seccomp, SECCOMP_SET_MODE_FILTER, 0, &fprog);

for (i = 0; i < 2; i++) { pid = syscall(__NR_clone, CLONE_NEWNET | SIGKILL, NULL, NULL, 0); if (pid == 0) return; } }

Fixes: 3a15fb6ed92c ("seccomp: release filter after task is fully dead") Reported-by: syzbot+ab17848fe269b573eb71@syzkaller.appspotmail.com Reported-by: Ayushman Dutta ayudutta@amazon.com Suggested-by: Kees Cook keescook@chromium.org Signed-off-by: Kuniyuki Iwashima kuniyu@amazon.com Reviewed-by: Christian Brauner (Microsoft) brauner@kernel.org Signed-off-by: Kees Cook keescook@chromium.org Link: https://lore.kernel.org/r/20220823154532.82913-1-kuniyu@amazon.com --- kernel/fork.c | 17 +++++++++++------ 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c index a5bc0c6a00fd..c6a289317e89 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -441,6 +441,9 @@ void put_task_stack(struct task_struct *tsk)

void free_task(struct task_struct *tsk) { +#ifdef CONFIG_SECCOMP + WARN_ON_ONCE(tsk->seccomp.filter); +#endif scs_release(tsk);

#ifndef CONFIG_THREAD_INFO_IN_TASK @@ -2248,12 +2251,6 @@ static __latent_entropy struct task_struct *copy_process(

spin_lock(&current->sighand->siglock);

- /* - * Copy seccomp details explicitly here, in case they were changed - * before holding sighand lock. - */ - copy_seccomp(p); - rseq_fork(p, clone_flags);

/* Don't start children in a dying pid namespace */ @@ -2268,6 +2265,14 @@ static __latent_entropy struct task_struct *copy_process( goto bad_fork_cancel_cgroup; }

+ /* No more failure paths after this point. */ + + /* + * Copy seccomp details explicitly here, in case they were changed + * before holding sighand lock. + */ + copy_seccomp(p); + init_task_pid_links(p); if (likely(p->pid)) { ptrace_init_task(p, (clone_flags & CLONE_PTRACE) || trace);