Re: [PATCH v2] seccomp: Move copy_seccomp() to no failure path.

[Christian Brauner]

On Mon, Aug 22, 2022 at 05:48:06PM -0700, Kuniyuki Iwashima wrote:
> 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=2809bb0ac77ad9aa3f4afe42d6a610aba594a987
> 
> [2]:
> #define _GNU_SOURCE
> #include <sched.h>
> #include <signal.h>
> #include <unistd.h>
> #include <sys/syscall.h>
> #include <linux/filter.h>
> #include <linux/seccomp.h>
> 
> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> Reported-by: Ayushman Dutta <ayudutta@xxxxxxxxxx>
> Suggested-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Signed-off-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
> ---

Looks good,
Reviewed-by: Christian Brauner (Microsoft) <brauner@xxxxxxxxxx>