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SubjectRe: [PATCH] kconfig: Add kernel config option for fuzz testing.
On Mon, Dec 16, 2019 at 11:01 AM Tetsuo Handa
<penguin-kernel@i-love.sakura.ne.jp> wrote:
>
> While syzkaller is finding many bugs, sometimes syzkaller examines
> stupid operations. But disabling operations using kernel config option
> is problematic because "kernel config option excludes whole module when
> there is still room for examining all but specific operation" and
> "the list of kernel config options becomes too complicated to maintain
> because such list changes over time". Thus, this patch introduces a
> kernel config option which allows disabling only specific operations.
> This kernel config option should be enabled only when building kernels
> for fuzz testing.
>
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> ---
> drivers/char/mem.c | 6 +++---
> drivers/tty/serial/8250/8250_port.c | 7 +++++--
> drivers/tty/vt/keyboard.c | 3 +++
> fs/ioctl.c | 5 +++++
> include/linux/uaccess.h | 12 ++++++++++++
> kernel/printk/printk.c | 8 ++++++++
> lib/Kconfig.debug | 10 ++++++++++
> lib/usercopy.c | 27 +++++++++++++++++++++++++++
> 8 files changed, 73 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/char/mem.c b/drivers/char/mem.c
> index 43dd0891ca1e..63aff3ae7c2b 100644
> --- a/drivers/char/mem.c
> +++ b/drivers/char/mem.c
> @@ -246,7 +246,7 @@ static ssize_t write_mem(struct file *file, const char __user *buf,
> return -EFAULT;
> }
>
> - copied = copy_from_user(ptr, buf, sz);
> + copied = copy_from_user_to_any(ptr, buf, sz);

FWIW we disable /dev/{mem,kmem} in the config now:
https://github.com/google/syzkaller/blob/master/dashboard/config/bits-syzbot.config#L169
And this looks like a reasonable thing to do for any fuzzing.


> unxlate_dev_mem_ptr(p, ptr);
> if (copied) {
> written += sz - copied;
> @@ -550,7 +550,7 @@ static ssize_t do_write_kmem(unsigned long p, const char __user *buf,
> if (!virt_addr_valid(ptr))
> return -ENXIO;
>
> - copied = copy_from_user(ptr, buf, sz);
> + copied = copy_from_user_to_any(ptr, buf, sz);
> if (copied) {
> written += sz - copied;
> if (written)
> @@ -604,7 +604,7 @@ static ssize_t write_kmem(struct file *file, const char __user *buf,
> err = -ENXIO;
> break;
> }
> - n = copy_from_user(kbuf, buf, sz);
> + n = copy_from_user_to_any(kbuf, buf, sz);
> if (n) {
> err = -EFAULT;
> break;
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index 90655910b0c7..367b92ad598b 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -519,11 +519,14 @@ serial_port_out_sync(struct uart_port *p, int offset, int value)
> case UPIO_MEM32:
> case UPIO_MEM32BE:
> case UPIO_AU:
> - p->serial_out(p, offset, value);
> + /* Writing to random kernel address causes crash. */
> + if (!IS_ENABLED(CONFIG_KERNEL_BUILT_FOR_FUZZ_TESTING))
> + p->serial_out(p, offset, value);

Does this do the same as LOCKDOWN_TIOCSSERIAL? How is it different?

> p->serial_in(p, UART_LCR); /* safe, no side-effects */
> break;
> default:
> - p->serial_out(p, offset, value);
> + if (!IS_ENABLED(CONFIG_KERNEL_BUILT_FOR_FUZZ_TESTING))
> + p->serial_out(p, offset, value);
> }
> }
>
> diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c
> index 15d33fa0c925..367820b8ff59 100644
> --- a/drivers/tty/vt/keyboard.c
> +++ b/drivers/tty/vt/keyboard.c
> @@ -633,6 +633,9 @@ static void k_spec(struct vc_data *vc, unsigned char value, char up_flag)
> kbd->kbdmode == VC_OFF) &&
> value != KVAL(K_SAK))
> return; /* SAK is allowed even in raw mode */
> + /* Repeating SysRq-t forever causes RCU stalls. */
> + if (IS_ENABLED(CONFIG_KERNEL_BUILT_FOR_FUZZ_TESTING))
> + return;
> fn_handler[value](vc);
> }
>
> diff --git a/fs/ioctl.c b/fs/ioctl.c
> index 2f5e4e5b97e1..f879aa94b118 100644
> --- a/fs/ioctl.c
> +++ b/fs/ioctl.c
> @@ -601,6 +601,11 @@ static int ioctl_fsfreeze(struct file *filp)
> if (sb->s_op->freeze_fs == NULL && sb->s_op->freeze_super == NULL)
> return -EOPNOTSUPP;
>
> +#ifdef CONFIG_KERNEL_BUILT_FOR_FUZZ_TESTING
> + /* Freezing filesystems causes hung tasks. */
> + return -EBUSY;
> +#endif
> +
> /* Freeze */
> if (sb->s_op->freeze_super)
> return sb->s_op->freeze_super(sb);
> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> index 67f016010aad..6e5ddd0fdcce 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -387,4 +387,16 @@ void __noreturn usercopy_abort(const char *name, const char *detail,
> unsigned long len);
> #endif
>
> +#ifdef CONFIG_KERNEL_BUILT_FOR_FUZZ_TESTING
> +unsigned long __must_check copy_from_user_to_any(void *to,
> + const void __user *from,
> + unsigned long n);
> +#else
> +static __always_inline unsigned long __must_check
> +copy_from_user_to_any(void *to, const void __user *from, unsigned long n)
> +{
> + return copy_from_user(to, from, n);
> +}
> +#endif
> +
> #endif /* __LINUX_UACCESS_H__ */
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 1ef6f75d92f1..9a2f95a78fef 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1198,6 +1198,14 @@ MODULE_PARM_DESC(ignore_loglevel,
>
> static bool suppress_message_printing(int level)
> {
> +#ifdef CONFIG_KERNEL_BUILT_FOR_FUZZ_TESTING
> + /*
> + * Changing console_loglevel causes "no output". But ignoring
> + * console_loglevel is easier than preventing change of
> + * console_loglevel.
> + */
> + return (level >= CONSOLE_LOGLEVEL_DEFAULT && !ignore_loglevel);
> +#endif
> return (level >= console_loglevel && !ignore_loglevel);
> }
>
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index d1842fe756d5..f9836cc23942 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -2184,4 +2184,14 @@ config HYPERV_TESTING
> help
> Select this option to enable Hyper-V vmbus testing.
>
> +config KERNEL_BUILT_FOR_FUZZ_TESTING
> + bool "Build kernel for fuzz testing"
> + default n
> + help
> + Say N unless you are building kernels for fuzz testing.
> + Saying Y here disables several things that legitimately cause
> + damage under a fuzzer workload (e.g. copying to arbitrary
> + user-specified kernel address, changing console loglevel,
> + freezing filesystems).
> +
> endmenu # Kernel hacking
> diff --git a/lib/usercopy.c b/lib/usercopy.c
> index cbb4d9ec00f2..f7d5d243a63d 100644
> --- a/lib/usercopy.c
> +++ b/lib/usercopy.c
> @@ -86,3 +86,30 @@ int check_zeroed_user(const void __user *from, size_t size)
> return -EFAULT;
> }
> EXPORT_SYMBOL(check_zeroed_user);
> +
> +#ifdef CONFIG_KERNEL_BUILT_FOR_FUZZ_TESTING
> +/*
> + * Since copying to arbitrary user-specified kernel address will crash
> + * the kernel trivially, do not copy to user-specified kernel address
> + * if the kernel was build for fuzz testing.
> + */
> +unsigned long __must_check copy_from_user_to_any(void *to,
> + const void __user *from,
> + unsigned long n)
> +{
> + static char dummybuf[PAGE_SIZE];
> + unsigned long ret = 0;
> +
> + while (n) {
> + unsigned long size = min(n, sizeof(dummybuf));
> +
> + ret = copy_from_user(dummybuf, from, size);
> + if (ret)
> + break;
> + from += size;
> + n -= size;
> + }
> + return ret;
> +}
> +EXPORT_SYMBOL(copy_from_user_to_any);
> +#endif
> --
> 2.18.1
>

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