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Date: Wed, 2 Jan 2019 08:11:26 -0800
From: Casey Schaufler <casey@...aufler-ca.com>
To: Jason Yan <yanaijie@...wei.com>, keescook@...omium.org,
kernel-hardening@...ts.openwall.com
Cc: zhaohongjiang@...wei.com, miaoxie@...wei.com,
Li Bin <huawei.libin@...wei.com>, Wei Yongjun <weiyongjun1@...wei.com>
Subject: Re: [PATCH] usercopy: skip the check if not a real usercopy
On 1/2/2019 1:31 AM, Jason Yan wrote:
> Some kernel codes use copy_to/from_user to copy data between kernel
> buffers by calling set_fs(KERNEL_DS). Hardened usercopy will check these
> objects and sometimes soft lockup may happen as follows:
>
> [ 96.314420] watchdog: BUG: soft lockup - CPU#4 stuck for 22s! [sh:356]
> .....
> [ 96.674904] Call Trace:
> [ 96.684489] __check_object_size+0x1f1/0x460
> [ 96.691669] __probe_kernel_write+0x195/0x390
> [ 96.696821] ftrace_write+0x67/0xa0
> [ 96.709086] ftrace_replace_code+0x3e2/0xa30
> [ 96.724418] ? ftrace_int3_handler+0x100/0x100
> [ 96.731570] ftrace_modify_all_code+0x1f6/0x2e0
> [ 96.741639] ? function_stack_trace_call+0x340/0x340
> [ 96.751778] arch_ftrace_update_code+0x3a/0x70
> [ 96.762062] ftrace_run_update_code+0x35/0xf0
> [ 96.763874] ftrace_startup_enable+0x7a/0xa0
> [ 96.770122] ftrace_startup+0x405/0x6a0
> [ 96.782269] register_ftrace_function+0x76/0x150
> [ 96.794676] function_trace_init+0x1bb/0x250
> [ 96.805203] tracing_set_tracer+0x4af/0xa10
> [ 96.817490] tracing_set_trace_write+0x40e/0x660
> [ 96.832344] ? tracing_set_tracer+0xa10/0xa10
> [ 96.843771] ? kasan_check_read+0x1d/0x30
> [ 96.855433] ? do_raw_spin_unlock+0x6c/0x300
> [ 96.864058] ? _raw_spin_unlock+0x44/0x70
> [ 96.873790] ? do_anonymous_page+0x6d3/0x1030
> [ 96.887520] ? tracing_set_tracer+0xa10/0xa10
> [ 96.897393] __vfs_write+0x11b/0x880
> [ 96.910798] ? kernel_read+0x150/0x150
> [ 96.918995] ? __lock_acquire+0x925/0x1770
> [ 96.925343] ? __lock_acquire+0x925/0x1770
> [ 96.933334] ? pmd_alloc+0x140/0x140
> [ 96.944556] ? __lock_is_held+0xe3/0x1a0
> [ 96.954812] ? kasan_check_read+0x1d/0x30
> [ 96.959380] ? rcu_read_lock_sched_held+0x1dd/0x210
> [ 96.967383] ? rcu_sync_lockdep_assert+0xf0/0x190
> [ 96.984740] ? __sb_start_write+0x1b3/0x3e0
> [ 96.999405] vfs_write+0x210/0x640
> [ 97.013153] ksys_write+0xe6/0x210
> [ 97.023623] ? __x64_sys_read+0xe0/0xe0
> [ 97.035021] ? trace_hardirqs_on_thunk+0x1a/0x1c
> [ 97.044914] ? do_syscall_64+0x98/0x2b0
> [ 97.050349] ? do_syscall_64+0x98/0x2b0
> [ 97.058009] __x64_sys_write+0x94/0xe0
> [ 97.063153] do_syscall_64+0x161/0x2b0
> [ 97.069358] entry_SYSCALL_64_after_hwframe+0x49/0xbe
> .....
>
> It's unnecessary to check these objects for copying between kernel buffers.
> So skip all hardened usercopy tests.
>
> Signed-off-by: Jason Yan <yanaijie@...wei.com>
> CC: Li Bin <huawei.libin@...wei.com>
> CC: Wei Yongjun <weiyongjun1@...wei.com>
I know it would be (a lot) more work, but wouldn't fixing
the code that calls copy_{to,from}_user be a better solution?
> ---
> mm/usercopy.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/mm/usercopy.c b/mm/usercopy.c
> index 852eb4e53f06..8a0a1854f564 100644
> --- a/mm/usercopy.c
> +++ b/mm/usercopy.c
> @@ -23,6 +23,7 @@
> #include <linux/atomic.h>
> #include <linux/jump_label.h>
> #include <asm/sections.h>
> +#include <linux/uaccess.h>
>
> /*
> * Checks if a given pointer and length is contained by the current
> @@ -255,6 +256,10 @@ void __check_object_size(const void *ptr, unsigned long n, bool to_user)
> if (static_branch_unlikely(&bypass_usercopy_checks))
> return;
>
> + /* Skip all tests if it is not a real usercopy. */
> + if (uaccess_kernel())
> + return;
> +
> /* Skip all tests if size is zero. */
> if (!n)
> return;
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