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Date: Wed, 28 Apr 2021 11:39:00 -0700
From: "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To: Borislav Petkov <bp@...en8.de>
Cc: x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
linux-doc@...r.kernel.org, linux-mm@...ck.org,
linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
Arnd Bergmann <arnd@...db.de>,
Andy Lutomirski <luto@...nel.org>,
Balbir Singh <bsingharora@...il.com>,
Cyrill Gorcunov <gorcunov@...il.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Eugene Syromiatnikov <esyr@...hat.com>,
Florian Weimer <fweimer@...hat.com>,
"H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
Jonathan Corbet <corbet@....net>,
Kees Cook <keescook@...omium.org>,
Mike Kravetz <mike.kravetz@...cle.com>,
Nadav Amit <nadav.amit@...il.com>,
Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
Peter Zijlstra <peterz@...radead.org>,
Randy Dunlap <rdunlap@...radead.org>,
"Ravi V. Shankar" <ravi.v.shankar@...el.com>,
Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
Dave Martin <Dave.Martin@....com>,
Weijiang Yang <weijiang.yang@...el.com>,
Pengfei Xu <pengfei.xu@...el.com>,
Haitao Huang <haitao.huang@...el.com>
Subject: Re: [PATCH v26 22/30] x86/cet/shstk: Add user-mode shadow stack
support
On 4/28/2021 10:52 AM, Borislav Petkov wrote:
> On Tue, Apr 27, 2021 at 01:43:07PM -0700, Yu-cheng Yu wrote:
>> @@ -535,6 +536,10 @@ struct thread_struct {
>>
>> unsigned int sig_on_uaccess_err:1;
>>
>> +#ifdef CONFIG_X86_SHADOW_STACK
>> + struct cet_status cet;
>
> A couple of versions ago I said:
>
> " struct shstk_desc shstk;
>
> or so"
>
> but no movement here. That thing is still called cet_status even though
> there's nothing status-related with it.
>
> So what's up?
>
Sorry about that. After that email thread, we went ahead to separate
shadow stack and ibt into different files. I thought about the struct,
the file names cet.h, etc. The struct still needs to include ibt
status, and if it is shstk_desc, the name is not entirely true. One
possible approach is, we don't make it a struct here, and put every item
directly in thread_struct. However, the benefit of putting all in a
struct is understandable (you might argue the opposite :-)). Please
make the call, and I will do the change.
>> +static unsigned long alloc_shstk(unsigned long size)
>> +{
>> + struct mm_struct *mm = current->mm;
>> + unsigned long addr, populate;
>> + int flags = MAP_ANONYMOUS | MAP_PRIVATE;
>
> The tip-tree preferred ordering of variable declarations at the
> beginning of a function is reverse fir tree order::
>
> struct long_struct_name *descriptive_name;
> unsigned long foo, bar;
> unsigned int tmp;
> int ret;
>
> The above is faster to parse than the reverse ordering::
>
> int ret;
> unsigned int tmp;
> unsigned long foo, bar;
> struct long_struct_name *descriptive_name;
>
> And even more so than random ordering::
>
> unsigned long foo, bar;
> int ret;
> struct long_struct_name *descriptive_name;
> unsigned int tmp;
>
> Please fix it up everywhere.
>
Ok!
>> + mmap_write_lock(mm);
>> + addr = do_mmap(NULL, 0, size, PROT_READ, flags, VM_SHADOW_STACK, 0,
>> + &populate, NULL);
>> + mmap_write_unlock(mm);
>> +
>> + return addr;
>> +}
>> +
>> +int shstk_setup(void)
>> +{
>> + unsigned long addr, size;
>> + struct cet_status *cet = ¤t->thread.cet;
>> +
>> + if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
>> + return -EOPNOTSUPP;
>> +
>> + size = round_up(min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G), PAGE_SIZE);
>> + addr = alloc_shstk(size);
>> + if (IS_ERR_VALUE(addr))
>> + return PTR_ERR((void *)addr);
>> +
>> + cet->shstk_base = addr;
>> + cet->shstk_size = size;
>> +
>> + start_update_msrs();
>> + wrmsrl(MSR_IA32_PL3_SSP, addr + size);
>> + wrmsrl(MSR_IA32_U_CET, CET_SHSTK_EN);
>> + end_update_msrs();
>
> <---- newline here.
>
>> + return 0;
>> +}
>> +
>> +void shstk_free(struct task_struct *tsk)
>> +{
>> + struct cet_status *cet = &tsk->thread.cet;
>> +
>> + if (!cpu_feature_enabled(X86_FEATURE_SHSTK) ||
>> + !cet->shstk_size ||
>> + !cet->shstk_base)
>> + return;
>> +
>> + if (!tsk->mm)
>> + return;
>
> Where are the comments you said you wanna add:
>
> https://lkml.kernel.org/r/b05ee7eb-1b5d-f84f-c8f3-bfe9426e8a7d@intel.com
>
> ?
>
Yes, the comments are in patch #23: Handle thread shadow stack. I
wanted to add that in the patch that takes the path.
>> +
>> + while (1) {
>> + int r;
>> +
>> + r = vm_munmap(cet->shstk_base, cet->shstk_size);
>
> int r = vm_munmap...
>
>> +
>> + /*
>> + * vm_munmap() returns -EINTR when mmap_lock is held by
>> + * something else, and that lock should not be held for a
>> + * long time. Retry it for the case.
>> + */
>> + if (r == -EINTR) {
>> + cond_resched();
>> + continue;
>> + }
>> + break;
>> + }
>
> vm_munmap() can return other negative error values, where are you
> handling those?
>
For other error values, the loop stops.
>> +
>> + cet->shstk_base = 0;
>> + cet->shstk_size = 0;
>> +}
>> +
>> +void shstk_disable(void)
>> +{
>> + struct cet_status *cet = ¤t->thread.cet;
>
> Same question as before: what guarantees that current doesn't change
> from under you here?
The actual reading/writing MSRs are protected by fpregs_lock().
>
> One of the worst thing to do is to ignore review comments. I'd strongly
> suggest you pay more attention and avoid that in the future.
>
> Thx.
>
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