From: Jeff Xu <jeffxu@google.com>
To: Dave Hansen <dave.hansen@intel.com>
Cc: jeffxu@chromium.org, luto@kernel.org, jorgelo@chromium.org,
keescook@chromium.org, groeck@chromium.org, jannh@google.com,
sroettger@google.com, akpm@linux-foundation.org,
linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
linux-mm@kvack.org, linux-hardening@vger.kernel.org
Subject: Re: [PATCH 2/6] PKEY: Add arch_check_pkey_enforce_api()
Date: Thu, 18 May 2023 15:51:38 -0700 [thread overview]
Message-ID: <CALmYWFtu-WzOTEs2aWU3zMW=KZUjaaL7OTE7hQtHjfwQfQNyEA@mail.gmail.com> (raw)
In-Reply-To: <6dbbc3da-78c9-8101-d52a-0be47da9d67e@intel.com>
On Thu, May 18, 2023 at 2:43 PM Dave Hansen <dave.hansen@intel.com> wrote:
>
> On 5/15/23 06:05, jeffxu@chromium.org wrote:
> > +static inline int __arch_check_vma_pkey_for_write(struct vm_area_struct *vma)
> > +{
> > + int pkey = vma_pkey(vma);
> > +
> > + if (mm_pkey_enforce_api(vma->vm_mm, pkey)) {
> > + if (!__pkru_allows_write(read_pkru(), pkey))
> > + return -EACCES;
> > + }
> > +
> > + return 0;
> > +}
>
> Please think very carefully about what I'm about to say:
>
> What connects vma->vm_mm to read_pkru() here?
>
> Now think about what happens when we have kthread_use_mm() or a ptrace()
> doing get_task_mm() and working on another process's mm or VMA.
>
> Look at arch_vma_access_permitted() and notice how it avoids read_pkru()
> for 'foreign' aka. 'remote' accesses:
>
> > static inline bool arch_vma_access_permitted(struct vm_area_struct *vma,
> > bool write, bool execute, bool foreign)
> > {
> ...
> > if (foreign || vma_is_foreign(vma))
> > return true;
> > return // check read_pkru()
> > }
>
> In other words, it lets all remote accesses right through. That's
> because there is *NOTHING* that fundamentally and tightly connects the
> PKRU value in this context to the VMA or the context that initiated this
> operation.
>
> If your security model depends on PKRU protection, this 'remote'
> disconnection is problematic. The PKRU enforcement inside the kernel is
> best-effort. That usually doesn't map into the security space very well.
>
> Do you have a solid handle on all call paths that will reach
> __arch_check_vma_pkey_for_write() and can you ensure they are all
> non-remote?
Is this about the attack scenario where the attacker uses ptrace()
into the chrome process ? if so it is not in our threat model, and
that is more related to sandboxing on the host.
Or is this about io_uring? Yes, io_uring kernel thread breaks our
expectations of PKRU & user space threads, however I thought the break
is not just for this - any syscall involved in memory operation will
break after into io_uring ?
Other than those, yes, I try to ensure the check is only used at the
beginning of syscall entry in all cases, which should be non-remote I
hope.
Thanks
-Jeff
next prev parent reply other threads:[~2023-05-18 22:52 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-15 13:05 [PATCH 0/6] Memory Mapping (VMA) protection using PKU - set 1 jeffxu
2023-05-15 13:05 ` [PATCH 1/6] PKEY: Introduce PKEY_ENFORCE_API flag jeffxu
2023-05-16 23:14 ` Dave Hansen
2023-05-16 23:55 ` Jeff Xu
2023-05-17 11:07 ` Stephen Röttger
2023-05-15 13:05 ` [PATCH 2/6] PKEY: Add arch_check_pkey_enforce_api() jeffxu
2023-05-18 21:43 ` Dave Hansen
2023-05-18 22:51 ` Jeff Xu [this message]
2023-05-19 0:00 ` Dave Hansen
2023-05-19 11:22 ` Stephen Röttger
2023-05-15 13:05 ` [PATCH 3/6] PKEY: Apply PKEY_ENFORCE_API to mprotect jeffxu
2023-05-16 20:07 ` Kees Cook
2023-05-16 22:23 ` Jeff Xu
2023-05-16 23:18 ` Dave Hansen
2023-05-16 23:36 ` Jeff Xu
2023-05-17 4:50 ` Jeff Xu
2023-05-15 13:05 ` [PATCH 4/6] PKEY:selftest pkey_enforce_api for mprotect jeffxu
2023-05-15 13:05 ` [PATCH 5/6] KEY: Apply PKEY_ENFORCE_API to munmap jeffxu
2023-05-16 20:06 ` Kees Cook
2023-05-16 22:24 ` Jeff Xu
2023-05-16 23:23 ` Dave Hansen
2023-05-17 0:08 ` Jeff Xu
2023-05-15 13:05 ` [PATCH 6/6] PKEY:selftest pkey_enforce_api for munmap jeffxu
2023-05-15 14:28 ` [PATCH 0/6] Memory Mapping (VMA) protection using PKU - set 1 Dave Hansen
2023-05-16 7:06 ` Stephen Röttger
2023-05-16 22:41 ` Dave Hansen
2023-05-17 10:51 ` Stephen Röttger
2023-05-17 15:07 ` Dave Hansen
2023-05-17 15:21 ` Jeff Xu
2023-05-17 15:29 ` Dave Hansen
2023-05-17 23:48 ` Jeff Xu
2023-05-18 15:37 ` Dave Hansen
2023-05-18 20:20 ` Jeff Xu
2023-05-18 21:04 ` Dave Hansen
2023-05-19 11:13 ` Stephen Röttger
2023-05-24 20:15 ` Jeff Xu
2023-06-01 1:39 ` Jeff Xu
2023-06-01 16:16 ` Dave Hansen
2023-05-31 23:02 ` Jeff Xu
2023-05-16 20:08 ` Kees Cook
2023-05-16 22:17 ` Jeff Xu
2023-05-16 22:30 ` Dave Hansen
2023-05-16 23:39 ` Jeff Xu
2023-05-17 10:49 ` Stephen Röttger
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