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From: Nadav Amit <namit@vmware.com>
To: Borislav Petkov <bp@alien8.de>
Cc: Rick Edgecombe <rick.p.edgecombe@intel.com>,
	Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Damian Tometzki <linux_dti@icloud.com>,
	linux-integrity <linux-integrity@vger.kernel.org>,
	LSM List <linux-security-module@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>,
	Linux-MM <linux-mm@kvack.org>, Will Deacon <will.deacon@arm.com>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Kristen Carlson Accardi <kristen@linux.intel.com>,
	"Dock, Deneen T" <deneen.t.dock@intel.com>,
	Kees Cook <keescook@chromium.org>,
	Dave Hansen <dave.hansen@intel.com>
Subject: Re: [PATCH v2 03/20] x86/mm: temporary mm struct
Date: Thu, 31 Jan 2019 22:19:54 +0000
Message-ID: <C481E605-E19A-4EA6-AB9A-6FF4229789E0@vmware.com> (raw)
In-Reply-To: <20190131112948.GE6749@zn.tnic>

> On Jan 31, 2019, at 3:29 AM, Borislav Petkov <bp@alien8.de> wrote:
> 
>> Subject: Re: [PATCH v2 03/20] x86/mm: temporary mm struct
> 
> Subject needs a verb: "Add a temporary... "
> 
> On Mon, Jan 28, 2019 at 04:34:05PM -0800, Rick Edgecombe wrote:
>> From: Andy Lutomirski <luto@kernel.org>
>> 
>> Sometimes we want to set a temporary page-table entries (PTEs) in one of
> 
> s/a //
> 
> Also, drop the "we" and make it impartial and passive:
> 
> "Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
>  instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy
>  to do frotz", as if you are giving orders to the codebase to change
>  its behaviour."
> 
>> the cores, without allowing other cores to use - even speculatively -
>> these mappings. There are two benefits for doing so:
>> 
>> (1) Security: if sensitive PTEs are set, temporary mm prevents their use
>> in other cores. This hardens the security as it prevents exploding a
> 
> exploding or exploiting? Or exposing? :)
> 
>> dangling pointer to overwrite sensitive data using the sensitive PTE.
>> 
>> (2) Avoiding TLB shootdowns: the PTEs do not need to be flushed in
>> remote page-tables.
> 
> Those belong in the code comments below, explaining what it is going to
> be used for.

I will add it to the code as well.

> 
>> To do so a temporary mm_struct can be used. Mappings which are private
>> for this mm can be set in the userspace part of the address-space.
>> During the whole time in which the temporary mm is loaded, interrupts
>> must be disabled.
>> 
>> The first use-case for temporary PTEs, which will follow, is for poking
>> the kernel text.
>> 
>> [ Commit message was written by Nadav ]
>> 
>> Cc: Kees Cook <keescook@chromium.org>
>> Cc: Dave Hansen <dave.hansen@intel.com>
>> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>> Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
>> Tested-by: Masami Hiramatsu <mhiramat@kernel.org>
>> Signed-off-by: Andy Lutomirski <luto@kernel.org>
>> Signed-off-by: Nadav Amit <namit@vmware.com>
>> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
>> ---
>> arch/x86/include/asm/mmu_context.h | 32 ++++++++++++++++++++++++++++++
>> 1 file changed, 32 insertions(+)
>> 
>> diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
>> index 19d18fae6ec6..cd0c29e494a6 100644
>> --- a/arch/x86/include/asm/mmu_context.h
>> +++ b/arch/x86/include/asm/mmu_context.h
>> @@ -356,4 +356,36 @@ static inline unsigned long __get_current_cr3_fast(void)
>> 	return cr3;
>> }
>> 
>> +typedef struct {
> 
> Why does it have to be a typedef?

Having a different struct can prevent the misuse of using mm_structs in
unuse_temporary_mm() that were not “used” using use_temporary_mm. The
typedef, I presume, can deter users from starting to play with the internal
“private” fields.

> That prev.prev below looks unnecessary, instead of just using prev.
> 
>> +	struct mm_struct *prev;
> 
> Why "prev”?

This is obviously the previous active mm. Feel free to suggest an
alternative name.

>> +} temporary_mm_state_t;
> 
> That's kinda long - it is longer than the function name below.
> temp_mm_state_t not enough?

I will change it.

> 
>> +
>> +/*
>> + * Using a temporary mm allows to set temporary mappings that are not accessible
>> + * by other cores. Such mappings are needed to perform sensitive memory writes
>> + * that override the kernel memory protections (e.g., W^X), without exposing the
>> + * temporary page-table mappings that are required for these write operations to
>> + * other cores.
>> + *
>> + * Context: The temporary mm needs to be used exclusively by a single core. To
>> + *          harden security IRQs must be disabled while the temporary mm is
> 			      ^
> 			      ,
> 
>> + *          loaded, thereby preventing interrupt handler bugs from override the
> 
> s/override/overriding/

I will fix all of these typos, comment. Thank you.

Meta-question: could you please review the entire patch-set? This is
actually v9 of this particular patch - it was part of a separate patch-set
before. I don’t think that the patch has changed since (the real) v1.

These sporadic comments after each version really makes it hard to get this
work completed.


  reply index

Thread overview: 71+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-01-29  0:34 [PATCH v2 00/20] Merge text_poke fixes and executable lockdowns Rick Edgecombe
2019-01-29  0:34 ` [PATCH v2 01/20] Fix "x86/alternatives: Lockdep-enforce text_mutex in text_poke*()" Rick Edgecombe
2019-01-29  0:34 ` [PATCH v2 02/20] x86/jump_label: Use text_poke_early() during early init Rick Edgecombe
2019-01-29  0:34 ` [PATCH v2 03/20] x86/mm: temporary mm struct Rick Edgecombe
2019-01-31 11:29   ` Borislav Petkov
2019-01-31 22:19     ` Nadav Amit [this message]
2019-02-01  0:08       ` Borislav Petkov
2019-02-01  0:25         ` Nadav Amit
2019-02-04 14:28       ` Borislav Petkov
2019-01-29  0:34 ` [PATCH v2 04/20] fork: provide a function for copying init_mm Rick Edgecombe
2019-02-05  8:53   ` Borislav Petkov
2019-02-05  9:03     ` Nadav Amit
2019-01-29  0:34 ` [PATCH v2 05/20] x86/alternative: initializing temporary mm for patching Rick Edgecombe
2019-02-05  9:18   ` Borislav Petkov
2019-02-11  0:39   ` Nadav Amit
2019-02-11  5:18     ` Andy Lutomirski
2019-02-11 18:04       ` Nadav Amit
2019-02-11 19:07         ` Andy Lutomirski
2019-02-11 19:18           ` Nadav Amit
2019-02-11 22:47             ` Andy Lutomirski
2019-02-12 18:23               ` Nadav Amit
2019-01-29  0:34 ` [PATCH v2 06/20] x86/alternative: use temporary mm for text poking Rick Edgecombe
2019-02-05  9:58   ` Borislav Petkov
2019-02-05 11:31     ` Peter Zijlstra
2019-02-05 12:35       ` Borislav Petkov
2019-02-05 13:25         ` Peter Zijlstra
2019-02-05 17:54         ` Nadav Amit
2019-02-05 13:29       ` Peter Zijlstra
2019-01-29  0:34 ` [PATCH v2 07/20] x86/kgdb: avoid redundant comparison of patched code Rick Edgecombe
2019-01-29  0:34 ` [PATCH v2 08/20] x86/ftrace: set trampoline pages as executable Rick Edgecombe
2019-01-29  0:34 ` [PATCH v2 09/20] x86/kprobes: instruction pages initialization enhancements Rick Edgecombe
2019-02-11 18:22   ` Borislav Petkov
2019-02-11 19:36     ` Nadav Amit
2019-01-29  0:34 ` [PATCH v2 10/20] x86: avoid W^X being broken during modules loading Rick Edgecombe
2019-02-11 18:29   ` Borislav Petkov
2019-02-11 18:45     ` Nadav Amit
2019-02-11 19:01       ` Borislav Petkov
2019-02-11 19:09         ` Nadav Amit
2019-02-11 19:10           ` Borislav Petkov
2019-02-11 19:27             ` Nadav Amit
2019-02-11 19:42               ` Borislav Petkov
2019-02-11 20:32                 ` Nadav Amit
2019-03-07 15:10                   ` [PATCH] x86/cpufeature: Remove __pure attribute to _static_cpu_has() Borislav Petkov
2019-03-07 16:43                     ` hpa
2019-03-07 17:02                       ` Borislav Petkov
2019-03-07  7:29                 ` [PATCH v2 10/20] x86: avoid W^X being broken during modules loading Borislav Petkov
2019-03-07 16:53                   ` hpa
2019-03-07 17:06                     ` Borislav Petkov
2019-03-07 20:02                       ` Andy Lutomirski
2019-03-07 20:25                         ` Borislav Petkov
2019-01-29  0:34 ` [PATCH v2 11/20] x86/jump-label: remove support for custom poker Rick Edgecombe
2019-02-11 18:37   ` Borislav Petkov
2019-01-29  0:34 ` [PATCH v2 12/20] x86/alternative: Remove the return value of text_poke_*() Rick Edgecombe
2019-01-29  0:34 ` [PATCH v2 13/20] Add set_alias_ function and x86 implementation Rick Edgecombe
2019-02-11 19:09   ` Borislav Petkov
2019-02-11 19:27     ` Edgecombe, Rick P
2019-02-11 22:59     ` Andy Lutomirski
2019-02-12  0:01       ` Edgecombe, Rick P
2019-01-29  0:34 ` [PATCH v2 14/20] mm: Make hibernate handle unmapped pages Rick Edgecombe
2019-02-19 11:04   ` Borislav Petkov
2019-02-19 21:28     ` Edgecombe, Rick P
2019-02-20 16:07       ` Borislav Petkov
2019-01-29  0:34 ` [PATCH v2 15/20] vmalloc: New flags for safe vfree on special perms Rick Edgecombe
2019-02-19 12:48   ` Borislav Petkov
2019-02-19 19:42     ` Edgecombe, Rick P
2019-02-20 16:14       ` Borislav Petkov
2019-01-29  0:34 ` [PATCH v2 16/20] modules: Use vmalloc special flag Rick Edgecombe
2019-01-29  0:34 ` [PATCH v2 17/20] bpf: " Rick Edgecombe
2019-01-29  0:34 ` [PATCH v2 18/20] x86/ftrace: " Rick Edgecombe
2019-01-29  0:34 ` [PATCH v2 19/20] x86/kprobes: " Rick Edgecombe
2019-01-29  0:34 ` [PATCH v2 20/20] x86/alternative: comment about module removal races Rick Edgecombe

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