Re: [PATCH] x86/asm/entry: Remove user_mode_ignore_vm86()

[Andy Lutomirski]

On Sun, Mar 29, 2015 at 5:13 AM, Borislav Petkov <bp@xxxxxxxxx> wrote:
> On Sun, Mar 29, 2015 at 11:02:34AM +0200, Ingo Molnar wrote:
>> So how about doing the patch below?
>>
>> Thanks,
>>
>>       Ingo
>>
>> ===================================>
>> From 6677d6f073cfda7f1036eb06d13faaad5c6742cc Mon Sep 17 00:00:00 2001
>> From: Ingo Molnar <mingo@xxxxxxxxxx>
>> Date: Sun, 29 Mar 2015 09:10:08 +0200
>> Subject: [PATCH] x86/asm/entry: Remove user_mode_ignore_vm86()
>>
>> user_mode_ignore_vm86() can be used instead of user_mode(), in
>> places where we have already done a v8086_mode() security
>> check of ptregs.
>>
>> But doing this check in the wrong place would be a bug that could
>> result in security problems, and also the naming still isn't very clear.
>>
>> Furthermore, it only affects 32-bit kernels, while most development
>> happens on 64-bit kernels.
>>
>> If we replace them with user_mode() checks then the cost is only a
>> very minor increase in various slowpaths:
>>
>>    text             data   bss     dec              hex    filename
>>    10573391         703562 1753042 13029995         c6d26b vmlinux.o.before
>>    10573423         703562 1753042 13030027         c6d28b vmlinux.o.after
>>
>> So lets get rid of this distinction once and for all.
>>
>> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
>> Cc: Borislav Petkov <bp@xxxxxxxxx>
>> Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
>> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
>> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
>> ---
>>  arch/x86/include/asm/ptrace.h    | 17 -----------------
>>  arch/x86/kernel/cpu/perf_event.c |  2 +-
>>  arch/x86/kernel/traps.c          |  6 +++---
>>  3 files changed, 4 insertions(+), 21 deletions(-)
>
> I had some doubts about people using user_mode_ignore_vm86() in the
> wrong way and thus introducing sec. bugs.
>
> Since this is only on the slow path, simplifying the code makes sense to
> me.
>
> Acked-by: Borislav Petkov <bp@xxxxxxx>
>

Ditto.

Acked-by: Andy Lutomirski <luto@xxxxxxxxxx>

> --
> Regards/Gruss,
>     Boris.
>
> ECO tip #101: Trim your mails when you reply.
> --



-- 
Andy Lutomirski
AMA Capital Management, LLC
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