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Subject: Re: [PATCH] drm/i915: Minimize uaccess exposure in
 i915_gem_execbuffer2_ioctl()
To:     Josh Poimboeuf <jpoimboe@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>
Cc:     Chris Wilson <chris@chris-wilson.co.uk>,
        linux-kernel@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        intel-gfx@lists.freedesktop.org
References: <ed52cfb852d2772bf20f48614d75f1d1b1451995.1582841072.git.jpoimboe@redhat.com>
 <20200227223542.GE23230@ZenIV.linux.org.uk>
 <20200228010342.3j3awgvvgvitif7z@treble>
From:   Randy Dunlap <rdunlap@infradead.org>
Message-ID: <ea7b1ae2-560d-e9ff-32bf-5068de05f954@infradead.org>
Date:   Thu, 27 Feb 2020 18:42:02 -0800
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On 2/27/20 5:03 PM, Josh Poimboeuf wrote:
> On Thu, Feb 27, 2020 at 10:35:42PM +0000, Al Viro wrote:
>> On Thu, Feb 27, 2020 at 04:08:26PM -0600, Josh Poimboeuf wrote:
>>> With CONFIG_CC_OPTIMIZE_FOR_SIZE, objtool reports:
>>>
>>>   drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool: i915_gem_execbuffer2_ioctl()+0x5b7: call to gen8_canonical_addr() with UACCESS enabled
>>>
>>> This means i915_gem_execbuffer2_ioctl() is calling gen8_canonical_addr()
>>> -- and indirectly, sign_extend64() -- from the user_access_begin/end
>>> critical region (i.e, with SMAP disabled).
>>>
>>> While it's probably harmless in this case, in general we like to avoid
>>> extra function calls in SMAP-disabled regions because it can open up
>>> inadvertent security holes.
>>>
>>> Fix it by moving the gen8_canonical_addr() conversion to a separate loop
>>> before user_access_begin() is called.
>>>
>>> Note that gen8_canonical_addr() is now called *before* masking off the
>>> PIN_OFFSET_MASK bits.  That should be ok because it just does a sign
>>> extension and ignores the masked lower bits anyway.
>>
>> How painful would it be to inline the damn thing?
>> <looks>
>> static inline u64 gen8_canonical_addr(u64 address)
>> {
>>         return sign_extend64(address, GEN8_HIGH_ADDRESS_BIT);
>> }
>> static inline __s64 sign_extend64(__u64 value, int index)
>> {
>>         __u8 shift = 63 - index;
>>         return (__s64)(value << shift) >> shift;
>> }
>>
>> What the hell?  Josh, what kind of .config do you have that these are
>> _not_ inlined?
> 
> I think this was seen with CONFIG_CC_OPTIMIZE_FOR_SIZE, which tends to

so the commit message correctly says.

> ignore inline.
> 
>> And why not mark gen8_canonical_addr() __always_inline?
> 
> Right, marking those two functions as __always_inline is the other
> option.  The problem is, if you keep doing it, eventually you end up
> with __always_inline-itis spreading all over the place.  And it affects
> all the other callers, at least in the CONFIG_CC_OPTIMIZE_FOR_SIZE case.
> At least this fix is localized.
> 
> But I agree my patch isn't ideal either.

fwiw,
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested


thanks.
-- 
~Randy