linux-kernel - Re: [PATCH] drm/i915: Minimize uaccess exposure in i915_gem_execbuffer2

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Date:   Thu, 27 Feb 2020 19:03:42 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Chris Wilson <chris@...is-wilson.co.uk>,
        linux-kernel@...r.kernel.org, Randy Dunlap <rdunlap@...radead.org>,
        Peter Zijlstra <peterz@...radead.org>,
        intel-gfx@...ts.freedesktop.org
Subject: Re: [PATCH] drm/i915: Minimize uaccess exposure in
 i915_gem_execbuffer2_ioctl()

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
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.

-- 
Josh