perf/core: Fix potential double-fetch bug

While examining the kernel source code, I found a dangerous operation that
could turn into a double-fetch situation (a race condition bug) where the same
userspace memory region are fetched twice into kernel with sanity checks after
the first fetch while missing checks after the second fetch.

  1. The first fetch happens in line 9573 get_user(size, &uattr->size).

  2. Subsequently the 'size' variable undergoes a few sanity checks and
     transformations (line 9577 to 9584).

  3. The second fetch happens in line 9610 copy_from_user(attr, uattr, size)

  4. Given that 'uattr' can be fully controlled in userspace, an attacker can
     race condition to override 'uattr->size' to arbitrary value (say, 0xFFFFFFFF)
     after the first fetch but before the second fetch. The changed value will be
     copied to 'attr->size'.

  5. There is no further checks on 'attr->size' until the end of this function,
     and once the function returns, we lose the context to verify that 'attr->size'
     conforms to the sanity checks performed in step 2 (line 9577 to 9584).

  6. My manual analysis shows that 'attr->size' is not used elsewhere later,
     so, there is no working exploit against it right now. However, this could
     easily turns to an exploitable one if careless developers start to use
     'attr->size' later.

To fix this, override 'attr->size' from the second fetch to the one from the
first fetch, regardless of what is actually copied in.

In this way, it is assured that 'attr->size' is consistent with the checks
performed after the first fetch.

Signed-off-by: Meng Xu <mengxu.gatech@gmail.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: acme@kernel.org
Cc: alexander.shishkin@linux.intel.com
Cc: meng.xu@gatech.edu
Cc: sanidhya@gatech.edu
Cc: taesoo@gatech.edu
Link: http://lkml.kernel.org/r/1503522470-35531-1-git-send-email-meng.xu@gatech.edu


Signed-off-by: Ingo Molnar <mingo@kernel.org>