powerpc/hugetlb: fix page rights verification in gup_hugepte()

[Michael Ellerman]

On Wed, 2017-07-12 at 15:03:42 UTC, Christophe Leroy wrote:
> gup_hugepte() checks if pages are present and readable, and
> when  'write' is set, also checks if the pages are writable.
> 
> Initially this was done by checking if _PAGE_PRESENT and
> _PAGE_READ were set. In addition, _PAGE_WRITE was verified for write
> accesses.
> 
> The problem is that we have to handle the three following cases:
> 1/ The target defines __PAGE_READ and __PAGE_WRITE
> 2/ The target defines __PAGE_RW
> 3/ The target defines __PAGE_RO
> 
> In case 1/, this is obvious
> In case 2/, __PAGE_READ is defined as 0 and __PAGE_WRITE as __PAGE_RW
> so it works as well.
> But in case 3, __PAGE_RW is defined as 0, which means __PAGE_WRITE is 0
> and then the test returns true (page writable) in all cases.
> 
> A first correction was attempted in commit 6b8cb66a6a7cc ("powerpc: Fix
> usage of _PAGE_RO in hugepage"), but that fix is wrong:
> instead of checking that the page is writable when write is requested,
> it checks that the page is NOT writable when write is NOT requested.
> 
> This patch adds a new pte_read() helper to check whether a page is
> readable or not. This avoids handling all possible cases in
> gup_hugepte().
> 
> Then gup_hugepte() is modified to use pte_present(), pte_read()
> and pte_write() instead of the raw flags.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy at c-s.fr>
> Reviewed-by: Aneesh Kumar K.V <aneesh.kumar at linux.vnet.ibm.com>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/ca8afd4046255ac046f8229d5159c6

cheers