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Date: Wed, 13 Jun 2018 05:24:01 -0700
From: Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH] mm: cma: honor __GFP_ZERO flag in cma_alloc()
Message-ID: <20180613122359.GA8695@bombadil.infradead.org>
References: <CGME20180613085851eucas1p20337d050face8ff8ea87674e16a9ccd2@eucas1p2.samsung.com>
 <20180613085851eucas1p20337d050face8ff8ea87674e16a9ccd2~3rI_9nj8b0455904559eucas1p2C@eucas1p2.samsung.com>
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To: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Andrew Morton <akpm@linux-foundation.org>, Michal Nazarewicz <mina86@mina86.com>, Joonsoo Kim <iamjoonsoo.kim@lge.com>, Vlastimil Babka <vbabka@suse.cz>

On Wed, Jun 13, 2018 at 10:58:37AM +0200, Marek Szyprowski wrote:
> cma_alloc() function has gfp mask parameter, so users expect that it
> honors typical memory allocation related flags. The most imporant from
> the security point of view is handling of __GFP_ZERO flag, because memory
> allocated by this function usually can be directly remapped to userspace
> by device drivers as a part of multimedia processing and ignoring this
> flag might lead to leaking some kernel structures to userspace.
> Some callers of this function (for example arm64 dma-iommu glue code)
> already assumed that the allocated buffers are cleared when this flag
> is set. To avoid such issues, add simple code for clearing newly
> allocated buffer when __GFP_ZERO flag is set. Callers will be then
> updated to skip implicit clearing or adjust passed gfp flags.

I think the documentation for this function needs improving.  For example,
GFP_ATOMIC does not work (it takes a mutex lock, so it can sleep).
At the very least, the kernel-doc needs:

 * Context: Process context (may sleep even if GFP flags indicate otherwise).

Unless someone wants to rework this allocator to use spinlocks instead
of mutexes ...