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Date: Mon, 5 Oct 2020 18:49:23 +0200 From: Jann Horn <jannh@...gle.com> To: Alexander Potapenko <glider@...gle.com> Cc: Mark Rutland <mark.rutland@....com>, Marco Elver <elver@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, "H. Peter Anvin" <hpa@...or.com>, "Paul E. McKenney" <paulmck@...nel.org>, Andrey Konovalov <andreyknvl@...gle.com>, Andrey Ryabinin <aryabinin@...tuozzo.com>, Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...en8.de>, Catalin Marinas <catalin.marinas@....com>, Christoph Lameter <cl@...ux.com>, Dave Hansen <dave.hansen@...ux.intel.com>, David Rientjes <rientjes@...gle.com>, Dmitry Vyukov <dvyukov@...gle.com>, Eric Dumazet <edumazet@...gle.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Hillf Danton <hdanton@...a.com>, Ingo Molnar <mingo@...hat.com>, Jonathan Cameron <Jonathan.Cameron@...wei.com>, Jonathan Corbet <corbet@....net>, Joonsoo Kim <iamjoonsoo.kim@....com>, Kees Cook <keescook@...omium.org>, Pekka Enberg <penberg@...nel.org>, Peter Zijlstra <peterz@...radead.org>, SeongJae Park <sjpark@...zon.com>, Thomas Gleixner <tglx@...utronix.de>, Vlastimil Babka <vbabka@...e.cz>, Will Deacon <will@...nel.org>, "the arch/x86 maintainers" <x86@...nel.org>, "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, kasan-dev <kasan-dev@...glegroups.com>, Linux ARM <linux-arm-kernel@...ts.infradead.org>, Linux Memory Management List <linux-mm@...ck.org> Subject: Re: [PATCH v3 01/10] mm: add Kernel Electric-Fence infrastructure On Mon, Oct 5, 2020 at 6:01 PM Alexander Potapenko <glider@...gle.com> wrote: > > On Tue, Sep 29, 2020 at 5:06 PM Mark Rutland <mark.rutland@....com> wrote: > > > > On Tue, Sep 29, 2020 at 04:51:29PM +0200, Marco Elver wrote: > > > On Tue, 29 Sep 2020 at 16:24, Mark Rutland <mark.rutland@....com> wrote: > > > [...] > > > > > > > > From other sub-threads it sounds like these addresses are not part of > > > > the linear/direct map. Having kmalloc return addresses outside of the > > > > linear map is going to break anything that relies on virt<->phys > > > > conversions, and is liable to make DMA corrupt memory. There were > > > > problems of that sort with VMAP_STACK, and this is why kvmalloc() is > > > > separate from kmalloc(). > > > > > > > > Have you tested with CONFIG_DEBUG_VIRTUAL? I'd expect that to scream. > > > > > > > > I strongly suspect this isn't going to be safe unless you always use an > > > > in-place carevout from the linear map (which could be the linear alias > > > > of a static carevout). > > > > > > That's an excellent point, thank you! Indeed, on arm64, a version with > > > naive static-pool screams with CONFIG_DEBUG_VIRTUAL. > > > > > > We'll try to put together an arm64 version using a carveout as you suggest. > > > > Great, thanks! > > > > Just to be clear, the concerns for DMA and virt<->phys conversions also > > apply to x86 (the x86 virt<->phys conversion behaviour is more forgiving > > in the common case, but still has cases that can go wrong). > > To clarify, shouldn't kmalloc/kmem_cache allocations used with DMA be > allocated with explicit GFP_DMA? > If so, how practical would it be to just skip such allocations in > KFENCE allocator? AFAIK GFP_DMA doesn't really mean "I will use this allocation for DMA"; it means "I will use this allocation for DMA using some ancient hardware (e.g. stuff on the ISA bus?) that only supports 16-bit physical addresses (or maybe different limits on other architectures)". There's also GFP_DMA32, which means the same thing, except with 32-bit physical addresses. You can see in e.g. __dma_direct_alloc_pages() that the GFP_DMA32 and GFP_DMA flags are only used if the hardware can't address the full physical address space supported by the CPU.
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