Date:   Wed, 28 Aug 2019 22:32:47 +0000
From:   Alistair Francis <Alistair.Francis@....com>
To:     "anup@...infault.org" <anup@...infault.org>,
        "paul.walmsley@...ive.com" <paul.walmsley@...ive.com>
CC:     "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Atish Patra <Atish.Patra@....com>,
        "palmer@...ive.com" <palmer@...ive.com>,
        Anup Patel <Anup.Patel@....com>,
        "hch@...radead.org" <hch@...radead.org>
Subject: Re: [PATCH v2] RISC-V: Fix FIXMAP area corruption on RV32 systems

On Tue, 2019-08-27 at 08:11 +0530, Anup Patel wrote:
> On Tue, Aug 27, 2019 at 5:43 AM Paul Walmsley <
> paul.walmsley@...ive.com> wrote:
> > Hello Anup,
> > 
> > On Mon, 19 Aug 2019, Anup Patel wrote:
> > 
> > > Currently, various virtual memory areas of Linux RISC-V are
> > > organized
> > > in increasing order of their virtual addresses is as follows:
> > > 1. User space area (This is lowest area and starts at 0x0)
> > > 2. FIXMAP area
> > > 3. VMALLOC area
> > > 4. Kernel area (This is highest area and starts at PAGE_OFFSET)
> > > 
> > > The maximum size of user space aread is represented by TASK_SIZE.
> > > 
> > > On RV32 systems, TASK_SIZE is defined as VMALLOC_START which
> > > causes the
> > > user space area to overlap the FIXMAP area. This allows user
> > > space apps
> > > to potentially corrupt the FIXMAP area and kernel OF APIs will
> > > crash
> > > whenever they access corrupted FDT in the FIXMAP area.
> > > 
> > > On RV64 systems, TASK_SIZE is set to fixed 256GB and no other
> > > areas
> > > happen to overlap so we don't see any FIXMAP area corruptions.
> > > 
> > > This patch fixes FIXMAP area corruption on RV32 systems by
> > > setting
> > > TASK_SIZE to FIXADDR_START.
> > 
> > This part -- the TASK_SIZE change -- makes sense to me.
> > 
> > However, the patch also changes FIXADDR_SIZE to be defined in terms
> > of
> > page table-related constants.  Previously, FIXADDR_SIZE was based
> > on
> > __end_of_fixed_addresses, as it is for most other architectures.
> > The part
> > of the patch that changes FIXADDR_SIZE seems unrelated to the
> > actual fix.
> > 
> > If that's indeed the case -- that the change to FIXADDR_SIZE is
> > unrelated
> > from the fix -- could you please split that into a separate patch,
> > with a
> > description of the rationale?  I think I understand why you're
> > proposing
> > it, but it seems odd to explicitly connect it to page table-related
> > constants, rather than the contents of "enum fixed_addresses", and
> > I'm
> > reluctant to merge that part of this patch without a bit more
> > discussion.
> 
> The FIXADDR_SIZE change is related to the TASK_SIZE requirement and
> it is not a separate change because:
> 
> 1. TASK_SIZE must be evenly divisible by PGDIR_SIZE. The
> FIXADDR_START
> is defined as (FIXADDR_TOP - FIXADDR_SIZE). The original FIXADDR_SIZE
> defined in-terms of __end_of_fixed_addresses is not a multiple of
> PGDIR_SIZE
> hence it makes sense to make FIXADDR_SIZE as PGDIR_SIZE.
> 
> 2. Let say we ignore point1 above then still we cannot continue to
> express
> FIXADDR_SIZE in-terms of __end_of_fixed_addresses because of cyclic
> header dependency where asm/fixmap.h includes asm/pgtable.h and
> __end_of_fixed_addresses is defined in asm/fixmap.h. We certainly
> need
> to move FIXADDR_TOP, FIXADDR_START, and FIXADDR_SIZE to
> asm/pgtable.h so that we can express TASK_SIZE as FIXADDR_START
> for RV32. If we don't simplify FIXADDR_SIZE then it will result in
> compile
> errors.

Ping!

Are we going to regress 32-bit support in 5.3?

Alistair

> 
> Regards,
> Anup
> 
> > 
> > > We also move FIXADDR_TOP, FIXADDR_SIZE, and FIXADDR_START defines
> > > to
> > > asm/pgtable.h so that we can avoid cyclic header includes.
> > > 
> > > Signed-off-by: Anup Patel <anup.patel@....com>
> > > Tested-by: Alistair Francis <alistair.francis@....com>
> > > Reviewed-by: Christoph Hellwig <hch@....de>
> > > ---
> > > Changes since v1:
> > > - Drop braces from "#define FIXADDR_TOP"
> > > ---
> > >  arch/riscv/include/asm/fixmap.h  |  4 ----
> > >  arch/riscv/include/asm/pgtable.h | 12 ++++++++++--
> > >  2 files changed, 10 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/arch/riscv/include/asm/fixmap.h
> > > b/arch/riscv/include/asm/fixmap.h
> > > index 9c66033c3a54..161f28d04a07 100644
> > > --- a/arch/riscv/include/asm/fixmap.h
> > > +++ b/arch/riscv/include/asm/fixmap.h
> > > @@ -30,10 +30,6 @@ enum fixed_addresses {
> > >       __end_of_fixed_addresses
> > >  };
> > > 
> > > -#define FIXADDR_SIZE         (__end_of_fixed_addresses *
> > > PAGE_SIZE)
> > > -#define FIXADDR_TOP          (VMALLOC_START)
> > > -#define FIXADDR_START                (FIXADDR_TOP -
> > > FIXADDR_SIZE)
> > > -
> > >  #define FIXMAP_PAGE_IO               PAGE_KERNEL
> > > 
> > >  #define __early_set_fixmap   __set_fixmap
> > > diff --git a/arch/riscv/include/asm/pgtable.h
> > > b/arch/riscv/include/asm/pgtable.h
> > > index a364aba23d55..c24a083b3e12 100644
> > > --- a/arch/riscv/include/asm/pgtable.h
> > > +++ b/arch/riscv/include/asm/pgtable.h
> > > @@ -420,14 +420,22 @@ static inline void pgtable_cache_init(void)
> > >  #define VMALLOC_END      (PAGE_OFFSET - 1)
> > >  #define VMALLOC_START    (PAGE_OFFSET - VMALLOC_SIZE)
> > > 
> > > +#define FIXADDR_TOP      VMALLOC_START
> > > +#ifdef CONFIG_64BIT
> > > +#define FIXADDR_SIZE     PMD_SIZE
> > > +#else
> > > +#define FIXADDR_SIZE     PGDIR_SIZE
> > > +#endif
> > > +#define FIXADDR_START    (FIXADDR_TOP - FIXADDR_SIZE)
> > > +
> > >  /*
> > > - * Task size is 0x4000000000 for RV64 or 0xb800000 for RV32.
> > > + * Task size is 0x4000000000 for RV64 or 0x9fc00000 for RV32.
> > >   * Note that PGDIR_SIZE must evenly divide TASK_SIZE.
> > >   */
> > >  #ifdef CONFIG_64BIT
> > >  #define TASK_SIZE (PGDIR_SIZE * PTRS_PER_PGD / 2)
> > >  #else
> > > -#define TASK_SIZE VMALLOC_START
> > > +#define TASK_SIZE FIXADDR_START
> > >  #endif
> > > 
> > >  #include <asm-generic/pgtable.h>
> > > --
> > > 2.17.1
> > > 
> > 
> > - Paul

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