Re: [PATCH] arm64: Add translation functions for /dev/mem read/write

[Will Deacon]

[adding some /dev/mem fans to cc]

On Tue, May 02, 2017 at 02:28:05PM -0600, Sameer Goel wrote:
> Port architecture specific xlate and unxlate functions for /dev/mem
> read/write. This sets up the mapping for a valid physical address if a
> kernel direct mapping is not already present.
> 
> This is a generic issue as a user space app should not be allowed to crash
> the kernel.

> This issue was observed when systemd tried to access performance
> pointer record from the FPDT table.

Why is it doing that? Is there not a way to get this via /sys?

> Ported from commit e045fb2a988a ("x86: PAT avoid aliasing in /dev/mem
> read/write")
> 
> Crash Signature:
>  Unable to handle kernel paging request at virtual address ffff800008ff0000
>  pgd = ffff8007de8b2200
>  [ffff800008ff0000] *pgd=0000000000000000, *pud=0000000000000000
>  Internal error: Oops: 96000007 [#1] SMP
> ................
>  CPU: 0 PID: 1 Comm: systemd Not tainted 4.10.0 #1
>  task: ffff8007c0820000 task.stack: ffff8007c0900000
>  PC is at __arch_copy_to_user+0xb4/0x280
>  LR is at read_mem+0xc0/0x138
>  pc : [<ffff0000084b3bb4>] lr : [<ffff00000869d178>]
> pstate: 80000145
>  sp : ffff8007c0903d40
> ....................
>  x3 : ffff800800000000 x2 : 0000000000000008
>  x1 : ffff800008ff0000 x0 : 0000fffff6fdac00
> ....................
>  Call trace:
>  Exception stack(0xffff8007c0903b70 to 0xffff8007c0903ca0)
>  [<ffff0000084b3bb4>] __arch_copy_to_user+0xb4/0x280
>  [<ffff0000082454d0>] __vfs_read+0x48/0x130
>  [<ffff0000082467dc>] vfs_read+0x8c/0x148
>  [<ffff000008247a34>] SyS_pread64+0x94/0xa8
>  [<ffff0000080833b0>] el0_svc_naked+0x24/0x28

So this certainly looks like a kernel bug, but I don't think your patch is
the right way to fix it.

>  Code: a88120c7 d503201f d503201f 36180082 (f8408423)
> 
> Signed-off-by: Sameer Goel <sgoel@xxxxxxxxxxxxxx>
> Tested-by: Shanker Donthineni <shankerd@xxxxxxxxxxxxxx>
> ---
>  arch/arm64/include/asm/io.h |  5 +++++
>  arch/arm64/mm/ioremap.c     | 31 +++++++++++++++++++++++++++++++
>  2 files changed, 36 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/io.h b/arch/arm64/include/asm/io.h
> index 0c00c87..c869ea4 100644
> --- a/arch/arm64/include/asm/io.h
> +++ b/arch/arm64/include/asm/io.h
> @@ -183,6 +183,11 @@ static inline u64 __raw_readq(const volatile void __iomem *addr)
>  #define iowrite32be(v,p)	({ __iowmb(); __raw_writel((__force __u32)cpu_to_be32(v), p); })
>  #define iowrite64be(v,p)	({ __iowmb(); __raw_writeq((__force __u64)cpu_to_be64(v), p); })
>  
> +extern void *xlate_dev_mem_ptr(phys_addr_t phys);
> +extern void unxlate_dev_mem_ptr(phys_addr_t phys, void *addr);
> +
> +#define xlate_dev_mem_ptr xlate_dev_mem_ptr
> +#define unxlate_dev_mem_ptr unxlate_dev_mem_ptr
>  #include <asm-generic/io.h>
>  
>  /*
> diff --git a/arch/arm64/mm/ioremap.c b/arch/arm64/mm/ioremap.c
> index c4c8cd4..ba7e63b 100644
> --- a/arch/arm64/mm/ioremap.c
> +++ b/arch/arm64/mm/ioremap.c
> @@ -24,6 +24,7 @@
>  #include <linux/mm.h>
>  #include <linux/vmalloc.h>
>  #include <linux/io.h>
> +#include <linux/memblock.h>
>  
>  #include <asm/fixmap.h>
>  #include <asm/tlbflush.h>
> @@ -105,6 +106,36 @@ void __iomem *ioremap_cache(phys_addr_t phys_addr, size_t size)
>  EXPORT_SYMBOL(ioremap_cache);
>  
>  /*
> + * Convert a physical pointer to a virtual kernel pointer for /dev/mem
> + * access
> + */
> +void *xlate_dev_mem_ptr(phys_addr_t phys)
> +{
> +	unsigned long start  = phys &  PAGE_MASK;
> +	unsigned long offset = phys & ~PAGE_MASK;
> +	void *vaddr;
> +
> +	/* If page is RAM, we can use __va. Otherwise ioremap and unmap. */
> +	if (page_is_ram(start >> PAGE_SHIFT) && memblock_is_memory(phys))
> +		return __va(phys);
> +
> +	vaddr = ioremap_cache(start, PAGE_SIZE);

Blindly using ioremap like this looks unsafe, since we could accidentally
set conflict with the attributes of a mapping used by something else (e.g.
firmware running on another CPU).

I'd like to understand more about the crash, so we can see work out how to
fix this properly.

Will