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

On 5/3/2017 2:18 PM, Leif Lindholm wrote:

On Wed, May 03, 2017 at 11:07:45AM -0600, Goel, Sameer wrote:

quoted

On 5/3/2017 5:26 AM, Will Deacon wrote:

quoted

[adding some /dev/mem fans to cc]

On Tue, May 02, 2017 at 02:28:05PM -0600, Sameer Goel wrote:

quoted

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.

quoted

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?

There is no ACPI FPDT implementation in the kernel, so the userspace
systemd code is getting the FPDT table contents from /sys
and parsing the entries. The performance pointer record is a
reserved address populated by UEFI and the userspace code tries to
access it using /dev/mem. The physical address is valid, so cannot
push back on the user space code.

OK, so then we need to add support for parsing this table in the
kernel and exposing the referred-to regions in a controllable fashion.
Maybe something that belongs under /sys/firmware/efi (adding Matt), or
maybe something that deserves its own driver.

The only two use-cases for /dev/mem on arm64 are:
- Implementing interfaces in the kernel takes up-front effort.
- Being able to accidentally panic the kernel from userland.

We will see this issue with any access using /dev/mem to a MEMBLOCK_NOMAP marked
memblock. The kernel crash issue has to be fixed irrespective of ACPI FPDT support.

- Sameer

/
    Leif

quoted

https://github.com/systemd/systemd/blob/master/src/shared/acpi-fpdt.c
http://www.uefi.org/sites/default/files/resources/ACPI_6.0.pdf  5.2.23 

quoted

quoted

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.

I agree that the reserved regions are not meant to be accessed by the kernel as system 
ram. So, another option was to to return a NULL for this translation.

Since, the same usage was working on other architectures I ported over the same code to
highlight the issue.

quoted

quoted

 Code: a88120c7 d503201f d503201f 36180082 (f8408423)

Signed-off-by: Sameer Goel <redacted>
Tested-by: Shanker Donthineni <redacted>
---
 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.

This does opens up access to any valid physical address. In the short term we 
can block this crash by return NULL from this function if the memblock is MEMBLOCK_NOMAP.

Eventually we might need to add another memory type to make sure that it can be mapped.
I have not though about the exact design here.

Thanks,
Sameer
 

quoted

Will

-- 
 Qualcomm Datacenter Technologies as an affiliate of Qualcomm Technologies, Inc. Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.

-- 
 Qualcomm Datacenter Technologies as an affiliate of Qualcomm Technologies, Inc. Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.