Re: [PATCH v3 3/3] PCI/sysfs: Prohibit unaligned access to I/O port on non-x86

[David Laight <david.laight.linux@gmail.com>]

On Thu, 8 Jan 2026 09:59:44 +0800
Ziming Du <duziming2@huawei.com> wrote:

> From: Yongqiang Liu <liuyongqiang13@huawei.com>
> 
> Unaligned access is harmful for non-x86 archs such as arm64. When we
> use pwrite or pread to access the I/O port resources with unaligned
> offset, system will crash as follows:
> 
> Unable to handle kernel paging request at virtual address fffffbfffe8010c1
> Internal error: Oops: 0000000096000061 [#1] SMP
> Call trace:
>  _outw include/asm-generic/io.h:594 [inline]
>  logic_outw+0x54/0x218 lib/logic_pio.c:305
>  pci_resource_io drivers/pci/pci-sysfs.c:1157 [inline]
>  pci_write_resource_io drivers/pci/pci-sysfs.c:1191 [inline]
>  pci_write_resource_io+0x208/0x260 drivers/pci/pci-sysfs.c:1181
>  sysfs_kf_bin_write+0x188/0x210 fs/sysfs/file.c:158
>  kernfs_fop_write_iter+0x2e8/0x4b0 fs/kernfs/file.c:338
>  vfs_write+0x7bc/0xac8 fs/read_write.c:586
>  ksys_write+0x12c/0x270 fs/read_write.c:639
>  __arm64_sys_write+0x78/0xb8 fs/read_write.c:648
> 
> Powerpc seems affected as well, so prohibit the unaligned access
> on non-x86 archs.

I'm not sure it makes any real sense for x86 either.
IIRC io space is just like memory space, so a 16bit io access looks the
same as two 8bit accesses to an 8bit device (some put the 'data fifo' on
addresses 0 and 1 so the code could use 16bit io accesses to speed things up).
The same will have applied to misaligned accesses.
But, in reality, all device registers are aligned.

I'm not sure EFAULT is the best error code though, EINVAL might be better.
(EINVAL is returned for other address/size errors.)
EFAULT is usually returned for errors accessing the user buffer, a least
one unix system raises SIGSEGV whenever EFAULT is returned.

	David

> 
> Fixes: 8633328be242 ("PCI: Allow read/write access to sysfs I/O port resources")
> Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>
> Signed-off-by: Ziming Du <duziming2@huawei.com>
> ---
>  drivers/pci/pci-sysfs.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
> index 7e697b82c5e1..11d8b7ec4263 100644
> --- a/drivers/pci/pci-sysfs.c
> +++ b/drivers/pci/pci-sysfs.c
> @@ -31,6 +31,7 @@
>  #include <linux/of.h>
>  #include <linux/aperture.h>
>  #include <linux/unaligned.h>
> +#include <linux/align.h>
>  #include "pci.h"
>  
>  #ifndef ARCH_PCI_DEV_GROUPS
> @@ -1166,12 +1167,20 @@ static ssize_t pci_resource_io(struct file *filp, struct kobject *kobj,
>  			*(u8 *)buf = inb(port);
>  		return 1;
>  	case 2:
> +		#if !defined(CONFIG_X86)
> +			if (!IS_ALIGNED(port, count))
> +				return -EFAULT;
> +		#endif
>  		if (write)
>  			outw(*(u16 *)buf, port);
>  		else
>  			*(u16 *)buf = inw(port);
>  		return 2;
>  	case 4:
> +		#if !defined(CONFIG_X86)
> +			if (!IS_ALIGNED(port, count))
> +				return -EFAULT;
> +		#endif
>  		if (write)
>  			outl(*(u32 *)buf, port);
>  		else