resource: fix region_intersects() vs add_memory_driver_managed()

In the Linux kernel, the following vulnerability has been resolved:

resource: fix region_intersects() vs add_memory_driver_managed()

On a system with CXL memory, the resource tree (/proc/iomem) related to

CXL memory may look like something as follows.

490000000-50fffffff : CXL Window 0

490000000-50fffffff : region0

490000000-50fffffff : dax0.0

490000000-50fffffff : System RAM (kmem)

Because drivers/dax/kmem.c calls add_memory_driver_managed() during

onlining CXL memory, which makes "System RAM (kmem)" a descendant of "CXL

Window X". This confuses region_intersects(), which expects all "System

RAM" resources to be at the top level of iomem_resource. This can lead to

bugs.

For example, when the following command line is executed to write some

memory in CXL memory range via /dev/mem,

$ dd if=data of=/dev/mem bs=$((1 << 10)) seek=$((0x490000000 >> 10)) count=1

dd: error writing '/dev/mem': Bad address

1+0 records in

0+0 records out

0 bytes copied, 0.0283507 s, 0.0 kB/s

the command fails as expected. However, the error code is wrong. It

should be "Operation not permitted" instead of "Bad address". More

seriously, the /dev/mem permission checking in devmem_is_allowed() passes

incorrectly. Although the accessing is prevented later because ioremap()

isn't allowed to map system RAM, it is a potential security issue. During

command executing, the following warning is reported in the kernel log for

calling ioremap() on system RAM.

ioremap on RAM at 0x0000000490000000 - 0x0000000490000fff

WARNING: CPU: 2 PID: 416 at arch/x86/mm/ioremap.c:216 __ioremap_caller.constprop.0+0x131/0x35d

Call Trace:

memremap+0xcb/0x184

xlate_dev_mem_ptr+0x25/0x2f

write_mem+0x94/0xfb

vfs_write+0x128/0x26d

ksys_write+0xac/0xfe

do_syscall_64+0x9a/0xfd

entry_SYSCALL_64_after_hwframe+0x4b/0x53

The details of command execution process are as follows. In the above

resource tree, "System RAM" is a descendant of "CXL Window 0" instead of a

top level resource. So, region_intersects() will report no System RAM

resources in the CXL memory region incorrectly, because it only checks the

top level resources. Consequently, devmem_is_allowed() will return 1

(allow access via /dev/mem) for CXL memory region incorrectly.

Fortunately, ioremap() doesn't allow to map System RAM and reject the

access.

So, region_intersects() needs to be fixed to work correctly with the

resource tree with "System RAM" not at top level as above. To fix it, if

we found a unmatched resource in the top level, we will continue to search

matched resources in its descendant resources. So, we will not miss any

matched resources in resource tree anymore.

In the new implementation, an example resource tree

|------------- "CXL Window 0" ------------|

|-- "System RAM" --|

will behave similar as the following fake resource tree for

region_intersects(, IORESOURCE_SYSTEM_RAM, ),

|-- "System RAM" --||-- "CXL Window 0a" --|

Where "CXL Window 0a" is part of the original "CXL Window 0" that

isn't covered by "System RAM".