CVE-2024-49878

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

resource: fix regionintersects() vs addmemorydrivermanaged()

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 addmemorydrivermanaged() during onlining CXL memory, which makes "System RAM (kmem)" a descendant of "CXL Window X". This confuses regionintersects(), 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 devmemisallowed() 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 _ioremapcaller.constprop.0+0x131/0x35d Call Trace: memremap+0xcb/0x184 xlatedevmemptr+0x25/0x2f writemem+0x94/0xfb vfswrite+0x128/0x26d ksyswrite+0xac/0xfe dosyscall64+0x9a/0xfd entrySYSCALL64afterhwframe+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, regionintersects() will report no System RAM resources in the CXL memory region incorrectly, because it only checks the top level resources. Consequently, devmemis_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 regionintersects(, IORESOURCESYSTEM_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".