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".
Link: https://lkml.kernel.org/r/20240906030713.204292-2-ying.huang@intel.com Fixes: c221c0b0308f ("device-dax: "Hotplug" persistent memory for use like normal RAM") Signed-off-by: "Huang, Ying" ying.huang@intel.com Cc: Dan Williams dan.j.williams@intel.com Cc: David Hildenbrand david@redhat.com Cc: Davidlohr Bueso dave@stgolabs.net Cc: Jonathan Cameron jonathan.cameron@huawei.com Cc: Dave Jiang dave.jiang@intel.com Cc: Alison Schofield alison.schofield@intel.com Cc: Vishal Verma vishal.l.verma@intel.com Cc: Ira Weiny ira.weiny@intel.com Cc: Alistair Popple apopple@nvidia.com Cc: Andy Shevchenko andriy.shevchenko@linux.intel.com Cc: Bjorn Helgaas bhelgaas@google.com Cc: Baoquan He bhe@redhat.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org --- kernel/resource.c | 58 ++++++++++++++++++++++++++++++++++++++++------- 1 file changed, 50 insertions(+), 8 deletions(-)
diff --git a/kernel/resource.c b/kernel/resource.c index cb441e3e7670..972bf1bf4d69 100644 --- a/kernel/resource.c +++ b/kernel/resource.c @@ -480,20 +480,62 @@ EXPORT_SYMBOL_GPL(page_is_ram); static int __region_intersects(resource_size_t start, size_t size, unsigned long flags, unsigned long desc) { - struct resource res; + resource_size_t ostart, oend; int type = 0; int other = 0; - struct resource *p; + struct resource *p, *dp; + bool is_type, covered; + struct resource res;
res.start = start; res.end = start + size - 1;
for (p = iomem_resource.child; p ; p = p->sibling) { - bool is_type = (((p->flags & flags) == flags) && - ((desc == IORES_DESC_NONE) || - (desc == p->desc))); - - if (resource_overlaps(p, &res)) - is_type ? type++ : other++; + if (!resource_overlaps(p, &res)) + continue; + is_type = (p->flags & flags) == flags && + (desc == IORES_DESC_NONE || desc == p->desc); + if (is_type) { + type++; + continue; + } + /* + * Continue to search in descendant resources as if the + * matched descendant resources cover some ranges of 'p'. + * + * |------------- "CXL Window 0" ------------| + * |-- "System RAM" --| + * + * will behave similar as the following fake resource + * tree when searching "System RAM". + * + * |-- "System RAM" --||-- "CXL Window 0a" --| + */ + covered = false; + ostart = max(res.start, p->start); + oend = min(res.end, p->end); + for (dp = p->child; dp; dp = next_resource(dp)) { + if (!resource_overlaps(dp, &res)) + continue; + is_type = (dp->flags & flags) == flags && + (desc == IORES_DESC_NONE || desc == dp->desc); + if (is_type) { + type++; + /* + * Range from 'ostart' to 'dp->start' + * isn't covered by matched resource. + */ + if (dp->start > ostart) + break; + if (dp->end >= oend) { + covered = true; + break; + } + /* Remove covered range */ + ostart = max(ostart, dp->end + 1); + } + } + if (!covered) + other++; }
if (type == 0)