CVE-2026-64417

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

mm: shrinker: fix NULL pointer dereference in debugfs

shrinkerdebugfsadd() creates both "count" and "scan" debugfs files unconditionally.

That assumes every shrinker implements both countobjects() and scanobjects(), which is not guaranteed. For example, the xen-backend shrinker sets countobjects() but leaves scanobjects() NULL, so writing to its scan file calls through a NULL function pointer and panics the kernel:

BUG: kernel NULL pointer dereference, address: 0000000000000000 RIP: 0010:0x0 Code: Unable to access opcode bytes at 0xffffffffffffffd6. Call Trace: <TASK> shrinkerdebugfsscanwrite+0x12e/0x270 fullproxywrite+0x5f/0x90 vfswrite+0xde/0x420 ? filpflush+0x75/0x90 ? filpclose+0x1d/0x30 ? dodup2+0xb8/0x120 ksyswrite+0x68/0xf0 ? filpflush+0x75/0x90 dosyscall64+0xb3/0x5b0 entrySYSCALL64after_hwframe+0x76/0x7e

The count path has the same issue in principle if a shrinker omits count_objects().

To fix it, only create "count" and "scan" debugfs files when the corresponding callbacks are present.