CVE-2026-64417mm: shrinker: fix NULL pointer dereference in debugfs

Official description

In the Linux kernel, the following vulnerability has been resolved: mm: shrinker: fix NULL pointer dereference in debugfs shrinker_debugfs_add() creates both "count" and "scan" debugfs files unconditionally. That assumes every shrinker implements both count_objects() and scan_objects(), which is not guaranteed. For example, the xen-backend shrinker sets count_objects() but leaves scan_objects() 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> shrinker_debugfs_scan_write+0x12e/0x270 full_proxy_write+0x5f/0x90 vfs_write+0xde/0x420 ? filp_flush+0x75/0x90 ? filp_close+0x1d/0x30 ? do_dup2+0xb8/0x120 ksys_write+0x68/0xf0 ? filp_flush+0x75/0x90 do_syscall_64+0xb3/0x5b0 entry_SYSCALL_64_after_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.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/ebb45c2648b1f60715fd283700f651e05e431231
• https://git.kernel.org/stable/c/09d2407985b8ce3e831f9d4310fe7ac06a6b3ae9
• https://git.kernel.org/stable/c/36f8534f461222291a74156ab91f3ba9f09b6f93
• https://git.kernel.org/stable/c/006467ab932698612398f853344a7405164541f4
• https://git.kernel.org/stable/c/b9beed2322f3538b0d2d53307062da4102b8d8d8
• https://git.kernel.org/stable/c/e30453c61e185e914fde83c650e268067b140218