[PATCH] mm: shrinker: fix NULL pointer dereference in debugfs

[Qi Zheng <qi.zheng@linux.dev>]

From: Qi Zheng <zhengqi.arch@bytedance.com>

The 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.

Fixes: bbf535fd6f06 ("mm: shrinkers: add scan interface for shrinker debugfs")
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
---
 mm/shrinker_debug.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/mm/shrinker_debug.c b/mm/shrinker_debug.c
index cda4e86428c8..cafb56630132 100644
--- a/mm/shrinker_debug.c
+++ b/mm/shrinker_debug.c
@@ -183,10 +183,12 @@ int shrinker_debugfs_add(struct shrinker *shrinker)
 	}
 	shrinker->debugfs_entry = entry;
 
-	debugfs_create_file("count", 0440, entry, shrinker,
-			    &shrinker_debugfs_count_fops);
-	debugfs_create_file("scan", 0220, entry, shrinker,
-			    &shrinker_debugfs_scan_fops);
+	if (shrinker->count_objects)
+		debugfs_create_file("count", 0440, entry, shrinker,
+				    &shrinker_debugfs_count_fops);
+	if (shrinker->scan_objects)
+		debugfs_create_file("scan", 0220, entry, shrinker,
+				    &shrinker_debugfs_scan_fops);
 	return 0;
 }
 
-- 
2.54.0