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mm/damon/sysfs-schemes: protect path kfree() with damon_sysfs_lock

damon_sysfs_quot_goal->path can be read and written by users, via DAMON
sysfs 'path' file. It can also be indirectly read, for the parameters
{on,off}line committing to DAMON. The reads for parameters committing are
protected by damon_sysfs_lock to avoid the sysfs files being destroyed
while any of the parameters are being read. But the user-driven direct
reads and writes are not protected by any lock, while the write is
deallocating the path-pointing buffer. As a result, the readers could
read the already freed buffer (user-after-free). Note that the user-reads
don't race when the same open file is used by the writer, due to kernfs's
open file locking. Nonetheless, doing the reads and writes with separate
open files would be common. Fix it by protecting both the user-direct
reads and writes with damon_sysfs_lock.

Link: https://lore.kernel.org/20260423150253.111520-3-sj@kernel.org
Fixes: c41e253a411e ("mm/damon/sysfs-schemes: implement path file under quota goal directory")
Co-developed-by: Junxi Qian <qjx1298677004@gmail.com>
Signed-off-by: Junxi Qian <qjx1298677004@gmail.com>
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: <stable@vger.kernel.org> # 6.19.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

authored by

SeongJae Park and committed by
Andrew Morton cf3b7142 1e68eb96

+11 -1
+11 -1
mm/damon/sysfs-schemes.c
··· 1197 1197 { 1198 1198 struct damos_sysfs_quota_goal *goal = container_of(kobj, 1199 1199 struct damos_sysfs_quota_goal, kobj); 1200 + int len; 1200 1201 1201 - return sysfs_emit(buf, "%s\n", goal->path ? goal->path : ""); 1202 + if (!mutex_trylock(&damon_sysfs_lock)) 1203 + return -EBUSY; 1204 + len = sysfs_emit(buf, "%s\n", goal->path ? goal->path : ""); 1205 + mutex_unlock(&damon_sysfs_lock); 1206 + return len; 1202 1207 } 1203 1208 1204 1209 static ssize_t path_store(struct kobject *kobj, ··· 1218 1213 return -ENOMEM; 1219 1214 1220 1215 strscpy(path, buf, count + 1); 1216 + if (!mutex_trylock(&damon_sysfs_lock)) { 1217 + kfree(path); 1218 + return -EBUSY; 1219 + } 1221 1220 kfree(goal->path); 1222 1221 goal->path = path; 1222 + mutex_unlock(&damon_sysfs_lock); 1223 1223 return count; 1224 1224 } 1225 1225