[PATCH 6.1.y 0/2] mm/damon/sysfs: fix pid leaks from wrong error handlings

[PATCH 6.1.y 0/2] mm/damon/sysfs: fix pid leaks from wrong error handlings

[SJ Park]

DAMON sysfs interface can leak pids due to wrong error handlings.  The
issue in the mainline has been fixed with code refactoring.  The
refactoring is done with commits including 7114bc5e01cf ("mm/damon/core:
add cleanup_target() ops callback").   Porting all the refactoring
changes to old stable kernels will be challenging.  Hence submitting
this non-mainline hotfix series.

Changes from RFC
- RFC: https://lore.kernel.org/20261028051104.154232-1-sj@kernel.org
- Drop RFC tag.

SJ Park (2):
  mm/damon/sysfs: put target pids for damon_start() failure
  mm/damon/sysfs: put target pids for apply_inputs() failure

 mm/damon/sysfs.c | 2 ++
 1 file changed, 2 insertions(+)


base-commit: 2d552c13645b4174829d2e131c6f6483a5111b6f
-- 
2.47.3

[PATCH 6.1.y 1/2] mm/damon/sysfs: put target pids for damon_start() failure

[SJ Park]

damon_sysfs_turn_damon_on() allocates and sets up damon_ctx to start.
If it is for virtual address space monitoring, the setup increases the
reference counts of the target pids.  After the setup, the function
invokes damon_start().  If damon_start() fails, it deallocates the
damon_ctx object without decreasing the reference counts of target pids.
As a result, pids can be leaked.

Fix the leak by putting pids using damon_sysfs_destroy_targets() before
deallocating the damon_ctx object.

Fixes: a61ea561c871 ("mm/damon/sysfs: link DAMON for virtual address spaces monitoring")
Cc: <stable@vger.kernel.org> # 5.18.x
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/sysfs.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index 9a918f8e4c6bf..4b28bb9ba0a24 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -2547,6 +2547,7 @@ static int damon_sysfs_turn_damon_on(struct damon_sysfs_kdamond *kdamond)
 		return PTR_ERR(ctx);
 	err = damon_start(&ctx, 1, false);
 	if (err) {
+		damon_sysfs_destroy_targets(ctx);
 		damon_destroy_ctx(ctx);
 		return err;
 	}
-- 
2.47.3

[PATCH 6.1.y 2/2] mm/damon/sysfs: put target pids for apply_inputs() failure

[SJ Park]

damon_sysfs_build_ctx() invokes damon_sysfs_apply_inputs() for setting
damon_ctx object with user inputs.  If it is configured for virtual
address spaces monitoring, the setup also increases reference counts of
monitoring target pids.  If the setup is failed, damon_sysfs_build_ctx()
deallocates the damon_ctx object without decreasing the reference counts
of the pids.  If the failure happened after increasing reference counts
of pids, the pids can be leaked.

Fix the leaks by decreasing the pid reference counts before deallocating
the damon_ctx object.

The issue was discovered [1] by Sashiko.

[1] https://lore.kernel.org/20260730040740.140836-1-sj@kernel.org

Fixes: 74bd8b7d2f8e ("mm/damon/sysfs: move targets setup code to a separated function")
Cc: <stable@vger.kernel.org> # 5.19.x
Signed-off-by: SJ Park <sj@kernel.org>
---
 mm/damon/sysfs.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index 4b28bb9ba0a24..b9b80d58e5bd6 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -2515,6 +2515,7 @@ static struct damon_ctx *damon_sysfs_build_ctx(
 
 	err = damon_sysfs_apply_inputs(ctx, sys_ctx);
 	if (err) {
+		damon_sysfs_destroy_targets(ctx);
 		damon_destroy_ctx(ctx);
 		return ERR_PTR(err);
 	}
-- 
2.47.3