[RFC PATCH v2 0/2] samples/damon/mtier: handle damon_{start,stop}() failures

[RFC PATCH v2 0/2] samples/damon/mtier: handle damon_{start,stop}() failures

[SeongJae Park]

DAMON sample module mtier is not correctly handling failures from
damon_start() and damon_stop().  As a result, it can leak memory,
disrupt next DAMON operations, and dereference freed and random memory.
Fix those.

Changes from RFC v1
- RFC v1: https://lore.kernel.org/20260609005443.2122-1-sj@kernel.org
- Add damon_stop() failure handling fix to the series.

SeongJae Park (2):
  samples/damon/mtier: handle damon_start() failure
  samples/damon/mtier: handle damon_stop() failure

 samples/damon/mtier.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)


base-commit: f822d98f51b6e2b3ab41cad2e8bc1013d06f93bb
-- 
2.47.3

[RFC PATCH v2 1/2] samples/damon/mtier: handle damon_start() failure

[SeongJae Park]

damon_sample_mtier_start() callers assume it will clean up resources
when it fails.  And the function does the cleanup for context buildup
failures.  However, it is not doing the cleanup for damon_start()
failure.

As a result, when damon_start() fails, it could leak the memory for
DAMON context.  Also, if damon_start() fails for only the second
context, the first context will indefinitely run, and avoid starting
other DAMON contexts since it is running in the exclusive mode.  Stop
possibly started DAMON context and free the contexts in case of the
failure to fix the issues.

Note that the issue can reliably be reproduced because the module calls
damon_start() in the exclusive mode.  For example,

    $ sudo damo start
    $ echo Y | sudo tee /sys/module/damon_sample_mtier/parameters/enabled
    $ sudo cat /proc/allocinfo | grep damon_new_ctx

Because the first command is running another DAMON instance, the second
command fails the damon_start() call because the new DAMON instance
cannot exclusively run.  And without this fix, by repeating the second
and the third commands above, we can show the memory consumption is only
increasing due to the leaks.  It requires the sudo permission though.

The issue was discovered [1] by Sashiko.

[1] https://lore.kernel.org/20260608112455.274231F00893@smtp.kernel.org

Fixes: 82a08bde3cf7 ("samples/damon: implement a DAMON module for memory tiering")
Cc: <stable@vger.kernel.org> # 6.16.x
Signed-off-by: SeongJae Park <sj@kernel.org>
---
 samples/damon/mtier.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/samples/damon/mtier.c b/samples/damon/mtier.c
index eb1143de8df17..66b591f2180fa 100644
--- a/samples/damon/mtier.c
+++ b/samples/damon/mtier.c
@@ -174,6 +174,7 @@ static struct damon_ctx *damon_sample_mtier_build_ctx(bool promote)
 static int damon_sample_mtier_start(void)
 {
 	struct damon_ctx *ctx;
+	int err;
 
 	ctx = damon_sample_mtier_build_ctx(true);
 	if (!ctx)
@@ -185,7 +186,15 @@ static int damon_sample_mtier_start(void)
 		return -ENOMEM;
 	}
 	ctxs[1] = ctx;
-	return damon_start(ctxs, 2, true);
+	err = damon_start(ctxs, 2, true);
+	if (!err)
+		return 0;
+
+	if (damon_is_running(ctxs[0]))
+		damon_stop(ctxs, 1);
+	damon_destroy_ctx(ctxs[0]);
+	damon_destroy_ctx(ctxs[1]);
+	return err;
 }
 
 static void damon_sample_mtier_stop(void)
-- 
2.47.3

[RFC PATCH v2 2/2] samples/damon/mtier: handle damon_stop() failure

[SeongJae Park]

damon_sample_mtier_stop() assumes its damon_stop() call will always
successfully stops the two DAMON contexts.  Hence it deallocates the two
DAMON contexts after the damon_stop() call.  However, if a given context
is already stopped, damon_stop() fails and returns an error while
letting the DAMON contexts that have not yet stopped keep running.  This
kind of unexpected early DAMON context stops could happen due to memory
allocation failures in kdamond_fn().  Because damon_sample_mtier_stop()
just deallocates all DAMON contexts with damon_target and damon_region
objects that are linked to the contexts, the execution of the unstopped
DAMON context (kdamond) ends up using the memory that freed
(use-after-free).  Fix the issue by separating the damon_stop() to be
invoked per context.

Note that DAMON_SYSFS also allows multiple DAMON contexts execution.
But, it calls damon_stop() for each context one by one.  Hence this
issue is only in mtier.

For the long term, it would be better to refactor damon_stop() to always
ensure stopping all contexts regardless of the failures in the middle.
Make this fix in the current way, though, to keep it simple and easy to
backport.  I will do the refactoring later.

The issue was discovered [1] by Sashiko.

[1] https://lore.kernel.org/20260609014219.3013-1-sj@kernel.org

Fixes: 82a08bde3cf7 ("samples/damon: implement a DAMON module for memory tiering")
Cc: <stable@vger.kernel.org> # 6.16.x
Signed-off-by: SeongJae Park <sj@kernel.org>
---
 samples/damon/mtier.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/samples/damon/mtier.c b/samples/damon/mtier.c
index 66b591f2180fa..faaaaa12e6206 100644
--- a/samples/damon/mtier.c
+++ b/samples/damon/mtier.c
@@ -199,7 +199,8 @@ static int damon_sample_mtier_start(void)
 
 static void damon_sample_mtier_stop(void)
 {
-	damon_stop(ctxs, 2);
+	damon_stop(ctxs, 1);
+	damon_stop(&ctxs[1], 1);
 	damon_destroy_ctx(ctxs[0]);
 	damon_destroy_ctx(ctxs[1]);
 }
-- 
2.47.3