[PATCH v2 1/3] mm/damon/sysfs: fix param_ctx leak on damon_sysfs_new_test_ctx() failure

[PATCH v2 1/3] mm/damon/sysfs: fix param_ctx leak on damon_sysfs_new_test_ctx() failure

[Josh Law]

When damon_sysfs_new_test_ctx() fails in damon_sysfs_commit_input(),
param_ctx is leaked because the early return skips the cleanup at the
out label. Destroy param_ctx before returning.

Fixes: f0c5118ebb0e ("mm/damon/sysfs: catch commit test ctx alloc failure")
Cc: <stable@vger.kernel.org> # 6.18.x
Signed-off-by: Josh Law <objecting@objecting.org>
Reviewed-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/sysfs.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index 576d1ddd736b..b573b9d60784 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -1524,8 +1524,10 @@ static int damon_sysfs_commit_input(void *data)
 	if (IS_ERR(param_ctx))
 		return PTR_ERR(param_ctx);
 	test_ctx = damon_sysfs_new_test_ctx(kdamond->damon_ctx);
-	if (!test_ctx)
+	if (!test_ctx) {
+		damon_destroy_ctx(param_ctx);
 		return -ENOMEM;
+	}
 	err = damon_commit_ctx(test_ctx, param_ctx);
 	if (err)
 		goto out;
-- 
2.34.1

[PATCH v2 2/3] mm/damon/sysfs: check contexts->nr before accessing contexts_arr[0]

[Josh Law]

Multiple sysfs command paths dereference contexts_arr[0] without first
verifying that nr_contexts >= 1.  A user can set nr_contexts to 0 via
sysfs while DAMON is running, causing NULL pointer dereferences.

Guard all commands (except OFF) at the entry point of
damon_sysfs_handle_cmd().

Fixes: 0ac32b8affb5 ("mm/damon/sysfs: support DAMOS stats")
Cc: <stable@vger.kernel.org>	# 5.18.x
Signed-off-by: Josh Law <objecting@objecting.org>
Reviewed-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/sysfs.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index b573b9d60784..ddc30586c0e6 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -1749,6 +1749,9 @@ static int damon_sysfs_update_schemes_tried_regions(
 static int damon_sysfs_handle_cmd(enum damon_sysfs_cmd cmd,
 		struct damon_sysfs_kdamond *kdamond)
 {
+	if (cmd != DAMON_SYSFS_CMD_OFF && kdamond->contexts->nr != 1)
+		return -EINVAL;
+
 	switch (cmd) {
 	case DAMON_SYSFS_CMD_ON:
 		return damon_sysfs_turn_damon_on(kdamond);
-- 
2.34.1

Re: [PATCH v2 2/3] mm/damon/sysfs: check contexts->nr before accessing contexts_arr[0]

[SeongJae Park]

On Fri, 20 Mar 2026 16:35:58 +0000 Josh Law <objecting@objecting.org> wrote:

> Multiple sysfs command paths dereference contexts_arr[0] without first
> verifying that nr_contexts >= 1.

Nit.  There is no 'nr_contexts' in the code.  Let's use kdamond->contexts->nr
or contexts->nr instead.

> A user can set nr_contexts to 0 via
> sysfs while DAMON is running, causing NULL pointer dereferences.

It would be nice to explain how users can reproducer the issue.  Could you
please add below to this part of the commit message?

'''
The issue can be triggered by privileged users like below.

First, start DAMON and make contexts directory empty (kdamond->contexts->nr ==
0).

    # damo start
    # cd /sys/kernel/mm/damon/admin/kdamonds/0
    # echo 0 > contexts/nr_contexts

Then, any of below commands will cause the NULL pointer dereference.

    # echo update_schemes_stats > state
    # echo update_schemes_tried_regions > state
    # echo update_schemes_tried_bytes > state
    # echo update_schemes_effective_quotas > state
    # echo update_tuned_intervals > state
'''

>
> Guard all commands (except OFF) at the entry point of
> damon_sysfs_handle_cmd().
>
> Fixes: 0ac32b8affb5 ("mm/damon/sysfs: support DAMOS stats")
> Cc: <stable@vger.kernel.org>  # 5.18.x
> Signed-off-by: Josh Law <objecting@objecting.org>
> Reviewed-by: SeongJae Park <sj@kernel.org>

I suggested [1] this patch.  But I didn't give you 'Revied-by:' tag.  So the
above Reviewed-by: is not valid.

And this patch looks good to me, so please take my valid Reviewed-by: tag.

Reviewed-by: SeongJae Park <sj@kernel.org>

> ---

From the next time, please add patch version changelog [2] here.

I added this patch into my tree after resolving the trivial things I mentioned
above.  I will post it as v3 tomorrow, unless Andrew add this to mm.git after
resolving the trivial things as I mentioned.

[1] https://lore.kernel.org/20260320155115.101025-1-sj@kernel.org
[2] https://docs.kernel.org/process/submitting-patches.html#commentary


Thanks,
SJ

[...]

[PATCH v2 3/3] mm/damon/sysfs: check contexts->nr in repeat_call_fn

[Josh Law]

damon_sysfs_repeat_call_fn() calls damon_sysfs_upd_tuned_intervals(),
damon_sysfs_upd_schemes_stats(), and
damon_sysfs_upd_schemes_effective_quotas() without checking
contexts->nr.  If nr_contexts is set to 0 via sysfs while DAMON is
running, these functions dereference contexts_arr[0] and cause a NULL
pointer dereference.  Add the missing check.

Fixes: d809a7c64ba8 ("mm/damon/sysfs: implement refresh_ms file internal work")
Cc: <stable@vger.kernel.org> # 6.17.x
Signed-off-by: Josh Law <objecting@objecting.org>
Reviewed-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/sysfs.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/damon/sysfs.c b/mm/damon/sysfs.c
index ddc30586c0e6..6a44a2f3d8fc 100644
--- a/mm/damon/sysfs.c
+++ b/mm/damon/sysfs.c
@@ -1620,9 +1620,12 @@ static int damon_sysfs_repeat_call_fn(void *data)
 
 	if (!mutex_trylock(&damon_sysfs_lock))
 		return 0;
+	if (sysfs_kdamond->contexts->nr != 1)
+		goto out;
 	damon_sysfs_upd_tuned_intervals(sysfs_kdamond);
 	damon_sysfs_upd_schemes_stats(sysfs_kdamond);
 	damon_sysfs_upd_schemes_effective_quotas(sysfs_kdamond);
+out:
 	mutex_unlock(&damon_sysfs_lock);
 	return 0;
 }
-- 
2.34.1

Re: [PATCH v2 3/3] mm/damon/sysfs: check contexts->nr in repeat_call_fn

[SeongJae Park]

On Fri, 20 Mar 2026 16:35:59 +0000 Josh Law <objecting@objecting.org> wrote:

> damon_sysfs_repeat_call_fn() calls damon_sysfs_upd_tuned_intervals(),
> damon_sysfs_upd_schemes_stats(), and
> damon_sysfs_upd_schemes_effective_quotas() without checking
> contexts->nr.  If nr_contexts is set to 0 via sysfs while DAMON is
> running, these functions dereference contexts_arr[0] and cause a NULL
> pointer dereference.  Add the missing check.
> 
> Fixes: d809a7c64ba8 ("mm/damon/sysfs: implement refresh_ms file internal work")
> Cc: <stable@vger.kernel.org> # 6.17.x
> Signed-off-by: Josh Law <objecting@objecting.org>
> Reviewed-by: SeongJae Park <sj@kernel.org>
> ---

From the next time, please add patch changelog here.

[...]
Sashiko also added comments [1] that are very same as those for the previous
version of this patch.  I replied [2] to those on the thread.  In short, nice
findings but orthogonal to this patch, and I will work on those separately.

[1] https://sashiko.dev/#/patchset/20260320163559.178101-3-objecting@objecting.org
[2] https://lore.kernel.org/20260320020630.962-1-sj@kernel.org


Thanks,
SJ

[...]

Re: [PATCH v2 1/3] mm/damon/sysfs: fix param_ctx leak on damon_sysfs_new_test_ctx() failure

[SeongJae Park]

On Fri, 20 Mar 2026 16:35:57 +0000 Josh Law <objecting@objecting.org> wrote:

> When damon_sysfs_new_test_ctx() fails in damon_sysfs_commit_input(),
> param_ctx is leaked because the early return skips the cleanup at the
> out label. Destroy param_ctx before returning.
> 
> Fixes: f0c5118ebb0e ("mm/damon/sysfs: catch commit test ctx alloc failure")
> Cc: <stable@vger.kernel.org> # 6.18.x
> Signed-off-by: Josh Law <objecting@objecting.org>
> Reviewed-by: SeongJae Park <sj@kernel.org>
> ---

From next time, please add patch changelog here.

[...]
Sashiko is adding comments [1] similar to those for the previous version of
this patch.  I replied [2] on the thread.  In short, it is good finding but
orthogonal to this patch, and I will work on it.

[1] https://sashiko.dev/#/patchset/20260320163559.178101-1-objecting@objecting.org
[2] https://lore.kernel.org/20260320020056.835-1-sj@kernel.org


Thanks,
SJ