[PATCH] mm/damon/core: avoid use of half-online-committed context

[SeongJae Park <sj@kernel.org>]

One major usage of damon_call() is online DAMON parameters update.  It
is done by calling damon_commit_ctx() inside the damon_call() callback
function.  damon_commit_ctx() can fail for two reasons: 1) invalid
parameters and 2) internal memory allocation failures.  In case of
failures, the damon_ctx that attempted to be updated (commit
destination) can be partially updated (or, corrupted from a
perspective), and therefore shouldn't be used anymore.  The function
only ensures the damon_ctx object can safely deallocated using
damon_destroy_ctx().

The API callers are, however, calling damon_commit_ctx() only after
asserting the parameters are valid, to avoid damon_commit_ctx() fails
due to invalid input parameters.  But it can still theoretically fail if
the internal memory allocation fails.  In the case, DAMON may run with
the partially updated damon_ctx.  This can result in unexpected
behaviors including even NULL pointer dereference in case of
damos_commit_dests() failure [1].  Such allocation failure is arguably
too small to fail, so the real world impact would be rare.  But, given
the bad consequence, this needs to be fixed.

Avoid such partially-committed (maybe-corrupted) damon_ctx use by saving
the damon_commit_ctx() failure on the damon_ctx object.  For this,
introduce damon_ctx->maybe_corrupted field.  damon_commit_ctx() sets it
when it is failed.  kdamond_call() checks if the field is set after each
damon_call_control->fn() is executed.  If it is set, ignore remaining
callback requests and return.  All kdamond_call() callers including
kdamond_fn() also check the maybe_corrupted field right after
kdamond_call() invocations.  If the field is set, break the
kdamond_fn() main loop so that DAMON sill doesn't use the context that
might be corrupted.

[1] https://lore.kernel.org/20260319043309.97966-1-sj@kernel.org

Fixes: 3301f1861d34 ("mm/damon/sysfs: handle commit command using damon_call()")
Cc: <stable@vger.kernel.org> # 6.15.x
Signed-off-by: SeongJae Park <sj@kernel.org>
---
 include/linux/damon.h | 6 ++++++
 mm/damon/core.c       | 8 ++++++++
 2 files changed, 14 insertions(+)

diff --git a/include/linux/damon.h b/include/linux/damon.h
index a4fea23da8576..be3d198043ff9 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -810,6 +810,12 @@ struct damon_ctx {
 	struct damos_walk_control *walk_control;
 	struct mutex walk_control_lock;
 
+	/*
+	 * indicate if this may be corrupted.  Currentonly this is set only for
+	 * damon_commit_ctx() failure.
+	 */
+	bool maybe_corrupted;
+
 	/* Working thread of the given DAMON context */
 	struct task_struct *kdamond;
 	/* Protects @kdamond field access */
diff --git a/mm/damon/core.c b/mm/damon/core.c
index c1d1091d307e4..37454e8c9c510 100644
--- a/mm/damon/core.c
+++ b/mm/damon/core.c
@@ -1252,6 +1252,7 @@ int damon_commit_ctx(struct damon_ctx *dst, struct damon_ctx *src)
 {
 	int err;
 
+	dst->maybe_corrupted = true;
 	if (!is_power_of_2(src->min_region_sz))
 		return -EINVAL;
 
@@ -1277,6 +1278,7 @@ int damon_commit_ctx(struct damon_ctx *dst, struct damon_ctx *src)
 	dst->addr_unit = src->addr_unit;
 	dst->min_region_sz = src->min_region_sz;
 
+	dst->maybe_corrupted = false;
 	return 0;
 }
 
@@ -2678,6 +2680,8 @@ static void kdamond_call(struct damon_ctx *ctx, bool cancel)
 			complete(&control->completion);
 		else if (control->canceled && control->dealloc_on_cancel)
 			kfree(control);
+		if (ctx->maybe_corrupted)
+			break;
 	}
 
 	mutex_lock(&ctx->call_controls_lock);
@@ -2707,6 +2711,8 @@ static int kdamond_wait_activation(struct damon_ctx *ctx)
 		kdamond_usleep(min_wait_time);
 
 		kdamond_call(ctx, false);
+		if (ctx->maybe_corrupted)
+			return -EINVAL;
 		damos_walk_cancel(ctx);
 	}
 	return -EBUSY;
@@ -2790,6 +2796,8 @@ static int kdamond_fn(void *data)
 		 * kdamond_merge_regions() if possible, to reduce overhead
 		 */
 		kdamond_call(ctx, false);
+		if (ctx->maybe_corrupted)
+			break;
 		if (!list_empty(&ctx->schemes))
 			kdamond_apply_schemes(ctx);
 		else

base-commit: 969615b6b5d178009a87abf4e4292f90c098978e
-- 
2.47.3