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

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.

[sj@kernel.org: let kdamond_call() with cancel regardless of maybe_corrupted]
Link: https://lkml.kernel.org/r/20260320031553.2479-1-sj@kernel.org
Link: https://sashiko.dev/#/patchset/20260319145218.86197-1-sj%40kernel.org
Link: https://lkml.kernel.org/r/20260319145218.86197-1-sj@kernel.org
Link: https://lore.kernel.org/20260319043309.97966-1-sj@kernel.org
Fixes: 3301f1861d34 ("mm/damon/sysfs: handle commit command using damon_call()")
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: <stable@vger.kernel.org>	[6.15+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/damon.h b/include/linux/damon.h
index a4fea23da8576aa7c901bede0bf037bad01f34f2..be3d198043ff9f8fad3e2f410a17c4095caa3fd7 100644 (file)
--- 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 c1d1091d307e4b122f49e33daf948a166090ff9d..3e1890d64d067a3c880d2a5bc9671a85afb759a4 100644 (file)
--- 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 (!cancel && 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