+ mm-damon-sysfs-check-contexts-nr-before-accessing-contexts_arr.patch added to mm-hotfixes-unstable branch

+ mm-damon-sysfs-check-contexts-nr-before-accessing-contexts_arr.patch added to mm-hotfixes-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm/damon/sysfs: check contexts->nr before accessing contexts_arr[0]
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-damon-sysfs-check-contexts-nr-before-accessing-contexts_arr.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-damon-sysfs-check-contexts-nr-before-accessing-contexts_arr.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Josh Law <objecting@objecting.org>
Subject: mm/damon/sysfs: check contexts->nr before accessing contexts_arr[0]
Date: Sat, 21 Mar 2026 10:54:25 -0700

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

In more detail, 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, each 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().

Link: https://lkml.kernel.org/r/20260321175427.86000-3-sj@kernel.org
Fixes: 0ac32b8affb5 ("mm/damon/sysfs: support DAMOS stats")
Signed-off-by: Josh Law <objecting@objecting.org>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: <stable@vger.kernel.org>	[5.18+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/damon/sysfs.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/mm/damon/sysfs.c~mm-damon-sysfs-check-contexts-nr-before-accessing-contexts_arr
+++ a/mm/damon/sysfs.c
@@ -1749,6 +1749,9 @@ static int damon_sysfs_update_schemes_tr
 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);
_

Patches currently in -mm which might be from objecting@objecting.org are

mm-damon-sysfs-fix-param_ctx-leak-on-damon_sysfs_new_test_ctx-failure.patch
mm-damon-sysfs-check-contexts-nr-before-accessing-contexts_arr.patch
mm-damon-sysfs-check-contexts-nr-in-repeat_call_fn.patch
lib-maple_tree-fix-swapped-arguments-in-mas_safe_pivot-call.patch
mm-damon-core-document-damos_commit_dests-failure-semantics.patch
lib-glob-fix-grammar-and-replace-non-inclusive-terminology.patch
lib-glob-add-explicit-include-for-exporth.patch
lib-glob-replace-bitwise-or-with-logical-operation-on-boolean.patch
lib-glob-clean-up-bool-abuse-in-pointer-arithmetic.patch
lib-uuid-fix-typo-reversion-to-revision-in-comment.patch
lib-inflate-fix-memory-leak-in-inflate_fixed-on-inflate_codes-failure.patch
lib-inflate-fix-memory-leak-in-inflate_dynamic-on-inflate_codes-failure.patch
lib-inflate-fix-grammar-in-comment-variable-to-variables.patch
lib-inflate-fix-typo-this-results-to-the-results-in-comment.patch
lib-bug-fix-inconsistent-capitalization-in-bug-message.patch
lib-bug-remove-unnecessary-variable-initializations.patch
lib-idr-fix-ida_find_first_range-missing-ids-across-chunk-boundaries.patch
lib-decompress_bunzip2-fix-32-bit-shift-undefined-behavior.patch
lib-ts_bm-fix-integer-overflow-in-pattern-length-calculation.patch
lib-ts_kmp-fix-integer-overflow-in-pattern-length-calculation.patch
lib-glob-initialize-back_str-to-silence-uninitialized-variable-warning.patch
lib-bch-fix-signed-left-shift-undefined-behavior.patch
lib-bch-fix-signed-shift-overflow-in-build_mod8_tables.patch