[PATCH 6.12.y] mm/hugetlb: fix hugetlb cgroup rsvd charge/uncharge mismatch

[Sasha Levin <sashal@kernel.org>]

From: Deepanshu Kartikey <kartikey406@gmail.com>

[ Upstream commit 15807d0ddde37407af72859426b654f3d1972b00 ]

In alloc_hugetlb_folio(), a single h_cg pointer is used for both the rsvd
and non-rsvd hugetlb cgroup charges.  When map_chg is set,
hugetlb_cgroup_charge_cgroup_rsvd() stores the charged cgroup in h_cg, but
the immediately following hugetlb_cgroup_charge_cgroup() overwrites h_cg
with the non-rsvd cgroup pointer.

As a result, hugetlb_cgroup_commit_charge_rsvd() stores the wrong
(non-rsvd) cgroup pointer into the folio's rsvd slot.

When the folio is later freed, free_huge_folio() unconditionally calls
both hugetlb_cgroup_uncharge_folio() and
hugetlb_cgroup_uncharge_folio_rsvd().  The rsvd uncharge reads back the
wrong cgroup from the folio and decrements a counter that was never
charged for that cgroup, causing a page_counter underflow:

  page_counter underflow: -512 nr_pages=512
  WARNING: mm/page_counter.c:61 at page_counter_cancel

Fix this by introducing a separate h_cg_rsvd pointer exclusively for the
rsvd charge path, keeping the rsvd and non-rsvd charges fully independent
through their charge, commit, and error uncharge paths.

Link: https://lore.kernel.org/20260328065534.346053-1-kartikey406@gmail.com
Fixes: 08cf9faf7558 ("hugetlb_cgroup: support noreserve mappings")
Reported-by: syzbot+226c1f947186f8fef796@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=226c1f947186f8fef796
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Cc: David Hildenbrand <david@kernel.org>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Mina Almasry <almasrymina@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 mm/hugetlb.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 17865215fda795..90faa4fd2ca272 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -2976,6 +2976,7 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
 	long gbl_chg;
 	int memcg_charge_ret, ret, idx;
 	struct hugetlb_cgroup *h_cg = NULL;
+	struct hugetlb_cgroup *h_cg_rsvd = NULL;
 	struct mem_cgroup *memcg;
 	bool deferred_reserve;
 	gfp_t gfp = htlb_alloc_mask(h) | __GFP_RETRY_MAYFAIL;
@@ -3019,7 +3020,7 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
 	deferred_reserve = map_chg || avoid_reserve;
 	if (deferred_reserve) {
 		ret = hugetlb_cgroup_charge_cgroup_rsvd(
-			idx, pages_per_huge_page(h), &h_cg);
+			idx, pages_per_huge_page(h), &h_cg_rsvd);
 		if (ret)
 			goto out_subpool_put;
 	}
@@ -3056,7 +3057,7 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
 	 */
 	if (deferred_reserve) {
 		hugetlb_cgroup_commit_charge_rsvd(idx, pages_per_huge_page(h),
-						  h_cg, folio);
+						  h_cg_rsvd, folio);
 	}
 
 	spin_unlock_irq(&hugetlb_lock);
@@ -3097,7 +3098,7 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
 out_uncharge_cgroup_reservation:
 	if (deferred_reserve)
 		hugetlb_cgroup_uncharge_cgroup_rsvd(idx, pages_per_huge_page(h),
-						    h_cg);
+						    h_cg_rsvd);
 out_subpool_put:
 	if (map_chg || avoid_reserve)
 		hugepage_subpool_put_pages(spool, 1);
-- 
2.53.0