[PATCH] mm: swap_cgroup: fix NULL deref in lookup_swap_cgroup_id on swapless host

[Jose Fernandez (Anthropic)]

lookup_swap_cgroup_id() passes swap_cgroup_ctrl[type].map to
__swap_cgroup_id_lookup() without checking that the type was ever
registered via swap_cgroup_swapon(). On a swapless host every
ctrl->map is NULL, so __swap_cgroup_id_lookup() dereferences
NULL + a scaled swp_offset().

Since commit bea67dcc5eea ("mm: attempt to batch free swap entries
for zap_pte_range()"), zap_pte_range() -> swap_pte_batch() calls
lookup_swap_cgroup_id() on any non-present, non-none PTE that
decodes as a real swap entry, without first validating it against
swap_info[]. A single PTE corrupted into a type-0 swap entry takes
the host down at process exit.

We hit this in production on a swapless 6.12.58 host: ~1s of
"get_swap_device: Bad swap file entry 3f800204222bb" (do_swap_page()
being correctly defensive about the same entry) followed by

  BUG: unable to handle page fault for address: 000003f800204220
  RIP: 0010:lookup_swap_cgroup_id+0x2b/0x60
  Call Trace:
   swap_pte_batch+0xbf/0x230
   zap_pte_range+0x4c8/0x780
   unmap_page_range+0x190/0x3e0
   exit_mmap+0xd9/0x3c0
   do_exit+0x20c/0x4b0

syzbot has reported the identical stack.

The source of the PTE corruption is a separate bug; this change
makes the teardown path as robust as the fault path already is.
Every other caller of lookup_swap_cgroup_id() is downstream of a
get_swap_device() that has already validated the entry, so the new
branch is cold.

Fixes: bea67dcc5eea ("mm: attempt to batch free swap entries for zap_pte_range()")
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: syzbot+e12bd9ca48157add237a@xxxxxxxxxxxxxxxxxxxxxxxxx
Link: https://lore.kernel.org/r/69859728.050a0220.3b3015.0033.GAE@xxxxxxxxxx
Assisted-by: Claude:unspecified
Signed-off-by: Jose Fernandez (Anthropic) <jose.fernandez@xxxxxxxxx>
---
 mm/swap_cgroup.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/swap_cgroup.c b/mm/swap_cgroup.c
index de779fed8c210..95c38e54dd587 100644
--- a/mm/swap_cgroup.c
+++ b/mm/swap_cgroup.c
@@ -124,6 +124,8 @@ unsigned short lookup_swap_cgroup_id(swp_entry_t ent)
 		return 0;
 
 	ctrl = &swap_cgroup_ctrl[swp_type(ent)];
+	if (unlikely(!ctrl->map))
+		return 0;
 	return __swap_cgroup_id_lookup(ctrl->map, swp_offset(ent));
 }
 

---
base-commit: 254f49634ee16a731174d2ae34bc50bd5f45e731
change-id: 20260504-swap-cgroup-fix-7-0-ed0fcdb8f103

Best regards,
--  
Jose Fernandez (Anthropic) <jose.fernandez@xxxxxxxxx>