[PATCH v3 3/5] slab: handle ERR_PTR values in kfree and hardened usercopy

[Karl Mehltretter <kmehltretter@gmail.com>]

Passing an ERR_PTR to kfree() currently reaches virt_to_page() and may
fault. Warn and return instead, leaving the bad caller visible without
using the pointer as allocator metadata.

Also reject ERR_PTR values in hardened usercopy. Keep both checks
separate from ZERO_OR_NULL_PTR(), whose exact matching is required by
krealloc().

Link: https://lore.kernel.org/r/CAG48ez05QVn6_gQ2TBrRa1a_DWQoaSSYubUsu5YMWxx-gqMijQ@mail.gmail.com
Link: https://lore.kernel.org/r/202608111716.0FA9DB17@keescook
Link: https://github.com/KSPP/linux/issues/93
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 mm/slub.c     | 3 +++
 mm/usercopy.c | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/mm/slub.c b/mm/slub.c
index f9b56cb439e7..027b44dd7f07 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -6780,6 +6780,9 @@ void kfree(const void *object)
 	if (unlikely(ZERO_OR_NULL_PTR(object)))
 		return;
 
+	if (WARN_ON(IS_ERR(object)))
+		return;
+
 	page = virt_to_page(object);
 	slab = page_slab(page);
 	if (!slab) {
diff --git a/mm/usercopy.c b/mm/usercopy.c
index 5de7a518b1b1..c8d8703544c6 100644
--- a/mm/usercopy.c
+++ b/mm/usercopy.c
@@ -157,6 +157,9 @@ static inline void check_bogus_address(const unsigned long ptr, unsigned long n,
 	/* Reject if NULL or ZERO-allocation. */
 	if (ZERO_OR_NULL_PTR(ptr))
 		usercopy_abort("null address", NULL, to_user, ptr, n);
+
+	if (IS_ERR_VALUE(ptr))
+		usercopy_abort("ERR_PTR", NULL, to_user, ptr, n);
 }
 
 static inline void check_heap_object(const void *ptr, unsigned long n,
-- 
2.53.0