userns: Add __counted_by_ptr attribute to struct uid_gid_map

The compiler attribute __counted_by_ptr associates a pointer field of a
struct with a sibling field within the same struct that specifies the
element count of the allocated memory. This enables KASAN and fortified
bounds-checking to detect out-of-bounds accesses to the pointer field at
runtime.

We can add the __counted_by_ptr attribute to the 'forward' and 'reverse'
pointer fields of 'struct uid_gid_map', which are counted by
'nr_extents'. Since 'nr_extents' is defined in a sibling anonymous
struct inside an anonymous union, the nearest common non-anonymous
struct level is 'struct uid_gid_map' itself, which is supported by the
compiler.

However, doing so has runtime implications. In the original
implementation of insert_extent(), elements are written to
map->forward[map->nr_extents] before map->nr_extents is incremented:

	if (map->nr_extents < UID_GID_MAP_MAX_BASE_EXTENTS)
		dest = &map->extent[map->nr_extents];
	else
		dest = &map->forward[map->nr_extents];

	*dest = *extent;
	map->nr_extents++;

At the time of writing to 'map->forward[map->nr_extents]',
map->nr_extents is still 5, but we are accessing index 5 (which is the
6th element). Under __counted_by_ptr(nr_extents), the compiler and
KASAN expect the accessed index to be strictly less than
map->nr_extents. Therefore, accessing index 5 when the count is 5
triggers an out-of-bounds panic/trap at runtime.

To resolve this, insert_extent() is refactored to increment
map->nr_extents first, and then use map->nr_extents - 1 as the index:

	map->nr_extents++;
	if (map->nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
		dest = &map->extent[map->nr_extents - 1];
	else
		dest = &map->forward[map->nr_extents - 1];

	*dest = *extent;

Signed-off-by: Bill Wendling <morbo@google.com>
Link: https://patch.msgid.link/20260908222734.3048684-1-morbo@google.com
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Bradley Morgan <brads@mainlining.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

2 files changed, 10 insertions, 6 deletions

diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index e38d9e60569f..2962256eddf7 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -29,8 +29,8 @@ struct uid_gid_map { /* 64 bytes -- 1 cache line */
 			u32 nr_extents;
 		};
 		struct {
-			struct uid_gid_extent *forward;
-			struct uid_gid_extent *reverse;
+			struct uid_gid_extent *forward __counted_by_ptr(nr_extents);
+			struct uid_gid_extent *reverse __counted_by_ptr(nr_extents);
 		};
 	};
 };
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index e9e04ce167df..7c8316c4271f 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -806,13 +806,17 @@ static int insert_extent(struct uid_gid_map *map, struct uid_gid_extent *extent)
 		map->reverse = NULL;
 	}
 
-	if (map->nr_extents < UID_GID_MAP_MAX_BASE_EXTENTS)
-		dest = &map->extent[map->nr_extents];
+	/*
+	 * nr_extents must be updated before the extent and forward arrays are
+	 * accessed, otherwise KSAN will assert an out-of-bounds error.
+	 */
+	map->nr_extents++;
+	if (map->nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
+		dest = &map->extent[map->nr_extents - 1];
 	else
-		dest = &map->forward[map->nr_extents];
+		dest = &map->forward[map->nr_extents - 1];
 
 	*dest = *extent;
-	map->nr_extents++;
 	return 0;
 }