bpf: Reject arena frees below the arena base

bpf_arena_free_pages() accepts scalar arena addresses. The runtime
masks the address to the low 32 bits and reconstructs a full user
address from the arena base before returning the range to the arena
free tree.

When the scalar value is below the low 32 bits of the arena base,
full_uaddr falls below user_vm_start. The existing upper-end clipping
then turns this into an out-of-range free-tree offset. A later
allocation can reuse that offset and return an address below the arena
mapping.

Reject such frees before computing the clipped range.

Fixes: 317460317a02a ("bpf: Introduce bpf_arena.")
Signed-off-by: Yiyang Chen <chenyy23@mails.tsinghua.edu.cn>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/bpf/20260717-c10-031-public-bpf-next-v2-b4-v2-1-54b555443a7c@mails.tsinghua.edu.cn
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
index f046e878f7ae..34f023a537fe 100644
--- a/kernel/bpf/arena.c
+++ b/kernel/bpf/arena.c
@@ -858,6 +858,8 @@ static void arena_free_pages(struct bpf_arena *arena, long uaddr, long page_cnt,
 	uaddr &= PAGE_MASK;
 	kaddr = bpf_arena_get_kern_vm_start(arena) + uaddr;
 	full_uaddr = clear_lo32(arena->user_vm_start) + uaddr;
+	if (full_uaddr < arena->user_vm_start)
+		return;
 	uaddr_end = min(arena->user_vm_end, full_uaddr + (page_cnt << PAGE_SHIFT));
 	if (full_uaddr >= uaddr_end)
 		return;