Merge branch 'bpf-reject-arena-frees-below-the-arena-base'

Yiyang Chen says:

====================
bpf: Reject arena frees below the arena base

bpf_arena_free_pages() can be called with a scalar arena address. The
runtime reconstructs a full user address from the arena base and the low
32 bits before returning the range to the arena free tree. A scalar one
page below the arena base can otherwise produce an out-of-domain free-tree
offset and make a later allocation return an address below the arena
mapping.

Patch 1 rejects frees whose reconstructed full user address is below
user_vm_start. Patch 2 adds verifier_arena coverage for the
scalar-below-base case.

Changes in v2:
- Add Reviewed-by tags from Emil Tsalapatis.
- Remove the empty inline asm from the selftest.

v1: https://lore.kernel.org/bpf/cover.1782813442.git.chenyy23@mails.tsinghua.edu.cn/
====================

Link: https://patch.msgid.link/20260717-c10-031-public-bpf-next-v2-b4-v2-0-54b555443a7c@mails.tsinghua.edu.cn
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

2 files changed, 37 insertions, 5 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;
diff --git a/tools/testing/selftests/bpf/progs/verifier_arena.c b/tools/testing/selftests/bpf/progs/verifier_arena.c
index df0e22d1a29b..b241bbcf54a8 100644
--- a/tools/testing/selftests/bpf/progs/verifier_arena.c
+++ b/tools/testing/selftests/bpf/progs/verifier_arena.c
@@ -12,15 +12,17 @@
 
 #define private(name) SEC(".bss." #name) __hidden __attribute__((aligned(8)))
 
+#ifdef __TARGET_ARCH_arm64
+#define ARENA_VM_START ((1ull << 32) | (~0u - __PAGE_SIZE * 2 + 1))
+#else
+#define ARENA_VM_START ((1ull << 44) | (~0u - __PAGE_SIZE * 2 + 1))
+#endif
+
 struct {
 	__uint(type, BPF_MAP_TYPE_ARENA);
 	__uint(map_flags, BPF_F_MMAPABLE);
 	__uint(max_entries, 2); /* arena of two pages close to 32-bit boundary*/
-#ifdef __TARGET_ARCH_arm64
-        __ulong(map_extra, (1ull << 32) | (~0u - __PAGE_SIZE * 2 + 1)); /* start of mmap() region */
-#else
-        __ulong(map_extra, (1ull << 44) | (~0u - __PAGE_SIZE * 2 + 1)); /* start of mmap() region */
-#endif
+	__ulong(map_extra, ARENA_VM_START); /* start of mmap() region */
 } arena SEC(".maps");
 
 SEC("socket")
@@ -93,6 +95,34 @@ int basic_alloc1(void *ctx)
 	return 0;
 }
 
+SEC("syscall")
+__success __retval(0)
+int free_scalar_below_arena(void *ctx)
+{
+	void __arena *page1, *page2, *page3;
+	__u64 bad_addr = ARENA_VM_START - __PAGE_SIZE;
+
+	page1 = bpf_arena_alloc_pages(&arena, NULL, 1, NUMA_NO_NODE, 0);
+	if (!page1)
+		return 1;
+
+	page2 = bpf_arena_alloc_pages(&arena, NULL, 1, NUMA_NO_NODE, 0);
+	if (!page2)
+		return 2;
+
+	page3 = bpf_arena_alloc_pages(&arena, NULL, 1, NUMA_NO_NODE, 0);
+	if (page3)
+		return 3;
+
+	bpf_arena_free_pages(&arena, (void __arena *)bad_addr, 1);
+
+	page3 = bpf_arena_alloc_pages(&arena, NULL, 1, NUMA_NO_NODE, 0);
+	if (page3)
+		return 4;
+
+	return 0;
+}
+
 SEC("socket")
 __success __retval(0)
 int basic_alloc2_nosleep(void *ctx)