bpf/arena: fix softlockup in arena_map_free on 64k page kernel

On an aarch64 kernel with CONFIG_PAGE_SIZE_64KB=y (64k pages),
arena_htab tests cause a segmentation fault and soft lockup.

$ sudo ./test_progs -t arena_htab
Caught signal #11!
Stack trace:
./test_progs(crash_handler+0x1c)[0x7bd4d8]
linux-vdso.so.1(__kernel_rt_sigreturn+0x0)[0xffffb34a0968]
./test_progs[0x420f74]
./test_progs(htab_lookup_elem+0x3c)[0x421090]
./test_progs[0x421320]
./test_progs[0x421bb8]
./test_progs(test_arena_htab+0x40)[0x421c14]
./test_progs[0x7bda84]
./test_progs(main+0x65c)[0x7bf670]
/usr/lib64/libc.so.6(+0x2caa0)[0xffffb31ecaa0]
/usr/lib64/libc.so.6(__libc_start_main+0x98)[0xffffb31ecb78]
./test_progs(_start+0x30)[0x41b4f0]

Message from syslogd@bpfol9aarch64 at Feb  4 08:50:09 ...
 kernel:watchdog: BUG: soft lockup - CPU#1 stuck for 26s! [kworker/u8:4:7589]

The same failure is not observed with 4k pages on aarch64.

Investigating further, it turns out arena_map_free() was calling
apply_to_existing_page_range() with the address returned by
bpf_arena_get_kern_vm_start().  If this address is not page-aligned -
as is the case for a 64k page kernel - we wind up calling apply_to_pte_range()
with that unaligned address.  The problem is apply_to_pte_range() implicitly
assumes that the addr passed in is page-aligned, specifically in this loop:

		do {
                        if (create || !pte_none(ptep_get(pte))) {
                                err = fn(pte++, addr, data);
                                if (err)
                                        break;
                        }
                } while (addr += PAGE_SIZE, addr != end);

If addr is _not_ page-aligned, it will never equal end exactly.

One solution is to round up the address returned by bpf_arena_get_kern_vm_start()
to a page-aligned value.  With that change in place the test passes:

$ sudo ./test_progs -t arena_htab
Summary: 1/1 PASSED, 1 SKIPPED, 0 FAILED

Reported-by: Colm Harrington <[email protected]>
Signed-off-by: Alan Maguire <[email protected]>

Author: alan-maguire

kernel/bpf/arena.c

@@ -54,7 +54,7 @@ struct bpf_arena {
 
 u64 bpf_arena_get_kern_vm_start(struct bpf_arena *arena)
 {
-	return arena ? (u64) (long) arena->kern_vm->addr + GUARD_SZ / 2 : 0;
+	return arena ? (u64) round_up((long) arena->kern_vm->addr + GUARD_SZ / 2, PAGE_SIZE) : 0;
 }
 
 u64 bpf_arena_get_user_vm_start(struct bpf_arena *arena)