[PATCH] bpf: fix overflow of bpf_jit_limit when PAGE_SIZE >= 64K

[Michael Roth <mdroth@linux.vnet.ibm.com>]

Commit ede95a63b5 introduced a bpf_jit_limit tuneable to limit BPF
JIT allocations. At compile time it defaults to PAGE_SIZE * 40000,
and is adjusted again at init time if MODULES_VADDR is defined.

For ppc64 kernels, MODULES_VADDR isn't defined, so we're stuck with
the compile-time default at boot-time, which is 0x9c400000 when
using 64K page size. This overflows the signed 32-bit bpf_jit_limit
value:

  root@ubuntu:/tmp# cat /proc/sys/net/core/bpf_jit_limit
  -1673527296

and can cause various unexpected failures throughout the network
stack. In one case `strace dhclient eth0` reported:

  setsockopt(5, SOL_SOCKET, SO_ATTACH_FILTER, {len=11, filter=0x105dd27f8}, 16) = -1 ENOTSUPP (Unknown error 524)

and similar failures can be seen with tools like tcpdump. This doesn't
always reproduce however, and I'm not sure why. The more consistent
failure I've seen is an Ubuntu 18.04 KVM guest booted on a POWER9 host
would time out on systemd/netplan configuring a virtio-net NIC with no
noticeable errors in the logs.

Fix this by limiting the compile-time default for bpf_jit_limit to
INT_MAX.

Fixes: ede95a63b5e8 ("bpf: add bpf_jit_limit knob to restrict unpriv allocations")
Cc: linuxppc-dev@ozlabs.org
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Sandipan Das <sandipan@linux.ibm.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
---
 kernel/bpf/core.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index b1a3545d0ec8..55de4746cdfd 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -365,7 +365,8 @@ void bpf_prog_kallsyms_del_all(struct bpf_prog *fp)
 }
 
 #ifdef CONFIG_BPF_JIT
-# define BPF_JIT_LIMIT_DEFAULT	(PAGE_SIZE * 40000)
+# define BPF_MIN(x, y) ((x) < (y) ? (x) : (y))
+# define BPF_JIT_LIMIT_DEFAULT	BPF_MIN((PAGE_SIZE * 40000), INT_MAX)
 
 /* All BPF JIT sysctl knobs here. */
 int bpf_jit_enable   __read_mostly = IS_BUILTIN(CONFIG_BPF_JIT_ALWAYS_ON);
-- 
2.17.1