[PATCH 5.4 17/85] bpf: Fix sysfs export of empty BTF section

From: Greg Kroah-Hartman
Date: Mon Oct 12 2020 - 09:42:19 EST


From: Tony Ambardar <tony.ambardar@xxxxxxxxx>

commit e23bb04b0c938588eae41b7f4712b722290ed2b8 upstream.

If BTF data is missing or removed from the ELF section it is still exported
via sysfs as a zero-length file:

root@OpenWrt:/# ls -l /sys/kernel/btf/vmlinux
-r--r--r-- 1 root root 0 Jul 18 02:59 /sys/kernel/btf/vmlinux

Moreover, reads from this file succeed and leak kernel data:

root@OpenWrt:/# hexdump -C /sys/kernel/btf/vmlinux|head -10
000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
000cc0 00 00 00 00 00 00 00 00 00 00 00 00 80 83 b0 80 |................|
000cd0 00 10 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000ce0 00 00 00 00 00 00 00 00 00 00 00 00 57 ac 6e 9d |............W.n.|
000cf0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
*
002650 00 00 00 00 00 00 00 10 00 00 00 01 00 00 00 01 |................|
002660 80 82 9a c4 80 85 97 80 81 a9 51 68 00 00 00 02 |..........Qh....|
002670 80 25 44 dc 80 85 97 80 81 a9 50 24 81 ab c4 60 |.%D.......P$...`|

This situation was first observed with kernel 5.4.x, cross-compiled for a
MIPS target system. Fix by adding a sanity-check for export of zero-length
data sections.

Fixes: 341dfcf8d78e ("btf: expose BTF info through sysfs")
Signed-off-by: Tony Ambardar <Tony.Ambardar@xxxxxxxxx>
Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Acked-by: John Fastabend <john.fastabend@xxxxxxxxx>
Acked-by: Andrii Nakryiko <andriin@xxxxxx>
Link: https://lore.kernel.org/bpf/b38db205a66238f70823039a8c531535864eaac5.1600417359.git.Tony.Ambardar@xxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
kernel/bpf/sysfs_btf.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

--- a/kernel/bpf/sysfs_btf.c
+++ b/kernel/bpf/sysfs_btf.c
@@ -30,15 +30,15 @@ static struct kobject *btf_kobj;

static int __init btf_vmlinux_init(void)
{
- if (!__start_BTF)
+ bin_attr_btf_vmlinux.size = __stop_BTF - __start_BTF;
+
+ if (!__start_BTF || bin_attr_btf_vmlinux.size == 0)
return 0;

btf_kobj = kobject_create_and_add("btf", kernel_kobj);
if (!btf_kobj)
return -ENOMEM;

- bin_attr_btf_vmlinux.size = __stop_BTF - __start_BTF;
-
return sysfs_create_bin_file(btf_kobj, &bin_attr_btf_vmlinux);
}