Re: A slab-out-of-bounds Read bug in __htab_map_lookup_and_delete_batch

From: Yonghong Song
Date: Thu Jan 06 2022 - 23:03:05 EST



On 1/6/22 7:25 PM, butt3rflyh4ck wrote:
Ok, I just reproduce the issue with the latest bpf-next tree.

I cannot reproduce with bpf-next tree. My bpf-next tree top commit is
70bc793382a0 selftests/bpf: Don't rely on preserving volatile in PT_REGS macros in loop3

The config difference between mine and the one you provided.

$ diff .config ~/crash-config
--- .config 2022-01-06 19:29:10.859839241 -0800
+++ /home/yhs/crash-config 2022-01-06 19:27:22.262595087 -0800
@@ -2,16 +2,17 @@
# Automatically generated file; DO NOT EDIT.
# Linux/x86 5.16.0-rc7 Kernel Configuration
#
-CONFIG_CC_VERSION_TEXT="gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-3)"
+CONFIG_CC_VERSION_TEXT="gcc (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0"
CONFIG_CC_IS_GCC=y
-CONFIG_GCC_VERSION=80500
+CONFIG_GCC_VERSION=90300
CONFIG_CLANG_VERSION=0
CONFIG_AS_IS_GNU=y
-CONFIG_AS_VERSION=23000
+CONFIG_AS_VERSION=23400
CONFIG_LD_IS_BFD=y
-CONFIG_LD_VERSION=23000
+CONFIG_LD_VERSION=23400
CONFIG_LLD_VERSION=0
CONFIG_CC_CAN_LINK=y
+CONFIG_CC_CAN_LINK_STATIC=y
CONFIG_CC_HAS_ASM_GOTO=y
CONFIG_CC_HAS_ASM_INLINE=y
CONFIG_CC_HAS_NO_PROFILE_FN_ATTR=y
@@ -117,7 +118,7 @@
CONFIG_BPF_UNPRIV_DEFAULT_OFF=y
CONFIG_USERMODE_DRIVER=y
CONFIG_BPF_PRELOAD=y
-CONFIG_BPF_PRELOAD_UMD=m
+CONFIG_BPF_PRELOAD_UMD=y
# CONFIG_BPF_LSM is not set
# end of BPF subsystem

@@ -8456,7 +8457,6 @@
# CONFIG_DEBUG_INFO_DWARF4 is not set
# CONFIG_DEBUG_INFO_DWARF5 is not set
# CONFIG_DEBUG_INFO_BTF is not set
-CONFIG_PAHOLE_HAS_SPLIT_BTF=y
# CONFIG_GDB_SCRIPTS is not set
CONFIG_FRAME_WARN=2048
# CONFIG_STRIP_ASM_SYMS is not set

The main difference is compiler and maybe a couple of other things
which I think should not impact the result.

On Fri, Jan 7, 2022 at 9:19 AM Yonghong Song <yhs@xxxxxx> wrote:



On 12/29/21 7:23 PM, butt3rflyh4ck wrote:
Hi, the attachment is a reproducer. Enjoy it.

Regards,
butt3rflyh4ck.


On Thu, Dec 30, 2021 at 10:23 AM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:

On Wed, Dec 29, 2021 at 2:10 AM butt3rflyh4ck
<butterflyhuangxx@xxxxxxxxx> wrote:

Hi, there is a slab-out-bounds Read bug in
__htab_map_lookup_and_delete_batch in kernel/bpf/hashtab.c
and I reproduce it in linux-5.16.rc7(upstream) and latest linux-5.15.11.

#carsh log
[ 166.945208][ T6897]
==================================================================
[ 166.947075][ T6897] BUG: KASAN: slab-out-of-bounds in _copy_to_user+0x87/0xb0
[ 166.948612][ T6897] Read of size 49 at addr ffff88801913f800 by
task __htab_map_look/6897
[ 166.950406][ T6897]
[ 166.950890][ T6897] CPU: 1 PID: 6897 Comm: __htab_map_look Not
tainted 5.16.0-rc7+ #30
[ 166.952521][ T6897] Hardware name: QEMU Standard PC (i440FX + PIIX,
1996), BIOS 1.13.0-1ubuntu1 04/01/2014
[ 166.954562][ T6897] Call Trace:
[ 166.955268][ T6897] <TASK>
[ 166.955918][ T6897] dump_stack_lvl+0x57/0x7d
[ 166.956875][ T6897] print_address_description.constprop.0.cold+0x93/0x347
[ 166.958411][ T6897] ? _copy_to_user+0x87/0xb0
[ 166.959356][ T6897] ? _copy_to_user+0x87/0xb0
[ 166.960272][ T6897] kasan_report.cold+0x83/0xdf
[ 166.961196][ T6897] ? _copy_to_user+0x87/0xb0
[ 166.962053][ T6897] kasan_check_range+0x13b/0x190
[ 166.962978][ T6897] _copy_to_user+0x87/0xb0
[ 166.964340][ T6897] __htab_map_lookup_and_delete_batch+0xdc2/0x1590
[ 166.965619][ T6897] ? htab_lru_map_update_elem+0xe70/0xe70
[ 166.966732][ T6897] bpf_map_do_batch+0x1fa/0x460
[ 166.967619][ T6897] __sys_bpf+0x99a/0x3860
[ 166.968443][ T6897] ? bpf_link_get_from_fd+0xd0/0xd0
[ 166.969393][ T6897] ? rcu_read_lock_sched_held+0x9c/0xd0
[ 166.970425][ T6897] ? lock_acquire+0x1ab/0x520
[ 166.971284][ T6897] ? find_held_lock+0x2d/0x110
[ 166.972208][ T6897] ? rcu_read_lock_sched_held+0x9c/0xd0
[ 166.973139][ T6897] ? rcu_read_lock_bh_held+0xb0/0xb0
[ 166.974096][ T6897] __x64_sys_bpf+0x70/0xb0
[ 166.974903][ T6897] ? syscall_enter_from_user_mode+0x21/0x70
[ 166.976077][ T6897] do_syscall_64+0x35/0xb0
[ 166.976889][ T6897] entry_SYSCALL_64_after_hwframe+0x44/0xae
[ 166.978027][ T6897] RIP: 0033:0x450f0d


In hashtable, if the elements' keys have the same jhash() value, the
elements will be put into the same bucket.
By putting a lot of elements into a single bucket, the value of
bucket_size can be increased to overflow.
but also we can increase bucket_cnt to out of bound Read.

But here bucket_size equals to bucket_cnt (the number of elements in a bucket), bucket_cnt has u32 type. The hash table max_entries maximum is
UINT_MAX, so bucket_cnt can at most be UINT_MAX. So I am not sure
how bucket_size/bucket_cnt could overflow. Even if bucket_cnt overflows,
it will wrap as 0 which should not cause issues either.

Maybe I missed something here. Since you can reproduce it, maybe you can help debug it a little bit more. It would be even better if you can provide a fix. Thanks.


I tried the attachment (reproducer) and cannot reproduce the issue
with latest bpf-next tree.
My config has kasan enabled. Could you send the matching .config file
as well so I could reproduce?


Can you be more specific?
If you can send a patch with a fix it would be even better.

the out of bound Read in __htab_map_lookup_and_delete_batch code:
```
...
if (bucket_cnt && (copy_to_user(ukeys + total * key_size, keys,
key_size * bucket_cnt) ||
copy_to_user(uvalues + total * value_size, values,
value_size * bucket_cnt))) {
ret = -EFAULT;
goto after_loop;
}
...
```
[...]