[syzbot] [kernel?] linux-next test error: UBSAN: array-index-out-of-bounds in alloc_pid

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 065efa589871 Add linux-next specific files for 20230517
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=17f27bb2280000
kernel config: https://syzkaller.appspot.com/x/.config?x=821eeb02ef201bcc
dashboard link: https://syzkaller.appspot.com/bug?extid=ac3b41786a2d0565b6d5
compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/dbbd691e9e5a/disk-065efa58.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/e5b9541c3979/vmlinux-065efa58.xz
kernel image: https://storage.googleapis.com/syzbot-assets/44cf3f3aaabb/bzImage-065efa58.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+ac3b41...@syzkaller.appspotmail.com

================================================================================
UBSAN: array-index-out-of-bounds in kernel/pid.c:244:15
index 1 is out of range for type 'upid [1]'
CPU: 1 PID: 5002 Comm: syz-executor.0 Not tainted 6.4.0-rc2-next-20230517-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/28/2023
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0x136/0x150 lib/dump_stack.c:106
ubsan_epilogue lib/ubsan.c:217 [inline]
__ubsan_handle_out_of_bounds+0xd5/0x140 lib/ubsan.c:348
alloc_pid+0xbe5/0xdd0 kernel/pid.c:244
copy_process+0x45da/0x7660 kernel/fork.c:2522
kernel_clone+0xeb/0x890 kernel/fork.c:2914
__do_sys_clone+0xba/0x100 kernel/fork.c:3057
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x39/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd
RIP: 0033:0x7f23be089e9b
Code: ed 0f 85 60 01 00 00 64 4c 8b 0c 25 10 00 00 00 45 31 c0 4d 8d 91 d0 02 00 00 31 d2 31 f6 bf 11 00 20 01 b8 38 00 00 00 0f 05 <48> 3d 00 f0 ff ff 0f 87 89 00 00 00 41 89 c5 85 c0 0f 85 90 00 00
RSP: 002b:00007ffe5c64b680 EFLAGS: 00000246 ORIG_RAX: 0000000000000038
RAX: ffffffffffffffda RBX: 00007ffe5c64bcd8 RCX: 00007f23be089e9b
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000001200011
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000555557393400
R10: 00005555573936d0 R11: 0000000000000246 R12: 0000000000000000
R13: 00007ffe5c64b770 R14: 00007f23be1ac9d8 R15: 000000000000000c
</TASK>
================================================================================


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[Christian Brauner]

On Wed, May 17, 2023 at 12:40:03AM -0700, syzbot wrote:
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: 065efa589871 Add linux-next specific files for 20230517
> git tree: linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=17f27bb2280000
> kernel config: https://syzkaller.appspot.com/x/.config?x=821eeb02ef201bcc
> dashboard link: https://syzkaller.appspot.com/bug?extid=ac3b41786a2d0565b6d5
> compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/dbbd691e9e5a/disk-065efa58.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/e5b9541c3979/vmlinux-065efa58.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/44cf3f3aaabb/bzImage-065efa58.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+ac3b41...@syzkaller.appspotmail.com
>
> ================================================================================
> UBSAN: array-index-out-of-bounds in kernel/pid.c:244:15

Only way I see this happening is if the logic in
kernel/pid_namespace.c:create_pid_cachep() which sets the object size
for the struct pid allocation of this pid namespace based on
parent_pid_namespace->level + 1 is broken. The way this works is:

struct pid
{
[snip]
struct upid numbers[1];
};

create_pid_namespace()
{
unsigned int level = parent_pid_ns->level + 1;
ns->pid_cachep = create_pid_cachep(level);
}

and then during fork:

alloc_pid()
{
pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
}

So effectively, the wrong level must've been set in
create_pid_namespace() so that the flexible array allocation is too
small.

I don't have time to debug this tbh. Ccing Kees maybe there's some
flexible array stuff going on I'm unaware of.

[Kees Cook]

I was *just* looking at this fake flex array during LSS last week. It was one of two core structs still using the ancient 1-element style.

> };
>
> create_pid_namespace()
> {
> unsigned int level = parent_pid_ns->level + 1;
> ns->pid_cachep = create_pid_cachep(level);
> }
>
>and then during fork:
>
> alloc_pid()
> {
> pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
> }
>
>So effectively, the wrong level must've been set in
>create_pid_namespace() so that the flexible array allocation is too
>small.
>
>I don't have time to debug this tbh. Ccing Kees maybe there's some
>flexible array stuff going on I'm unaware of.

Yes, I think it's due to:
https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=for-next/hardening&id=2d47c6956ab3c8b580a59d7704aab3e2a4882b6c

This makes the sanitizer treat only [0]-arrays as flex arrays.

Though I wonder why Clang hasn't warned about this yet.

Regardless, we'll need to fix struct pid. Since it uses a static initializer for "numbers[0]", this will need a bit of a tweak, but I've got patches for this. I hadn't sent them yet because I was still studying the use of the "levels" member which is off by one for the count of "numbers" elements, which some code already has to work around (using "<=" when iterating and "+ 1" for some outputs)...



--
Kees Cook