[Bug] WARNING: zero-size vmalloc in sel_write_load

[Sam Sun]

Dear developers and maintainers,

We encountered a warning in function sel_write_load(). It is tested on
kernel 6.8.0-rc7. Bug report is listed below.

```
WARNING: CPU: 1 PID: 8109 at mm/vmalloc.c:3247
__vmalloc_node_range+0x10a0/0x1490 mm/vmalloc.c:3247
Modules linked in:
CPU: 1 PID: 8109 Comm: syz-executor370 Not tainted 6.7.0-rc7 #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
RIP: 0010:__vmalloc_node_range+0x10a0/0x1490 mm/vmalloc.c:3247
Code: 65 48 2b 04 25 28 00 00 00 0f 85 98 02 00 00 48 81 c4 70 01 00
00 4c 89 e0 5b 5d 41 5c 41 5d 41 5e 41 5f c3 e8 71 43 b7 ff 90 <0f> 0b
90 45 31 e4 eb a1 e8 63 43 b7 ff 48 8b 4c 24 40 31 f6 45 31
RSP: 0018:ffffc90002adf9c0 EFLAGS: 00010293
RAX: 0000000000000000 RBX: dffffc0000000000 RCX: ffffffff81cdc194
RDX: ffff888022124ec0 RSI: ffffffff81cdd16f RDI: 0000000000000007
RBP: 0000000000000000 R08: 0000000000000007 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000001 R12: ffff888107373a48
R13: dffffc0000000000 R14: 0000000000000000 R15: ffffc90002adfec8
FS:  00005555560953c0(0000) GS:ffff888135c00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020000010 CR3: 000000010503d000 CR4: 0000000000750ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
PKRU: 55555554
Call Trace:
 <TASK>
 __vmalloc_node mm/vmalloc.c:3385 [inline]
 vmalloc+0x6b/0x80 mm/vmalloc.c:3418
 sel_write_load+0x27f/0x19d0 security/selinux/selinuxfs.c:603
 vfs_write+0x2a9/0xd80 fs/read_write.c:582
 ksys_pwrite64 fs/read_write.c:699 [inline]
 __do_sys_pwrite64 fs/read_write.c:709 [inline]
 __se_sys_pwrite64 fs/read_write.c:706 [inline]
 __x64_sys_pwrite64+0x1f3/0x250 fs/read_write.c:706
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0x40/0x110 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x63/0x6b
RIP: 0033:0x7f40e7728f8d
Code: 28 c3 e8 46 1e 00 00 66 0f 1f 44 00 00 f3 0f 1e fa 48 89 f8 48
89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d
01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fff5bf39508 EFLAGS: 00000246 ORIG_RAX: 0000000000000012
RAX: ffffffffffffffda RBX: 00007fff5bf39708 RCX: 00007f40e7728f8d
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000003
RBP: 0000000000000001 R08: 0000000000000000 R09: 00007fff5bf39708
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
R13: 00007fff5bf396f8 R14: 00007f40e77a6530 R15: 0000000000000001
 </TASK>
```

The cause of this bug is that in sel_write_load(), parameter "count"
is controlled by user, which could be zero. It is passed to vmalloc()
as an argument.

If you have any questions, please contact us.
Reported by: Yue Sun <samsun1006219@xxxxxxxxx>
Reported by: xingwei lee <xrivendell7@xxxxxxxxx>

Best Regards,
Yue

Attachment: config
Description: Binary data

// autogenerated by syzkaller (https://github.com/google/syzkaller)

#define _GNU_SOURCE

#include <endian.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>

uint64_t r[1] = {0xffffffffffffffff};

int main(void)
{
  syscall(__NR_mmap, /*addr=*/0x1ffff000ul, /*len=*/0x1000ul, /*prot=*/0ul,
          /*flags=*/0x32ul, /*fd=*/-1, /*offset=*/0ul);
  syscall(__NR_mmap, /*addr=*/0x20000000ul, /*len=*/0x1000000ul, /*prot=*/7ul,
          /*flags=*/0x32ul, /*fd=*/-1, /*offset=*/0ul);
  syscall(__NR_mmap, /*addr=*/0x21000000ul, /*len=*/0x1000ul, /*prot=*/0ul,
          /*flags=*/0x32ul, /*fd=*/-1, /*offset=*/0ul);
  intptr_t res = 0;
  memcpy((void*)0x20000000, "/sys/fs/selinux/load\000", 21);
  res = syscall(__NR_openat, /*fd=*/0xffffffffffffff9cul, /*file=*/0x20000000ul,
                /*flags=*/1ul, /*mode=*/0ul);
  if (res != -1)
    r[0] = res;
  syscall(__NR_pwrite64, /*fd=*/r[0], /*buf=*/0ul, /*count=*/0ul, /*pos=*/0ul);
  return 0;
}