ceph_start_decoding() accepts struct_len=0 as valid:
ceph_decode_need(p, end, 0, bad) always passes. When a malicious or
compromised OSD sends a cls_lock_get_info_reply with struct_len=0,
ceph_start_decoding() returns success with p == end, leaving zero
bytes guaranteed for subsequent reads.

The immediately following bare ceph_decode_32(p) in decode_lockers()
has no preceding bounds check. With p == end this is a 4-byte read
past the validated buffer boundary. The garbage value is then passed
to kzalloc_objs() as the locker count.

The sibling function decode_watchers() in osd_client.c already uses
the safe variant ceph_decode_32_safe() after its own
ceph_start_decoding() call. decode_lockers() is the only site using
the bare variant, confirming an oversight.

Fix by replacing ceph_decode_32(p) with ceph_decode_32_safe(p, end,
*num_lockers, err_inval), adding a new err_inval label that returns
-EINVAL directly without attempting to free an uninitialized lockers
pointer.

KASAN report (kernel 7.0.0-rc7, QEMU/x86_64, KASLR disabled):
  ==================================================================
  BUG: KASAN: slab-out-of-bounds in ceph_oob3_init+0x251/0xff0 [ceph_oob3_poc]
  Read of size 4 at addr ffff88800a29b76e by task insmod/58

  CPU: 0 UID: 0 PID: 58 Comm: insmod Tainted: G           O        7.0.0-rc7-g9c2abf69da83-dirty #15 PREEMPT(lazy)
  Tainted: [O]=OOT_MODULE
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-debian-1.17.0-1 04/01/2014
  Call Trace:
   <TASK>
   dump_stack_lvl+0x4d/0x70
   print_report+0x170/0x4f3
   kasan_report+0xda/0x110
   ceph_oob3_init+0x251/0xff0 [ceph_oob3_poc]
   do_one_initcall+0x9a/0x3a0
   do_init_module+0x27c/0x790
   load_module+0x4a9a/0x6350
   init_module_from_file+0x15c/0x180
   idempotent_init_module+0x21f/0x750
   __x64_sys_finit_module+0xba/0x120
   do_syscall_64+0xe2/0x570
   entry_SYSCALL_64_after_hwframe+0x77/0x7f

  Allocated by task 58:
   kasan_save_stack+0x30/0x50
   kasan_save_track+0x14/0x30
   __kasan_kmalloc+0x7f/0x90
   ceph_oob3_init+0x4d/0xff0 [ceph_oob3_poc]
   do_one_initcall+0x9a/0x3a0
   do_init_module+0x27c/0x790
   load_module+0x4a9a/0x6350
   init_module_from_file+0x15c/0x180
   idempotent_init_module+0x21f/0x750
   __x64_sys_finit_module+0xba/0x120
   do_syscall_64+0xe2/0x570
   entry_SYSCALL_64_after_hwframe+0x77/0x7f

  The buggy address belongs to the object at ffff88800a29a000
   which belongs to the cache kmalloc-8k of size 8192
  The buggy address is located 5998 bytes inside of
   allocated 6000-byte region [ffff88800a29a000, ffff88800a29b770)

  Memory state around the buggy address:
   ffff88800a29b600: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
   ffff88800a29b680: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  >ffff88800a29b700: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 fc fc
                                                               ^
   ffff88800a29b780: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
  ==================================================================

  num_lockers=0xccccaaaa (OOB garbage from KASAN redzone)

Attacker model: a malicious or compromised OSD in a multi-tenant Ceph
deployment can trigger this against any kernel client that issues the
lock.get_info class method (e.g. during RBD exclusive lock acquisition)
without any further privileges beyond OSD session establishment.

Fixes: d4ed4a530562 ("libceph: support for lock.lock_info")
Cc: stable@vger.kernel.org
Signed-off-by: Pavitra Jha <jhapavitra98@gmail.com>
---
 net/ceph/cls_lock_client.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/ceph/cls_lock_client.c b/net/ceph/cls_lock_client.c
index c6956f1df..78276273c 100644
--- a/net/ceph/cls_lock_client.c
+++ b/net/ceph/cls_lock_client.c
@@ -299,7 +299,7 @@ static int decode_lockers(void **p, void *end, u8 *type, char **tag,
 	if (ret)
 		return ret;
 
-	*num_lockers = ceph_decode_32(p);
+	ceph_decode_32_safe(p, end, *num_lockers, err_inval);
 	*lockers = kzalloc_objs(**lockers, *num_lockers, GFP_NOIO);
 	if (!*lockers)
 		return -ENOMEM;
@@ -320,6 +320,8 @@ static int decode_lockers(void **p, void *end, u8 *type, char **tag,
 	*tag = s;
 	return 0;
 
+err_inval:
+	return -EINVAL;
 err_free_lockers:
 	ceph_free_lockers(*lockers, *num_lockers);
 	return ret;
-- 
2.53.0