[PATCH v3] ceph: fix two unsafe bare decodes in decode_lockers()

[Pavitra Jha <jhapavitra98@gmail.com>]

decode_lockers() in cls_lock_client.c contains two bare decode operations
that allow a malicious or compromised OSD to trigger slab-out-of-bounds
reads:

1. ceph_decode_32(p) at the num_lockers field has no preceding bounds
   check. ceph_start_decoding() accepts struct_len=0 as valid -- the
   internal ceph_decode_need(p, end, 0, bad) always passes -- so when an
   OSD sends struct_len=0, ceph_start_decoding() returns success with
   p == end. The immediately following bare ceph_decode_32(p) then reads
   4 bytes past the validated buffer boundary. The garbage value is
   passed directly to kzalloc_objs() as the locker count.

   The sibling function decode_watchers() in osd_client.c already uses
   ceph_decode_32_safe() after its own ceph_start_decoding() call.
   decode_lockers() was the only site using the bare variant.

2. ceph_decode_8(p) after the decode_locker() loop has no preceding
   bounds check. If an OSD crafts num_lockers such that the loop
   advances p exactly to end, the subsequent bare ceph_decode_8(p) reads
   one byte past the validated buffer boundary. The result is passed
   directly into *type, which is used as a lock type discriminator by
   callers, giving an OSD-controlled one-byte OOB read with direct
   influence over the lock type field.

Fix both by replacing bare operations with their safe variants:
  ceph_decode_32(p) -> ceph_decode_32_safe(p, end, *num_lockers,
                                           err_inval)
  ceph_decode_8(p)  -> ceph_decode_8_safe(p, end, *type,
                                          err_free_lockers)

The goto targets differ intentionally:
  err_inval: is a new label returning -EINVAL directly. It is used for
  the pre-allocation failure path where *lockers is not yet allocated
  and must not be passed to ceph_free_lockers().

  err_free_lockers: is the existing label. It is used for the
  post-allocation failure path where *lockers is allocated and must
  be freed.

ret is set to -EINVAL before ceph_decode_8_safe() so that
err_free_lockers returns the correct error code on bounds violation.
Without this, err_free_lockers would return a stale ret value (0 from
the successful decode_locker() loop), silently swallowing the error.

-EINVAL is correct for both failure paths. The data received from the
OSD is structurally malformed. -ENOMEM would misrepresent the failure
class to callers and to stable@ backporters triaging error paths.

KASAN report for bug 1 (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)

Bug 2 (ceph_decode_8) follows from the identical precondition. A
dedicated PoC is available on request.

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>
---
v3: Combine both fixes (ceph_decode_32 and ceph_decode_8) into a single
    patch per Viacheslav Dubeyko's review. Set ret = -EINVAL before
    ceph_decode_8_safe() so err_free_lockers returns the correct error
    code, not stale ret (caught by Dan Carpenter / smatch). Clarify
    err_inval vs err_free_lockers goto selection rationale and
    -EINVAL justification.
---
 net/ceph/cls_lock_client.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/net/ceph/cls_lock_client.c b/net/ceph/cls_lock_client.c
index c6956f1df..4e6a6d3e4 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;
@@ -310,7 +310,8 @@ static int decode_lockers(void **p, void *end, u8 *type, char **tag,
 			goto err_free_lockers;
 	}
 
-	*type = ceph_decode_8(p);
+	ret = -EINVAL;
+	ceph_decode_8_safe(p, end, *type, err_free_lockers);
 	s = ceph_extract_encoded_string(p, end, NULL, GFP_NOIO);
 	if (IS_ERR(s)) {
 		ret = PTR_ERR(s);
@@ -320,6 +321,8 @@ 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