ceph_start_decoding() validates that struct_len bytes remain in the
buffer after the encoding header, but accepts struct_len=0 as valid:
ceph_decode_need(p, end, 0, bad) always passes. When a malicious or
compromised OSD sends an obj_list_watch_response_t reply with
struct_len=0, ceph_start_decoding() returns success with p == end,
leaving zero bytes guaranteed for subsequent reads.

The immediately following ceph_decode_32(p) in decode_watchers() 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
directly to kzalloc_objs() as the watcher count.

The sibling function decode_watcher() already uses the safe variants
(ceph_decode_copy_safe, ceph_decode_64_safe, ceph_decode_skip_32)
after its own ceph_start_decoding() call. decode_watchers() is the
only site that uses the bare variant, confirming an oversight.

Fix by replacing ceph_decode_32(p) with ceph_decode_32_safe(p, end,
*num_watchers, e_inval), consistent with the established pattern.

KASAN report (kernel 7.0.0-rc7, QEMU/x86_64, KASLR disabled):

  [   72.047085] ceph_oob_poc: buf=ffff8880085936c8 end=ffff8880085936ce
  [   72.048685] ceph_oob_poc: ceph_start_decoding OK: struct_v=1
  struct_len=0 p==end: 1
  [   72.049477] ceph_oob_poc: triggering OOB read past slab boundary...
  [   72.050699] ==================================================
  [   72.051427] BUG: KASAN: slab-out-of-bounds in
  ceph_oob_init+0x128/0xff0 [ceph_oob_poc]
  [   72.051427] Read of size 4 at addr ffff8880085936ce by task insmod/61
  [   72.051427] CPU: 0 UID: 0 PID: 61 Comm: insmod Tainted: G O
  [   72.051427]  7.0.0-rc7-g9c2abf69da83-dirty #14 PREEMPT(lazy)
  [   72.051427] Call Trace:
  [   72.051427]  dump_stack_lvl+0x4d/0x70
  [   72.051427]  print_report+0x170/0x4f3
  [   72.051427]  kasan_report+0xda/0x110
  [   72.051427]  kasan_check_range+0x125/0x200
  [   72.051427]  ceph_oob_init+0x128/0xff0 [ceph_oob_poc]
  [   72.051427]  do_one_initcall+0x9a/0x310
  [   72.051427]  do_init_module+0x186/0x410
  [   72.051427]  load_module+0x2ba7/0x2e50
  [   72.051427]  init_module_from_file+0x15c/0x180
  [   72.051427]  idempotent_init_module+0x19f/0x430
  [   72.051427]  __x64_sys_finit_module+0x78/0xc0
  [   72.051427]  do_syscall_64+0xe2/0x570
  [   72.051427]  entry_SYSCALL_64_after_hwframe+0x77/0x7f
  [   72.051427] The buggy address belongs to the object at ffff8880085936c8
  [   72.051427]  which belongs to the cache kmalloc-8 of size 8
  [   72.051427] The buggy address is located 0 bytes to the right of
  [   72.051427]  allocated 6-byte region [ffff8880085936c8, ffff8880085936ce)
  [   72.051427] Memory state around the buggy address:
  [   72.051427] >ffff888008593680: fc fc fc fc fc fc fc fc fc 06 fc fc fc fc fc fc
  [   72.051427]                                               ^
  [   72.051427] ==================================================
  [   72.129720] ceph_oob_poc: num_watchers=3435973836 (OOB garbage)

  0xCCCCCCCC (3435973836) is KASAN redzone poison, confirming the read
  landed in the slab redzone immediately past the 6-byte allocation.

Attacker model: a malicious or compromised OSD in a multi-tenant Ceph
deployment (e.g. cloud) can trigger this against any kernel client
that calls CEPH_OSD_OP_LIST_WATCHERS, without any further privileges
beyond OSD session establishment.

Fixes: a4ed38d7a180 ("libceph: support for CEPH_OSD_OP_LIST_WATCHERS")
Cc: stable@vger.kernel.org
Signed-off-by: Pavitra Jha <jhapavitra98@gmail.com>
---
v2: Correct commit message. v1 overstated the impact:

  - "unbounded alloc": retracted. kzalloc_objs() uses size_mul()
    internally which returns SIZE_MAX on overflow, causing kmalloc
    to return NULL. The large garbage value from the OOB read will
    simply fail allocation with -ENOMEM.

  - "decode_watcher() writing attacker-controlled data into it":
    retracted. ceph_start_decoding() calls ceph_decode_need() for
    its 6-byte header, which catches p==end and returns -ERANGE
    before any copy occurs. Verified with a follow-up KASAN harness.

  The fix itself is unchanged.
---
 net/ceph/osd_client.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/ceph/osd_client.c b/net/ceph/osd_client.c
index 2ff00070c..0148e4c40 100644
--- a/net/ceph/osd_client.c
+++ b/net/ceph/osd_client.c
@@ -5030,7 +5030,7 @@ static int decode_watchers(void **p, void *end,
 	if (ret)
 		return ret;
 
-	*num_watchers = ceph_decode_32(p);
+	ceph_decode_32_safe(p, end, *num_watchers, e_inval);
 	*watchers = kzalloc_objs(**watchers, *num_watchers, GFP_NOIO);
 	if (!*watchers)
 		return -ENOMEM;
@@ -5044,6 +5044,9 @@ static int decode_watchers(void **p, void *end,
 	}
 
 	return 0;
+
+e_inval:
+	return -EINVAL;
 }
 
 /*
-- 
2.53.0