Re: [PATCH v4] ceph: fix OOB read in decode_watchers() via missing bounds check

[Ilya Dryomov]

On Wed, Jul 8, 2026 at 7:40 AM Pavitra Jha <jhapavitra98@xxxxxxxxx> wrote:
>
> 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, bad), 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@xxxxxxxxxxxxxxx
> Reviewed-by: Viacheslav Dubeyko <Slava.Dubeyko@xxxxxxx>
> Signed-off-by: Pavitra Jha <jhapavitra98@xxxxxxxxx>
> ---
> v4: Rebase against current linux/master and ceph-testing/testing.
>     No functional changes from Slava's reviewed v3.
> v3: Rename error label e_inval -> bad per Slava Dubeyko's review.
> v2: Correct commit message; retracted overstated impact claims,
>     verified with follow-up KASAN harness.
> ---
>  net/ceph/osd_client.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/net/ceph/osd_client.c b/net/ceph/osd_client.c
> index 2ff00070c..2cdac81a6 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, bad);
>         *watchers = kzalloc_objs(**watchers, *num_watchers, GFP_NOIO);
>         if (!*watchers)
>                 return -ENOMEM;
> @@ -5044,6 +5044,8 @@ static int decode_watchers(void **p, void *end,
>         }
>
>         return 0;
> +bad:
> +       return -EINVAL;
>  }
>
>  /*
> --
> 2.53.0
>

Applied.

Thanks,

                Ilya