Re: [PATCH v3] ceph: fix multiple unsafe decodes in decode_locker()

On Fri, Aug 7, 2026 at 5:15 PM Pavitra Jha <[email protected]> wrote:
>
> Ilya,
>
> This has been Reviewed-by'd by Slava since Jun 2 (v3), no outstanding
> comments or NACKs on this one. It's been quiet for two months now.
> Any blocker to applying, or should I rebase and resend?

Hi Pavitra,

On a second look, I didn't see an actual Reviewed-by tag from Slava
on any of the revisions, so formally this patch was still unreviewed.
I went ahead and reviewed it myself, it's now applied with a small
modification to use ceph_decode_skip_string() helper instead of two
separate calls to ceph_decode_32_safe() and ceph_decode_skip_n():

https://github.com/ceph/ceph-client/commit/437b6551cfcc235eea1d735a874f9d421f555e17

Thanks,

                Ilya

>
> thanks,
> Pavitra
>
> On Tue, 2 Jun 2026 at 10:32, Pavitra Jha <[email protected]> wrote:
> >
> > decode_locker() in cls_lock_client.c contains three unsafe decode
> > operations that allow a malicious or compromised OSD to trigger
> > slab-out-of-bounds reads:
> >
> > 1. ceph_decode_copy() at the locker_id_t name field has no preceding
> >    bounds check. With p == end after ceph_start_decoding() accepts
> >    struct_len=0, this reads sizeof(ceph_entity_name) = 9 bytes past
> >    the validated buffer boundary.
> >
> > 2. *p += sizeof(struct ceph_timespec) after the locker_info_t header
> >    is an unchecked pointer advance. A malicious OSD can position p
> >    past end, causing all subsequent _safe checks to pass against a
> >    bogus boundary.
> >
> > 3. len = ceph_decode_32(p) has no preceding bounds check, and the
> >    immediately following *p += len is uncapped. A malicious OSD can
> >    send len=0xffffffff, advancing p gigabytes past end and escaping
> >    the decode window entirely.
> >
> > Fix all three by replacing bare operations with their safe variants:
> >   ceph_decode_copy   -> ceph_decode_copy_safe
> >   *p += sizeof(...)  -> ceph_decode_skip_n
> >   ceph_decode_32(p)  -> ceph_decode_32_safe
> >   *p += len          -> ceph_decode_skip_n
> >
> > A new out_bad: label is added to return -EINVAL on any bounds
> > violation. -EINVAL is appropriate here: the data received from the OSD
> > is structurally malformed, which is an invalid argument to the decode
> > contract regardless of whether the caller or the wire is at fault.
> >
> > KASAN report (kernel 7.0.0-rc7, QEMU/x86_64, KASLR disabled):
> >
> >   [   26.183969] ceph_oob4_poc: buf=ffff888009e31000 end=ffff888009e31fa0
> >   [   26.186087] ceph_oob4_poc: struct_v=1 struct_len=0 p==end: 1
> >   [   26.186738] ceph_oob4_poc: triggering bare ceph_decode_32 past slab boundary...
> >   [   26.187679] ==================================================================
> >   [   26.188236] BUG: KASAN: slab-out-of-bounds in ceph_oob4_init+0x22b/0xff0 [ceph_oob4_poc]
> >   [   26.188236] Read of size 4 at addr ffff888009e31fa0 by task insmod/59
> >   [   26.188236] CPU: 0 UID: 0 PID: 59 Comm: insmod Tainted: G           O        7.0.0-rc7-g9c2abf69da83-dirty #15 PREEMPT(lazy)
> >   [   26.188236] Tainted: [O]=OOT_MODULE
> >   [   26.188236] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-debian-1.17.0-1 04/01/2014
> >   [   26.188236] Call Trace:
> >   [   26.188236]  <TASK>
> >   [   26.188236]  dump_stack_lvl+0x4d/0x70
> >   [   26.188236]  print_report+0x170/0x4f3
> >   [   26.188236]  kasan_report+0xda/0x110
> >   [   26.188236]  ceph_oob4_init+0x22b/0xff0 [ceph_oob4_poc]
> >   [   26.188236]  do_one_initcall+0x9a/0x3a0
> >   [   26.188236]  do_init_module+0x27c/0x790
> >   [   26.188236]  load_module+0x4a9a/0x6350
> >   [   26.188236]  init_module_from_file+0x15c/0x180
> >   [   26.188236]  idempotent_init_module+0x21f/0x750
> >   [   26.188236]  __x64_sys_finit_module+0xba/0x120
> >   [   26.188236]  do_syscall_64+0xe2/0x570
> >   [   26.188236]  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> >   [   26.188236]  </TASK>
> >   [   26.188236] The buggy address belongs to the object at ffff888009e31000
> >   [   26.188236]  which belongs to the cache kmalloc-4k of size 4096
> >   [   26.188236] The buggy address is located 0 bytes to the right of
> >   [   26.188236]  allocated 4000-byte region [ffff888009e31000, ffff888009e31fa0)
> >   [   26.188236]  ffff888009e31f80: 00 00 00 00 fc fc fc fc fc fc fc fc fc fc fc fc
> >   [   26.188236]                                ^
> >   [   26.188236]  ffff888009e32000: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> >   [   26.188236] ==================================================================
> >   [   26.255513] ceph_oob4_poc: len=0xcccccccc (OOB garbage from KASAN redzone)
> >
> >   0xCCCCCCCC is KASAN redzone poison, confirming the read landed in
> >   the slab redzone immediately past the 4000-byte allocation.
> >
> > 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: [email protected]
> > Signed-off-by: Pavitra Jha <[email protected]>
> > ---
> > v3: Split ceph_decode_copy_safe call to fit 80-column limit,
> >     per Viacheslav Dubeyko's review of v2.
> > v2: Move inline comments above ceph_decode_skip_n calls, rename
> >     label bad -> out_bad, per Viacheslav Dubeyko's review.
> > ---
> >  net/ceph/cls_lock_client.c | 13 +++++++++----
> >  1 file changed, 9 insertions(+), 4 deletions(-)
> >
> > diff --git a/net/ceph/cls_lock_client.c b/net/ceph/cls_lock_client.c
> > index 4e6a6d3e4..79a449897 100644
> > --- a/net/ceph/cls_lock_client.c
> > +++ b/net/ceph/cls_lock_client.c
> > @@ -259,7 +259,8 @@ static int decode_locker(void **p, void *end, struct ceph_locker *locker)
> >         if (ret)
> >                 return ret;
> >
> > -       ceph_decode_copy(p, &locker->id.name, sizeof(locker->id.name));
> > +       ceph_decode_copy_safe(p, end, &locker->id.name,
> > +                             sizeof(locker->id.name), out_bad);
> >         s = ceph_extract_encoded_string(p, end, NULL, GFP_NOIO);
> >         if (IS_ERR(s))
> >                 return PTR_ERR(s);
> > @@ -270,19 +271,23 @@ static int decode_locker(void **p, void *end, struct ceph_locker *locker)
> >         if (ret)
> >                 return ret;
> >
> > -       *p += sizeof(struct ceph_timespec); /* skip expiration */
> > +       /* skip expiration */
> > +       ceph_decode_skip_n(p, end, sizeof(struct ceph_timespec), out_bad);
> >
> >         ret = ceph_decode_entity_addr(p, end, &locker->info.addr);
> >         if (ret)
> >                 return ret;
> >
> > -       len = ceph_decode_32(p);
> > -       *p += len; /* skip description */
> > +       ceph_decode_32_safe(p, end, len, out_bad);
> > +       /* skip description */
> > +       ceph_decode_skip_n(p, end, len, out_bad);
> >
> >         dout("%s %s%llu cookie %s addr %s\n", __func__,
> >              ENTITY_NAME(locker->id.name), locker->id.cookie,
> >              ceph_pr_addr(&locker->info.addr));
> >         return 0;
> > +out_bad:
> > +       return -EINVAL;
> >  }
> >
> >  static int decode_lockers(void **p, void *end, u8 *type, char **tag,
> > --
> > 2.53.0
> >