Re: [PATCH v3] ceph: fix OOB read in ceph_osdc_list_watchers via uncapped outdata_len
From: Viacheslav Dubeyko
Date: Tue Jun 02 2026 - 13:00:37 EST
On Tue, 2026-06-02 at 00:54 -0400, Pavitra Jha wrote:
> The OSD reply header field op->payload_len is wire-controlled and is
> copied directly into m->outdata_len[i] without any bounds check:
>
> m->outdata_len[i] = le32_to_cpu(op->payload_len);
>
> This value propagates unchecked to req->r_ops[0].outdata_len and is
> then used to set the decode boundary in ceph_osdc_list_watchers():
>
> void *const end = p + req->r_ops[0].outdata_len;
>
> The actual data allocation is always exactly one page:
> ceph_alloc_page_vector(1, GFP_NOIO)
> ceph_osd_data_pages_init(..., PAGE_SIZE, ...)
>
> The messenger caps the copy to PAGE_SIZE bytes, but the decode window
> end is set from the uncapped wire value. A malicious OSD can send
> outdata_len=0x10000, causing _safe decoder boundary checks to pass
> while the physical reads cross the slab allocation boundary.
>
> KASAN report (kernel 7.0.0-rc7, QEMU/x86_64, KASLR disabled):
>
> ==================================================================
> BUG: KASAN: slab-out-of-bounds in ceph_oob2_init+0x23d/0xff0
> [ceph_oob2_poc]
> Read of size 4 at addr ffff88800a229f9e by task insmod/57
>
> CPU: 0 UID: 0 PID: 57 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
> ? __pfx__raw_spin_lock_irqsave+0x10/0x10
> kasan_report+0xda/0x110
> ? ceph_oob2_init+0x23d/0xff0 [ceph_oob2_poc]
> ? ceph_oob2_init+0x23d/0xff0 [ceph_oob2_poc]
> ? __pfx_ceph_oob2_init+0x10/0x10 [ceph_oob2_poc]
> ceph_oob2_init+0x23d/0xff0 [ceph_oob2_poc]
> do_one_initcall+0x9a/0x3a0
> ? __pfx_do_one_initcall+0x10/0x10
> ? kasan_unpoison+0x44/0x70
> do_init_module+0x27c/0x790
> ? __pfx_do_init_module+0x10/0x10
> ? __kasan_slab_free+0x47/0x70
> ? kfree+0x15f/0x3b0
> load_module+0x4a9a/0x6350
> ? __pfx_load_module+0x10/0x10
> ? security_file_permission+0x24/0x50
> ? kernel_read_file+0x2ed/0x770
> ? init_module_from_file+0x15c/0x180
> init_module_from_file+0x15c/0x180
> ? __pfx_init_module_from_file+0x10/0x10
> ? tick_nohz_handler+0x2a3/0x640
> ? _raw_spin_lock+0x7e/0xd0
> idempotent_init_module+0x21f/0x750
> ? __pfx_idempotent_init_module+0x10/0x10
> ? fdget+0x4e/0x4a0
> ? fdget+0x4e/0x4a0
> __x64_sys_finit_module+0xba/0x120
> do_syscall_64+0xe2/0x570
> ? exc_page_fault+0x66/0xb0
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> Allocated by task 57:
> kasan_save_stack+0x30/0x50
> kasan_save_track+0x14/0x30
> __kasan_kmalloc+0x7f/0x90
> ceph_oob2_init+0x44/0xff0 [ceph_oob2_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 ffff88800a229000
> which belongs to the cache kmalloc-4k of size 4096
> The buggy address is located 3998 bytes inside of
> allocated 4000-byte region [ffff88800a229000, ffff88800a229fa0)
>
> Memory state around the buggy address:
> ffff88800a229e80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> ffff88800a229f00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> >ffff88800a229f80: 00 00 00 00 fc fc fc fc fc fc fc fc fc fc fc fc
> ^
> ffff88800a22a000: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ffff88800a22a080: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ==================================================================
>
> val=0xccccaaaa (OOB garbage from KASAN redzone)
>
> Fix by introducing buf_len to hold the allocation size, using it in
> both ceph_osd_data_pages_init() and the min_t() decode boundary cap,
> so the two are guaranteed to stay in sync if the buffer size changes.
>
> Attacker model: a malicious or compromised OSD in a multi-tenant
> Ceph deployment can trigger this against any client issuing
> CEPH_OSD_OP_LIST_WATCHERS without further privileges beyond OSD
> session establishment.
>
> Fixes: a4ed38d7a180 ("libceph: support for
> CEPH_OSD_OP_LIST_WATCHERS")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Pavitra Jha <jhapavitra98@xxxxxxxxx>
> ---
> v3: Split overlong min_t() line to fit 80-column limit,
> per Viacheslav Dubeyko's review of v2.
> v2: Introduce buf_len variable instead of hardcoding PAGE_SIZE
> independently in ceph_osd_data_pages_init() and the min_t() cap,
> per Viacheslav Dubeyko's review.
> ---
> net/ceph/osd_client.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/net/ceph/osd_client.c b/net/ceph/osd_client.c
> index a67093cf4..0a55bc1f9 100644
> --- a/net/ceph/osd_client.c
> +++ b/net/ceph/osd_client.c
> @@ -5063,6 +5063,7 @@ int ceph_osdc_list_watchers(struct
> ceph_osd_client *osdc,
> struct ceph_osd_request *req;
> struct page **pages;
> int ret;
> + const size_t buf_len = PAGE_SIZE;
>
> req = ceph_osdc_alloc_request(osdc, NULL, 1, false,
> GFP_NOIO);
> if (!req)
> @@ -5081,7 +5082,7 @@ int ceph_osdc_list_watchers(struct
> ceph_osd_client *osdc,
> osd_req_op_init(req, 0, CEPH_OSD_OP_LIST_WATCHERS, 0);
> ceph_osd_data_pages_init(osd_req_op_data(req, 0,
> list_watchers,
> response_data),
> - pages, PAGE_SIZE, 0, false, true);
> + pages, buf_len, 0, false, true);
>
> ret = ceph_osdc_alloc_messages(req, GFP_NOIO);
> if (ret)
> @@ -5091,7 +5092,8 @@ int ceph_osdc_list_watchers(struct
> ceph_osd_client *osdc,
> ret = ceph_osdc_wait_request(osdc, req);
> if (ret >= 0) {
> void *p = page_address(pages[0]);
> - void *const end = p + min_t(u32, req-
> >r_ops[0].outdata_len, PAGE_SIZE);
> + void *const end = p +
> + min_t(u32, req->r_ops[0].outdata_len,
> buf_len);
Now, min_t() worries me slightly because req->r_ops[0].outdata_len is
u32 data type, but buf_len is size_t. Could we have the same data type
for both variables?
Thanks,
Slava.
>
> ret = decode_watchers(&p, end, watchers,
> num_watchers);
> }