Re: [PATCH v2 3/4] ceph: bound num_export_targets array for mds info v2/v3

[Viacheslav Dubeyko]

On Sat, 2026-06-06 at 15:00 -0400, Michael Bommarito wrote:
> ceph_mdsmap_decode() in fs/ceph/mdsmap.c reads num_export_targets
> from
> each per-mds info record and advances the decode cursor by
> num_export_targets * sizeof(u32) without first checking that many
> bytes
> remain. The only upper-bound check that catches a runaway cursor
> (*p > info_end) is gated on info_v >= 4, because info_end is left
> NULL
> for info_v 2 and 3. When the monitor sends an MDS map whose per-mds
> info version is 2 or 3 with an oversized num_export_targets, the
> cursor
> moves past the message front buffer and the later export-targets loop
> calls the unchecked ceph_decode_32() on out-of-bounds memory.
> 
> A kernel client processes CEPH_MSG_MDS_MAP from its monitor session
> (net/ceph/mon_client.c dispatches it; fs/ceph/super.c routes it to
> ceph_mdsc_handle_mdsmap(), which sets end to the front buffer bound
> and
> calls ceph_mdsmap_decode()). A malicious or compromised monitor, or
> an
> on-path attacker on an unsigned/unencrypted messenger session, can
> therefore drive an out-of-bounds read in the client kernel; on x86_64
> with KASAN it is reported as a slab-out-of-bounds read in
> ceph_mdsmap_decode(). The decoded values land in the internal
> info->export_targets[] array, so the consequence is a kernel
> out-of-bounds read, not an information leak to the attacker.
> 
> Impact: a malicious or compromised Ceph monitor sending an MDS map
> with
> a per-mds info version of 2 or 3 and an oversized num_export_targets
> field triggers an out-of-bounds read in the CephFS client kernel.
> 
> Add a ceph_decode_need() for the export-targets array before
> advancing
> the cursor, so the bound is enforced for every info_v >= 2, not only
> info_v >= 4. This mirrors the count-then-need idiom already used for
> m_data_pg_pools later in the same function.
> 
> Compute the export-targets byte count with size_mul() and reuse that
> checked length when advancing the cursor, so the attacker-controlled
> num_export_targets multiplication fails closed on overflow rather
> than
> relying on the later kcalloc() guard.
> 
> Fixes: d463a43d69f4 ("ceph: CEPH_FEATURE_MDSENC support")
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Michael Bommarito <michael.bommarito@xxxxxxxxx>
> ---
> v2:
> - Compute the export-targets byte count with size_mul() and reuse the
>   checked length when advancing the cursor.
> 
>  fs/ceph/mdsmap.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ceph/mdsmap.c b/fs/ceph/mdsmap.c
> index d8e46eb7e5eb5..c0f63f0460f67 100644
> --- a/fs/ceph/mdsmap.c
> +++ b/fs/ceph/mdsmap.c
> @@ -3,6 +3,7 @@
>  
>  #include <linux/bug.h>
>  #include <linux/err.h>
> +#include <linux/overflow.h>
>  #include <linux/random.h>
>  #include <linux/slab.h>
>  #include <linux/types.h>
> @@ -126,6 +127,7 @@ struct ceph_mdsmap *ceph_mdsmap_decode(struct
> ceph_mds_client *mdsc, void **p,
>  	u8 mdsmap_v;
>  	u16 mdsmap_ev;
>  	u32 target;
> +	size_t export_targets_len;
>  
>  	m = kzalloc_obj(*m, GFP_NOFS);
>  	if (!m)
> @@ -224,8 +226,11 @@ struct ceph_mdsmap *ceph_mdsmap_decode(struct
> ceph_mds_client *mdsc, void **p,
>  		*p += namelen;
>  		if (info_v >= 2) {
>  			ceph_decode_32_safe(p, end,
> num_export_targets, bad);
> +			export_targets_len =
> size_mul(num_export_targets,
> +						      sizeof(u32));
> +			ceph_decode_need(p, end, export_targets_len,
> bad);
>  			pexport_targets = *p;
> -			*p += num_export_targets * sizeof(u32);
> +			*p += export_targets_len;
>  		} else {
>  			num_export_targets = 0;
>  		}

Looks good.

Reviewed-by: Viacheslav Dubeyko <slava@xxxxxxxxxxx>

Thanks,
Slava.