Re: [PATCH V5] nfsd: fix nfsd4_create_reclaim_record_grace crp null-ptr-deref in nfs4recover

[Chuck Lever]

On Thu, Aug 6, 2026, at 3:37 AM, Lai Zewei wrote:
> nfs4_client_to_reclaim() may return NULL if alloc_reclaim() fails. 
> The caller __nfsd4_create_reclaim_record_grace() then unconditionally
> dereferences the returned pointer via crp->cr_clp = clp, leading to a
> null-ptr-deref crash.
>
> Add a NULL check before assignment. If crp is NULL, just return.
>
> Fixes: 4552f4e3f2c9 ("nfsd: change nfs4_client_to_reclaim() to allocate data")
> Signed-off-by: Lai Zewei <laizewei3@xxxxxxxxxx>

First a process note: I don't see v1 through v4 on linux-nfs, and this
posting carries no changelog. If those were internal review rounds, you
should post the first public version as v1, or add a changelog saying
what changed between the previous revisions.

The code change is nominally correct. Commit 4552f4e3f2c9 moved the
allocations into nfs4_client_to_reclaim() and removed this caller's
NULL check along with the kfree() that check guarded. Restoring it
keeps this caller consistent with load_recdir() and
__cld_pipe_inprogress_downcall(), and both handle a NULL return.

However, IMO a UAF crash is not reachable. struct nfs4_client_reclaim
is just 48 bytes, and a GFP_KERNEL allocation of that size is almost
guaranteed to succeed 100% of the time on production systems. (I'll
redact the analysis of the allocator code path here).

The Fixes: tag then asks stable maintainers to backport a fix for it,
and I wonder if there's any need to.

Please send a v6 whose description says the check was dropped by
commit 4552f4e3f2c9 and is being restored, removes the Fixes: tag, and
states that the NULL return is not reachable from this caller. If the
defect was found using a static analysis tool or from slab fault
injection rather than observed in production, mention that in the
description too.

In addition, the subject names nfsd4_create_reclaim_record_grace, but
the function is __nfsd4_create_reclaim_record_grace; and "in
nfs4recover" repeats what the "nfsd:" prefix already says. Something
like "nfsd: restore nfs4_client_to_reclaim() error check" would be
more accurate.

Do let me know if I've missed something that makes this patch more
than a clean-up.


-- 
Chuck Lever